diff --git a/.mailmap b/.mailmap
index 8b504084266bb..d6b7ab51a48c6 100644
--- a/.mailmap
+++ b/.mailmap
@@ -113,10 +113,14 @@ Hanna Kruppe <hanna.kruppe@gmail.com> <robin.kruppe@gmail.com>
 Heather <heather@cynede.net> <Cynede@Gentoo.org>
 Heather <heather@cynede.net> <Heather@cynede.net>
 Herman J. Radtke III <herman@hermanradtke.com> Herman J. Radtke III <hermanradtke@gmail.com>
+Ian Jackson <ijackson@chiark.greenend.org.uk> <ian.jackson@citrix.com>
+Ian Jackson <ijackson@chiark.greenend.org.uk> <ijackson+github@slimy.greenend.org.uk>
+Ian Jackson <ijackson@chiark.greenend.org.uk> <iwj@xenproject.org>
 Ilyong Cho <ilyoan@gmail.com>
 inquisitivecrystal <22333129+inquisitivecrystal@users.noreply.github.com>
 Ivan Ivaschenko <defuz.net@gmail.com>
 J. J. Weber <jjweber@gmail.com>
+Jacob Pratt <jacob@jhpratt.dev> <the.z.cuber@gmail.com>
 Jakub Adam Wieczorek <jakub.adam.wieczorek@gmail.com> <jakub.bukaj@yahoo.com>
 Jakub Adam Wieczorek <jakub.adam.wieczorek@gmail.com> <jakub@jakub.cc>
 Jakub Adam Wieczorek <jakub.adam.wieczorek@gmail.com> <jakubw@jakubw.net>
diff --git a/compiler/rustc_ast/src/util/comments/tests.rs b/compiler/rustc_ast/src/util/comments/tests.rs
index 98ab653e45f70..6d137f3774fe7 100644
--- a/compiler/rustc_ast/src/util/comments/tests.rs
+++ b/compiler/rustc_ast/src/util/comments/tests.rs
@@ -1,9 +1,9 @@
 use super::*;
-use rustc_span::with_default_session_globals;
+use rustc_span::create_default_session_globals_then;
 
 #[test]
 fn test_block_doc_comment_1() {
-    with_default_session_globals(|| {
+    create_default_session_globals_then(|| {
         let comment = "\n * Test \n **  Test\n *   Test\n";
         let stripped = beautify_doc_string(Symbol::intern(comment));
         assert_eq!(stripped.as_str(), " Test \n*  Test\n   Test");
@@ -12,7 +12,7 @@ fn test_block_doc_comment_1() {
 
 #[test]
 fn test_block_doc_comment_2() {
-    with_default_session_globals(|| {
+    create_default_session_globals_then(|| {
         let comment = "\n * Test\n *  Test\n";
         let stripped = beautify_doc_string(Symbol::intern(comment));
         assert_eq!(stripped.as_str(), " Test\n  Test");
@@ -21,7 +21,7 @@ fn test_block_doc_comment_2() {
 
 #[test]
 fn test_block_doc_comment_3() {
-    with_default_session_globals(|| {
+    create_default_session_globals_then(|| {
         let comment = "\n let a: *i32;\n *a = 5;\n";
         let stripped = beautify_doc_string(Symbol::intern(comment));
         assert_eq!(stripped.as_str(), " let a: *i32;\n *a = 5;");
@@ -30,7 +30,7 @@ fn test_block_doc_comment_3() {
 
 #[test]
 fn test_line_doc_comment() {
-    with_default_session_globals(|| {
+    create_default_session_globals_then(|| {
         let stripped = beautify_doc_string(Symbol::intern(" test"));
         assert_eq!(stripped.as_str(), " test");
         let stripped = beautify_doc_string(Symbol::intern("! test"));
diff --git a/compiler/rustc_ast_pretty/src/pprust/tests.rs b/compiler/rustc_ast_pretty/src/pprust/tests.rs
index 8abd85a5a5214..6c8d42f33eb5a 100644
--- a/compiler/rustc_ast_pretty/src/pprust/tests.rs
+++ b/compiler/rustc_ast_pretty/src/pprust/tests.rs
@@ -1,8 +1,8 @@
 use super::*;
 
 use rustc_ast as ast;
+use rustc_span::create_default_session_globals_then;
 use rustc_span::symbol::Ident;
-use rustc_span::with_default_session_globals;
 
 fn fun_to_string(
     decl: &ast::FnDecl,
@@ -24,7 +24,7 @@ fn variant_to_string(var: &ast::Variant) -> String {
 
 #[test]
 fn test_fun_to_string() {
-    with_default_session_globals(|| {
+    create_default_session_globals_then(|| {
         let abba_ident = Ident::from_str("abba");
 
         let decl =
@@ -39,7 +39,7 @@ fn test_fun_to_string() {
 
 #[test]
 fn test_variant_to_string() {
-    with_default_session_globals(|| {
+    create_default_session_globals_then(|| {
         let ident = Ident::from_str("principal_skinner");
 
         let var = ast::Variant {
diff --git a/compiler/rustc_error_codes/src/error_codes/E0542.md b/compiler/rustc_error_codes/src/error_codes/E0542.md
index 7cb58f9d0cb74..7fecfeaa57c28 100644
--- a/compiler/rustc_error_codes/src/error_codes/E0542.md
+++ b/compiler/rustc_error_codes/src/error_codes/E0542.md
@@ -10,7 +10,7 @@ Erroneous code example:
 fn _stable_fn() {}
 
 #[rustc_const_stable(feature = "_stable_const_fn")] // invalid
-fn _stable_const_fn() {}
+const fn _stable_const_fn() {}
 
 #[stable(feature = "_deprecated_fn", since = "0.1.0")]
 #[rustc_deprecated(
@@ -29,7 +29,7 @@ To fix this issue, you need to provide the `since` field. Example:
 fn _stable_fn() {}
 
 #[rustc_const_stable(feature = "_stable_const_fn", since = "1.0.0")] // ok!
-fn _stable_const_fn() {}
+const fn _stable_const_fn() {}
 
 #[stable(feature = "_deprecated_fn", since = "0.1.0")]
 #[rustc_deprecated(
diff --git a/compiler/rustc_error_codes/src/error_codes/E0545.md b/compiler/rustc_error_codes/src/error_codes/E0545.md
index 9fb935a3ab1ab..7aba084f4d3aa 100644
--- a/compiler/rustc_error_codes/src/error_codes/E0545.md
+++ b/compiler/rustc_error_codes/src/error_codes/E0545.md
@@ -10,7 +10,7 @@ Erroneous code example:
 fn _unstable_fn() {}
 
 #[rustc_const_unstable(feature = "_unstable_const_fn", issue = "0")] // invalid
-fn _unstable_const_fn() {}
+const fn _unstable_const_fn() {}
 ```
 
 To fix this issue, you need to provide a correct value in the `issue` field.
@@ -24,7 +24,7 @@ Example:
 fn _unstable_fn() {}
 
 #[rustc_const_unstable(feature = "_unstable_const_fn", issue = "1")] // ok!
-fn _unstable_const_fn() {}
+const fn _unstable_const_fn() {}
 ```
 
 See the [How Rust is Made and “Nightly Rust”][how-rust-made-nightly] appendix
diff --git a/compiler/rustc_error_codes/src/error_codes/E0547.md b/compiler/rustc_error_codes/src/error_codes/E0547.md
index 1aa4b35424884..4950325df6400 100644
--- a/compiler/rustc_error_codes/src/error_codes/E0547.md
+++ b/compiler/rustc_error_codes/src/error_codes/E0547.md
@@ -10,7 +10,7 @@ Erroneous code example:
 fn _unstable_fn() {}
 
 #[rustc_const_unstable(feature = "_unstable_const_fn")] // invalid
-fn _unstable_const_fn() {}
+const fn _unstable_const_fn() {}
 ```
 
 To fix this issue, you need to provide the `issue` field. Example:
@@ -26,7 +26,7 @@ fn _unstable_fn() {}
     feature = "_unstable_const_fn",
     issue = "none"
 )] // ok!
-fn _unstable_const_fn() {}
+const fn _unstable_const_fn() {}
 ```
 
 See the [How Rust is Made and “Nightly Rust”][how-rust-made-nightly] appendix
diff --git a/compiler/rustc_errors/src/json/tests.rs b/compiler/rustc_errors/src/json/tests.rs
index e69e868c8ede5..d055937ac36e3 100644
--- a/compiler/rustc_errors/src/json/tests.rs
+++ b/compiler/rustc_errors/src/json/tests.rs
@@ -39,16 +39,11 @@ impl<T: Write> Write for Shared<T> {
     }
 }
 
-fn with_default_session_globals(f: impl FnOnce()) {
-    let session_globals = rustc_span::SessionGlobals::new(rustc_span::edition::DEFAULT_EDITION);
-    rustc_span::SESSION_GLOBALS.set(&session_globals, f);
-}
-
 /// Test the span yields correct positions in JSON.
 fn test_positions(code: &str, span: (u32, u32), expected_output: SpanTestData) {
     let expected_output = TestData { spans: vec![expected_output] };
 
-    with_default_session_globals(|| {
+    rustc_span::create_default_session_globals_then(|| {
         let sm = Lrc::new(SourceMap::new(FilePathMapping::empty()));
         sm.new_source_file(Path::new("test.rs").to_owned().into(), code.to_owned());
 
diff --git a/compiler/rustc_expand/src/mut_visit/tests.rs b/compiler/rustc_expand/src/mut_visit/tests.rs
index 7e7155ad27876..0068539fb3bd4 100644
--- a/compiler/rustc_expand/src/mut_visit/tests.rs
+++ b/compiler/rustc_expand/src/mut_visit/tests.rs
@@ -3,8 +3,8 @@ use crate::tests::{matches_codepattern, string_to_crate};
 use rustc_ast as ast;
 use rustc_ast::mut_visit::MutVisitor;
 use rustc_ast_pretty::pprust;
+use rustc_span::create_default_session_globals_then;
 use rustc_span::symbol::Ident;
-use rustc_span::with_default_session_globals;
 
 // This version doesn't care about getting comments or doc-strings in.
 fn print_crate_items(krate: &ast::Crate) -> String {
@@ -38,7 +38,7 @@ macro_rules! assert_pred {
 // Make sure idents get transformed everywhere.
 #[test]
 fn ident_transformation() {
-    with_default_session_globals(|| {
+    create_default_session_globals_then(|| {
         let mut zz_visitor = ToZzIdentMutVisitor;
         let mut krate =
             string_to_crate("#[a] mod b {fn c (d : e, f : g) {h!(i,j,k);l;m}}".to_string());
@@ -55,7 +55,7 @@ fn ident_transformation() {
 // Make sure idents get transformed even inside macro defs.
 #[test]
 fn ident_transformation_in_defs() {
-    with_default_session_globals(|| {
+    create_default_session_globals_then(|| {
         let mut zz_visitor = ToZzIdentMutVisitor;
         let mut krate = string_to_crate(
             "macro_rules! a {(b $c:expr $(d $e:token)f+ => \
diff --git a/compiler/rustc_expand/src/parse/tests.rs b/compiler/rustc_expand/src/parse/tests.rs
index 288efe22982fa..6402a81e7c1a5 100644
--- a/compiler/rustc_expand/src/parse/tests.rs
+++ b/compiler/rustc_expand/src/parse/tests.rs
@@ -10,9 +10,9 @@ use rustc_errors::PResult;
 use rustc_parse::new_parser_from_source_str;
 use rustc_parse::parser::ForceCollect;
 use rustc_session::parse::ParseSess;
+use rustc_span::create_default_session_globals_then;
 use rustc_span::source_map::FilePathMapping;
 use rustc_span::symbol::{kw, sym, Symbol};
-use rustc_span::with_default_session_globals;
 use rustc_span::{BytePos, FileName, Pos, Span};
 
 use std::path::PathBuf;
@@ -51,7 +51,7 @@ fn string_to_item(source_str: String) -> Option<P<ast::Item>> {
 #[should_panic]
 #[test]
 fn bad_path_expr_1() {
-    with_default_session_globals(|| {
+    create_default_session_globals_then(|| {
         string_to_expr("::abc::def::return".to_string());
     })
 }
@@ -59,7 +59,7 @@ fn bad_path_expr_1() {
 // Checks the token-tree-ization of macros.
 #[test]
 fn string_to_tts_macro() {
-    with_default_session_globals(|| {
+    create_default_session_globals_then(|| {
         let tts: Vec<_> =
             string_to_stream("macro_rules! zip (($a)=>($a))".to_string()).trees().collect();
         let tts: &[TokenTree] = &tts[..];
@@ -96,7 +96,7 @@ fn string_to_tts_macro() {
 
 #[test]
 fn string_to_tts_1() {
-    with_default_session_globals(|| {
+    create_default_session_globals_then(|| {
         let tts = string_to_stream("fn a (b : i32) { b; }".to_string());
 
         let expected = TokenStream::new(vec![
@@ -131,7 +131,7 @@ fn string_to_tts_1() {
 
 #[test]
 fn parse_use() {
-    with_default_session_globals(|| {
+    create_default_session_globals_then(|| {
         let use_s = "use foo::bar::baz;";
         let vitem = string_to_item(use_s.to_string()).unwrap();
         let vitem_s = item_to_string(&vitem);
@@ -146,7 +146,7 @@ fn parse_use() {
 
 #[test]
 fn parse_extern_crate() {
-    with_default_session_globals(|| {
+    create_default_session_globals_then(|| {
         let ex_s = "extern crate foo;";
         let vitem = string_to_item(ex_s.to_string()).unwrap();
         let vitem_s = item_to_string(&vitem);
@@ -184,7 +184,7 @@ fn get_spans_of_pat_idents(src: &str) -> Vec<Span> {
 
 #[test]
 fn span_of_self_arg_pat_idents_are_correct() {
-    with_default_session_globals(|| {
+    create_default_session_globals_then(|| {
         let srcs = [
             "impl z { fn a (&self, &myarg: i32) {} }",
             "impl z { fn a (&mut self, &myarg: i32) {} }",
@@ -208,7 +208,7 @@ fn span_of_self_arg_pat_idents_are_correct() {
 
 #[test]
 fn parse_exprs() {
-    with_default_session_globals(|| {
+    create_default_session_globals_then(|| {
         // just make sure that they parse....
         string_to_expr("3 + 4".to_string());
         string_to_expr("a::z.froob(b,&(987+3))".to_string());
@@ -217,7 +217,7 @@ fn parse_exprs() {
 
 #[test]
 fn attrs_fix_bug() {
-    with_default_session_globals(|| {
+    create_default_session_globals_then(|| {
         string_to_item(
             "pub fn mk_file_writer(path: &Path, flags: &[FileFlag])
                 -> Result<Box<Writer>, String> {
@@ -238,7 +238,7 @@ let mut fflags: c_int = wb();
 
 #[test]
 fn crlf_doc_comments() {
-    with_default_session_globals(|| {
+    create_default_session_globals_then(|| {
         let sess = sess();
 
         let name_1 = FileName::Custom("crlf_source_1".to_string());
@@ -272,7 +272,7 @@ fn ttdelim_span() {
         new_parser_from_source_str(sess, name, source).parse_expr()
     }
 
-    with_default_session_globals(|| {
+    create_default_session_globals_then(|| {
         let sess = sess();
         let expr = parse_expr_from_source_str(
             PathBuf::from("foo").into(),
@@ -300,7 +300,7 @@ fn ttdelim_span() {
 // See `recurse_into_file_modules` in the parser.
 #[test]
 fn out_of_line_mod() {
-    with_default_session_globals(|| {
+    create_default_session_globals_then(|| {
         let item = parse_item_from_source_str(
             PathBuf::from("foo").into(),
             "mod foo { struct S; mod this_does_not_exist; }".to_owned(),
diff --git a/compiler/rustc_expand/src/tests.rs b/compiler/rustc_expand/src/tests.rs
index f2345ff2707e9..ed3aa1eaca84e 100644
--- a/compiler/rustc_expand/src/tests.rs
+++ b/compiler/rustc_expand/src/tests.rs
@@ -2,8 +2,8 @@ use rustc_ast as ast;
 use rustc_ast::tokenstream::TokenStream;
 use rustc_parse::{new_parser_from_source_str, parser::Parser, source_file_to_stream};
 use rustc_session::parse::ParseSess;
+use rustc_span::create_default_session_if_not_set_then;
 use rustc_span::source_map::{FilePathMapping, SourceMap};
-use rustc_span::with_default_session_globals;
 use rustc_span::{BytePos, MultiSpan, Span};
 
 use rustc_data_structures::sync::Lrc;
@@ -124,7 +124,7 @@ impl<T: Write> Write for Shared<T> {
 }
 
 fn test_harness(file_text: &str, span_labels: Vec<SpanLabel>, expected_output: &str) {
-    with_default_session_globals(|| {
+    create_default_session_if_not_set_then(|_| {
         let output = Arc::new(Mutex::new(Vec::new()));
 
         let source_map = Lrc::new(SourceMap::new(FilePathMapping::empty()));
diff --git a/compiler/rustc_expand/src/tokenstream/tests.rs b/compiler/rustc_expand/src/tokenstream/tests.rs
index 8b546e7e4a343..31052bfb54ce8 100644
--- a/compiler/rustc_expand/src/tokenstream/tests.rs
+++ b/compiler/rustc_expand/src/tokenstream/tests.rs
@@ -2,7 +2,7 @@ use crate::tests::string_to_stream;
 
 use rustc_ast::token;
 use rustc_ast::tokenstream::{Spacing, TokenStream, TokenStreamBuilder, TokenTree};
-use rustc_span::with_default_session_globals;
+use rustc_span::create_default_session_globals_then;
 use rustc_span::{BytePos, Span, Symbol};
 use smallvec::smallvec;
 
@@ -20,7 +20,7 @@ fn joint(tree: TokenTree) -> TokenStream {
 
 #[test]
 fn test_concat() {
-    with_default_session_globals(|| {
+    create_default_session_globals_then(|| {
         let test_res = string_to_ts("foo::bar::baz");
         let test_fst = string_to_ts("foo::bar");
         let test_snd = string_to_ts("::baz");
@@ -33,7 +33,7 @@ fn test_concat() {
 
 #[test]
 fn test_to_from_bijection() {
-    with_default_session_globals(|| {
+    create_default_session_globals_then(|| {
         let test_start = string_to_ts("foo::bar(baz)");
         let test_end = test_start.trees().collect();
         assert_eq!(test_start, test_end)
@@ -42,7 +42,7 @@ fn test_to_from_bijection() {
 
 #[test]
 fn test_eq_0() {
-    with_default_session_globals(|| {
+    create_default_session_globals_then(|| {
         let test_res = string_to_ts("foo");
         let test_eqs = string_to_ts("foo");
         assert_eq!(test_res, test_eqs)
@@ -51,7 +51,7 @@ fn test_eq_0() {
 
 #[test]
 fn test_eq_1() {
-    with_default_session_globals(|| {
+    create_default_session_globals_then(|| {
         let test_res = string_to_ts("::bar::baz");
         let test_eqs = string_to_ts("::bar::baz");
         assert_eq!(test_res, test_eqs)
@@ -60,7 +60,7 @@ fn test_eq_1() {
 
 #[test]
 fn test_eq_3() {
-    with_default_session_globals(|| {
+    create_default_session_globals_then(|| {
         let test_res = string_to_ts("");
         let test_eqs = string_to_ts("");
         assert_eq!(test_res, test_eqs)
@@ -69,7 +69,7 @@ fn test_eq_3() {
 
 #[test]
 fn test_diseq_0() {
-    with_default_session_globals(|| {
+    create_default_session_globals_then(|| {
         let test_res = string_to_ts("::bar::baz");
         let test_eqs = string_to_ts("bar::baz");
         assert_eq!(test_res == test_eqs, false)
@@ -78,7 +78,7 @@ fn test_diseq_0() {
 
 #[test]
 fn test_diseq_1() {
-    with_default_session_globals(|| {
+    create_default_session_globals_then(|| {
         let test_res = string_to_ts("(bar,baz)");
         let test_eqs = string_to_ts("bar,baz");
         assert_eq!(test_res == test_eqs, false)
@@ -87,7 +87,7 @@ fn test_diseq_1() {
 
 #[test]
 fn test_is_empty() {
-    with_default_session_globals(|| {
+    create_default_session_globals_then(|| {
         let test0: TokenStream = Vec::<TokenTree>::new().into_iter().collect();
         let test1: TokenStream =
             TokenTree::token(token::Ident(Symbol::intern("a"), false), sp(0, 1)).into();
@@ -101,7 +101,7 @@ fn test_is_empty() {
 
 #[test]
 fn test_dotdotdot() {
-    with_default_session_globals(|| {
+    create_default_session_globals_then(|| {
         let mut builder = TokenStreamBuilder::new();
         builder.push(joint(TokenTree::token(token::Dot, sp(0, 1))));
         builder.push(joint(TokenTree::token(token::Dot, sp(1, 2))));
diff --git a/compiler/rustc_hir/src/lang_items.rs b/compiler/rustc_hir/src/lang_items.rs
index bab685006eafc..28ae08030e630 100644
--- a/compiler/rustc_hir/src/lang_items.rs
+++ b/compiler/rustc_hir/src/lang_items.rs
@@ -348,7 +348,4 @@ language_item_table! {
     Range,                   sym::Range,               range_struct,               Target::Struct;
     RangeToInclusive,        sym::RangeToInclusive,    range_to_inclusive_struct,  Target::Struct;
     RangeTo,                 sym::RangeTo,             range_to_struct,            Target::Struct;
-    Send,                    sym::send,                send_trait,                 Target::Trait;
-    UnwindSafe,              sym::unwind_safe,         unwind_safe_trait,          Target::Trait;
-    RefUnwindSafe,           sym::ref_unwind_safe,     ref_unwind_safe_trait,      Target::Trait;
 }
diff --git a/compiler/rustc_interface/src/interface.rs b/compiler/rustc_interface/src/interface.rs
index a1090ee316db4..086c49c73972f 100644
--- a/compiler/rustc_interface/src/interface.rs
+++ b/compiler/rustc_interface/src/interface.rs
@@ -76,7 +76,7 @@ impl Compiler {
 
 /// Converts strings provided as `--cfg [cfgspec]` into a `crate_cfg`.
 pub fn parse_cfgspecs(cfgspecs: Vec<String>) -> FxHashSet<(String, Option<String>)> {
-    rustc_span::with_default_session_globals(move || {
+    rustc_span::create_default_session_if_not_set_then(move |_| {
         let cfg = cfgspecs
             .into_iter()
             .map(|s| {
diff --git a/compiler/rustc_interface/src/tests.rs b/compiler/rustc_interface/src/tests.rs
index b252409a92ada..a053253ec16e0 100644
--- a/compiler/rustc_interface/src/tests.rs
+++ b/compiler/rustc_interface/src/tests.rs
@@ -107,7 +107,7 @@ fn assert_non_crate_hash_different(x: &Options, y: &Options) {
 // When the user supplies --test we should implicitly supply --cfg test
 #[test]
 fn test_switch_implies_cfg_test() {
-    rustc_span::with_default_session_globals(|| {
+    rustc_span::create_default_session_globals_then(|| {
         let matches = optgroups().parse(&["--test".to_string()]).unwrap();
         let (sess, cfg) = mk_session(matches);
         let cfg = build_configuration(&sess, to_crate_config(cfg));
@@ -118,7 +118,7 @@ fn test_switch_implies_cfg_test() {
 // When the user supplies --test and --cfg test, don't implicitly add another --cfg test
 #[test]
 fn test_switch_implies_cfg_test_unless_cfg_test() {
-    rustc_span::with_default_session_globals(|| {
+    rustc_span::create_default_session_globals_then(|| {
         let matches = optgroups().parse(&["--test".to_string(), "--cfg=test".to_string()]).unwrap();
         let (sess, cfg) = mk_session(matches);
         let cfg = build_configuration(&sess, to_crate_config(cfg));
@@ -130,20 +130,20 @@ fn test_switch_implies_cfg_test_unless_cfg_test() {
 
 #[test]
 fn test_can_print_warnings() {
-    rustc_span::with_default_session_globals(|| {
+    rustc_span::create_default_session_globals_then(|| {
         let matches = optgroups().parse(&["-Awarnings".to_string()]).unwrap();
         let (sess, _) = mk_session(matches);
         assert!(!sess.diagnostic().can_emit_warnings());
     });
 
-    rustc_span::with_default_session_globals(|| {
+    rustc_span::create_default_session_globals_then(|| {
         let matches =
             optgroups().parse(&["-Awarnings".to_string(), "-Dwarnings".to_string()]).unwrap();
         let (sess, _) = mk_session(matches);
         assert!(sess.diagnostic().can_emit_warnings());
     });
 
-    rustc_span::with_default_session_globals(|| {
+    rustc_span::create_default_session_globals_then(|| {
         let matches = optgroups().parse(&["-Adead_code".to_string()]).unwrap();
         let (sess, _) = mk_session(matches);
         assert!(sess.diagnostic().can_emit_warnings());
diff --git a/compiler/rustc_interface/src/util.rs b/compiler/rustc_interface/src/util.rs
index d1d0eee365d48..4f51ce620427b 100644
--- a/compiler/rustc_interface/src/util.rs
+++ b/compiler/rustc_interface/src/util.rs
@@ -150,7 +150,7 @@ pub fn setup_callbacks_and_run_in_thread_pool_with_globals<F: FnOnce() -> R + Se
     crate::callbacks::setup_callbacks();
 
     let main_handler = move || {
-        rustc_span::with_session_globals(edition, || {
+        rustc_span::create_session_globals_then(edition, || {
             io::set_output_capture(stderr.clone());
             f()
         })
@@ -171,12 +171,13 @@ unsafe fn handle_deadlock() {
     rustc_data_structures::sync::assert_sync::<tls::ImplicitCtxt<'_, '_>>();
     let icx: &tls::ImplicitCtxt<'_, '_> = &*(context as *const tls::ImplicitCtxt<'_, '_>);
 
-    let session_globals = rustc_span::SESSION_GLOBALS.with(|sg| sg as *const _);
+    let session_globals = rustc_span::with_session_globals(|sg| sg as *const _);
     let session_globals = &*session_globals;
     thread::spawn(move || {
         tls::enter_context(icx, |_| {
-            rustc_span::SESSION_GLOBALS
-                .set(session_globals, || tls::with(|tcx| tcx.queries.deadlock(tcx, &registry)))
+            rustc_span::set_session_globals_then(session_globals, || {
+                tls::with(|tcx| tcx.queries.deadlock(tcx, &registry))
+            })
         });
     });
 }
@@ -203,13 +204,13 @@ pub fn setup_callbacks_and_run_in_thread_pool_with_globals<F: FnOnce() -> R + Se
 
     let with_pool = move |pool: &rayon::ThreadPool| pool.install(f);
 
-    rustc_span::with_session_globals(edition, || {
-        rustc_span::SESSION_GLOBALS.with(|session_globals| {
+    rustc_span::create_session_globals_then(edition, || {
+        rustc_span::with_session_globals(|session_globals| {
             // The main handler runs for each Rayon worker thread and sets up
             // the thread local rustc uses. `session_globals` is captured and set
             // on the new threads.
             let main_handler = move |thread: rayon::ThreadBuilder| {
-                rustc_span::SESSION_GLOBALS.set(session_globals, || {
+                rustc_span::set_session_globals_then(session_globals, || {
                     io::set_output_capture(stderr.clone());
                     thread.run()
                 })
diff --git a/compiler/rustc_lint/src/tests.rs b/compiler/rustc_lint/src/tests.rs
index a50c88aa0f736..fc9d6f636b214 100644
--- a/compiler/rustc_lint/src/tests.rs
+++ b/compiler/rustc_lint/src/tests.rs
@@ -1,21 +1,23 @@
 use crate::context::parse_lint_and_tool_name;
-use rustc_span::{with_default_session_globals, Symbol};
+use rustc_span::{create_default_session_globals_then, Symbol};
 
 #[test]
 fn parse_lint_no_tool() {
-    with_default_session_globals(|| assert_eq!(parse_lint_and_tool_name("foo"), (None, "foo")));
+    create_default_session_globals_then(|| {
+        assert_eq!(parse_lint_and_tool_name("foo"), (None, "foo"))
+    });
 }
 
 #[test]
 fn parse_lint_with_tool() {
-    with_default_session_globals(|| {
+    create_default_session_globals_then(|| {
         assert_eq!(parse_lint_and_tool_name("clippy::foo"), (Some(Symbol::intern("clippy")), "foo"))
     });
 }
 
 #[test]
 fn parse_lint_multiple_path() {
-    with_default_session_globals(|| {
+    create_default_session_globals_then(|| {
         assert_eq!(
             parse_lint_and_tool_name("clippy::foo::bar"),
             (Some(Symbol::intern("clippy")), "foo::bar")
diff --git a/compiler/rustc_mir/src/transform/coverage/tests.rs b/compiler/rustc_mir/src/transform/coverage/tests.rs
index b04c2d542d459..e5b3059a5995f 100644
--- a/compiler/rustc_mir/src/transform/coverage/tests.rs
+++ b/compiler/rustc_mir/src/transform/coverage/tests.rs
@@ -22,7 +22,7 @@
 //! are unrelated to the `TyCtxt` global. Without initializing the `Span` session globals, some
 //! basic, coverage-specific features would be impossible to test, but thankfully initializing these
 //! globals is comparatively simpler. The easiest way is to wrap the test in a closure argument
-//! to: `rustc_span::with_default_session_globals(|| { test_here(); })`.
+//! to: `rustc_span::create_default_session_globals_then(|| { test_here(); })`.
 
 use super::counters;
 use super::debug;
@@ -677,7 +677,7 @@ fn synthesize_body_span_from_terminators(mir_body: &Body<'_>) -> Span {
 
 #[test]
 fn test_make_bcb_counters() {
-    rustc_span::with_default_session_globals(|| {
+    rustc_span::create_default_session_globals_then(|| {
         let mir_body = goto_switchint();
         let body_span = synthesize_body_span_from_terminators(&mir_body);
         let mut basic_coverage_blocks = graph::CoverageGraph::from_mir(&mir_body);
diff --git a/compiler/rustc_parse_format/src/tests.rs b/compiler/rustc_parse_format/src/tests.rs
index 9fd0497fffee5..b7693a85ad955 100644
--- a/compiler/rustc_parse_format/src/tests.rs
+++ b/compiler/rustc_parse_format/src/tests.rs
@@ -144,8 +144,7 @@ fn format_align_fill() {
 }
 #[test]
 fn format_counts() {
-    use rustc_span::{edition, SessionGlobals, SESSION_GLOBALS};
-    SESSION_GLOBALS.set(&SessionGlobals::new(edition::DEFAULT_EDITION), || {
+    rustc_span::create_default_session_globals_then(|| {
         same(
             "{:10x}",
             &[NextArgument(Argument {
diff --git a/compiler/rustc_passes/src/lang_items.rs b/compiler/rustc_passes/src/lang_items.rs
index d8abffc5f6400..3a88d1932a80a 100644
--- a/compiler/rustc_passes/src/lang_items.rs
+++ b/compiler/rustc_passes/src/lang_items.rs
@@ -257,9 +257,6 @@ impl LanguageItemCollector<'tcx> {
             | LangItem::Unpin
             | LangItem::Termination
             | LangItem::Try
-            | LangItem::Send
-            | LangItem::UnwindSafe
-            | LangItem::RefUnwindSafe
                 => Some(0),
 
             // Not a trait
diff --git a/compiler/rustc_passes/src/stability.rs b/compiler/rustc_passes/src/stability.rs
index 5830245e837e4..cd8dd6984d5b9 100644
--- a/compiler/rustc_passes/src/stability.rs
+++ b/compiler/rustc_passes/src/stability.rs
@@ -20,6 +20,7 @@ use rustc_session::parse::feature_err;
 use rustc_session::Session;
 use rustc_span::symbol::{sym, Symbol};
 use rustc_span::{Span, DUMMY_SP};
+use rustc_target::spec::abi::Abi;
 
 use std::cmp::Ordering;
 use std::iter;
@@ -95,10 +96,12 @@ struct Annotator<'a, 'tcx> {
 impl<'a, 'tcx> Annotator<'a, 'tcx> {
     // Determine the stability for a node based on its attributes and inherited
     // stability. The stability is recorded in the index and used as the parent.
+    // If the node is a function, `fn_sig` is its signature
     fn annotate<F>(
         &mut self,
         hir_id: HirId,
         item_sp: Span,
+        fn_sig: Option<&'tcx hir::FnSig<'tcx>>,
         kind: AnnotationKind,
         inherit_deprecation: InheritDeprecation,
         inherit_const_stability: InheritConstStability,
@@ -163,13 +166,30 @@ impl<'a, 'tcx> Annotator<'a, 'tcx> {
         }
 
         let (stab, const_stab) = attr::find_stability(&self.tcx.sess, attrs, item_sp);
+        let mut const_span = None;
 
-        let const_stab = const_stab.map(|(const_stab, _)| {
+        let const_stab = const_stab.map(|(const_stab, const_span_node)| {
             let const_stab = self.tcx.intern_const_stability(const_stab);
             self.index.const_stab_map.insert(hir_id, const_stab);
+            const_span = Some(const_span_node);
             const_stab
         });
 
+        // If the current node is a function, has const stability attributes and if it doesn not have an intrinsic ABI,
+        // check if the function/method is const or the parent impl block is const
+        if let (Some(const_span), Some(fn_sig)) = (const_span, fn_sig) {
+            if fn_sig.header.abi != Abi::RustIntrinsic
+                && fn_sig.header.abi != Abi::PlatformIntrinsic
+                && !fn_sig.header.is_const()
+            {
+                if !self.in_trait_impl
+                    || (self.in_trait_impl && !self.tcx.is_const_fn_raw(hir_id.owner.to_def_id()))
+                {
+                    missing_const_err(&self.tcx.sess, fn_sig.span, const_span);
+                }
+            }
+        }
+
         // `impl const Trait for Type` items forward their const stability to their
         // immediate children.
         if const_stab.is_none() {
@@ -367,6 +387,8 @@ impl<'a, 'tcx> Visitor<'tcx> for Annotator<'a, 'tcx> {
         let orig_in_trait_impl = self.in_trait_impl;
         let mut kind = AnnotationKind::Required;
         let mut const_stab_inherit = InheritConstStability::No;
+        let mut fn_sig = None;
+
         match i.kind {
             // Inherent impls and foreign modules serve only as containers for other items,
             // they don't have their own stability. They still can be annotated as unstable
@@ -387,6 +409,7 @@ impl<'a, 'tcx> Visitor<'tcx> for Annotator<'a, 'tcx> {
                     self.annotate(
                         ctor_hir_id,
                         i.span,
+                        None,
                         AnnotationKind::Required,
                         InheritDeprecation::Yes,
                         InheritConstStability::No,
@@ -395,12 +418,16 @@ impl<'a, 'tcx> Visitor<'tcx> for Annotator<'a, 'tcx> {
                     )
                 }
             }
+            hir::ItemKind::Fn(ref item_fn_sig, _, _) => {
+                fn_sig = Some(item_fn_sig);
+            }
             _ => {}
         }
 
         self.annotate(
             i.hir_id(),
             i.span,
+            fn_sig,
             kind,
             InheritDeprecation::Yes,
             const_stab_inherit,
@@ -411,9 +438,15 @@ impl<'a, 'tcx> Visitor<'tcx> for Annotator<'a, 'tcx> {
     }
 
     fn visit_trait_item(&mut self, ti: &'tcx hir::TraitItem<'tcx>) {
+        let fn_sig = match ti.kind {
+            hir::TraitItemKind::Fn(ref fn_sig, _) => Some(fn_sig),
+            _ => None,
+        };
+
         self.annotate(
             ti.hir_id(),
             ti.span,
+            fn_sig,
             AnnotationKind::Required,
             InheritDeprecation::Yes,
             InheritConstStability::No,
@@ -427,9 +460,16 @@ impl<'a, 'tcx> Visitor<'tcx> for Annotator<'a, 'tcx> {
     fn visit_impl_item(&mut self, ii: &'tcx hir::ImplItem<'tcx>) {
         let kind =
             if self.in_trait_impl { AnnotationKind::Prohibited } else { AnnotationKind::Required };
+
+        let fn_sig = match ii.kind {
+            hir::ImplItemKind::Fn(ref fn_sig, _) => Some(fn_sig),
+            _ => None,
+        };
+
         self.annotate(
             ii.hir_id(),
             ii.span,
+            fn_sig,
             kind,
             InheritDeprecation::Yes,
             InheritConstStability::No,
@@ -444,6 +484,7 @@ impl<'a, 'tcx> Visitor<'tcx> for Annotator<'a, 'tcx> {
         self.annotate(
             var.id,
             var.span,
+            None,
             AnnotationKind::Required,
             InheritDeprecation::Yes,
             InheritConstStability::No,
@@ -453,6 +494,7 @@ impl<'a, 'tcx> Visitor<'tcx> for Annotator<'a, 'tcx> {
                     v.annotate(
                         ctor_hir_id,
                         var.span,
+                        None,
                         AnnotationKind::Required,
                         InheritDeprecation::Yes,
                         InheritConstStability::No,
@@ -470,6 +512,7 @@ impl<'a, 'tcx> Visitor<'tcx> for Annotator<'a, 'tcx> {
         self.annotate(
             s.hir_id,
             s.span,
+            None,
             AnnotationKind::Required,
             InheritDeprecation::Yes,
             InheritConstStability::No,
@@ -484,6 +527,7 @@ impl<'a, 'tcx> Visitor<'tcx> for Annotator<'a, 'tcx> {
         self.annotate(
             i.hir_id(),
             i.span,
+            None,
             AnnotationKind::Required,
             InheritDeprecation::Yes,
             InheritConstStability::No,
@@ -498,6 +542,7 @@ impl<'a, 'tcx> Visitor<'tcx> for Annotator<'a, 'tcx> {
         self.annotate(
             md.hir_id(),
             md.span,
+            None,
             AnnotationKind::Required,
             InheritDeprecation::Yes,
             InheritConstStability::No,
@@ -517,6 +562,7 @@ impl<'a, 'tcx> Visitor<'tcx> for Annotator<'a, 'tcx> {
         self.annotate(
             p.hir_id,
             p.span,
+            None,
             kind,
             InheritDeprecation::No,
             InheritConstStability::No,
@@ -687,6 +733,7 @@ fn stability_index(tcx: TyCtxt<'tcx>, (): ()) -> Index<'tcx> {
         annotator.annotate(
             hir::CRATE_HIR_ID,
             krate.item.inner,
+            None,
             AnnotationKind::Required,
             InheritDeprecation::Yes,
             InheritConstStability::No,
@@ -969,3 +1016,15 @@ fn duplicate_feature_err(sess: &Session, span: Span, feature: Symbol) {
     struct_span_err!(sess, span, E0636, "the feature `{}` has already been declared", feature)
         .emit();
 }
+
+fn missing_const_err(session: &Session, fn_sig_span: Span, const_span: Span) {
+    const ERROR_MSG: &'static str = "attributes `#[rustc_const_unstable]` \
+         and `#[rustc_const_stable]` require \
+         the function or method to be `const`";
+
+    session
+        .struct_span_err(fn_sig_span, ERROR_MSG)
+        .span_help(fn_sig_span, "make the function or method const")
+        .span_label(const_span, "attribute specified here")
+        .emit();
+}
diff --git a/compiler/rustc_span/src/hygiene.rs b/compiler/rustc_span/src/hygiene.rs
index 913aeeca78beb..b2da51f8f38d2 100644
--- a/compiler/rustc_span/src/hygiene.rs
+++ b/compiler/rustc_span/src/hygiene.rs
@@ -26,7 +26,7 @@
 
 use crate::edition::Edition;
 use crate::symbol::{kw, sym, Symbol};
-use crate::SESSION_GLOBALS;
+use crate::with_session_globals;
 use crate::{BytePos, CachingSourceMapView, ExpnIdCache, SourceFile, Span, DUMMY_SP};
 
 use crate::def_id::{CrateNum, DefId, DefPathHash, CRATE_DEF_INDEX, LOCAL_CRATE};
@@ -201,7 +201,7 @@ impl HygieneData {
     }
 
     pub fn with<T, F: FnOnce(&mut HygieneData) -> T>(f: F) -> T {
-        SESSION_GLOBALS.with(|session_globals| f(&mut *session_globals.hygiene_data.borrow_mut()))
+        with_session_globals(|session_globals| f(&mut *session_globals.hygiene_data.borrow_mut()))
     }
 
     fn fresh_expn(&mut self, mut expn_data: Option<ExpnData>) -> ExpnId {
@@ -1367,8 +1367,9 @@ fn update_disambiguator(expn_id: ExpnId) {
         }
     }
 
-    let source_map = SESSION_GLOBALS
-        .with(|session_globals| session_globals.source_map.borrow().as_ref().unwrap().clone());
+    let source_map = with_session_globals(|session_globals| {
+        session_globals.source_map.borrow().as_ref().unwrap().clone()
+    });
 
     let mut ctx =
         DummyHashStableContext { caching_source_map: CachingSourceMapView::new(&source_map) };
diff --git a/compiler/rustc_span/src/lev_distance/tests.rs b/compiler/rustc_span/src/lev_distance/tests.rs
index 90e20afc8f538..11822e9ef9742 100644
--- a/compiler/rustc_span/src/lev_distance/tests.rs
+++ b/compiler/rustc_span/src/lev_distance/tests.rs
@@ -21,8 +21,8 @@ fn test_lev_distance() {
 
 #[test]
 fn test_find_best_match_for_name() {
-    use crate::with_default_session_globals;
-    with_default_session_globals(|| {
+    use crate::create_default_session_globals_then;
+    create_default_session_globals_then(|| {
         let input = vec![Symbol::intern("aaab"), Symbol::intern("aaabc")];
         assert_eq!(
             find_best_match_for_name(&input, Symbol::intern("aaaa"), None),
diff --git a/compiler/rustc_span/src/lib.rs b/compiler/rustc_span/src/lib.rs
index a8c28c8e46949..3ddb10d2a06de 100644
--- a/compiler/rustc_span/src/lib.rs
+++ b/compiler/rustc_span/src/lib.rs
@@ -97,19 +97,65 @@ impl SessionGlobals {
     }
 }
 
-pub fn with_session_globals<R>(edition: Edition, f: impl FnOnce() -> R) -> R {
+#[inline]
+pub fn create_session_globals_then<R>(edition: Edition, f: impl FnOnce() -> R) -> R {
+    assert!(
+        !SESSION_GLOBALS.is_set(),
+        "SESSION_GLOBALS should never be overwritten! \
+         Use another thread if you need another SessionGlobals"
+    );
     let session_globals = SessionGlobals::new(edition);
     SESSION_GLOBALS.set(&session_globals, f)
 }
 
-pub fn with_default_session_globals<R>(f: impl FnOnce() -> R) -> R {
-    with_session_globals(edition::DEFAULT_EDITION, f)
+#[inline]
+pub fn set_session_globals_then<R>(session_globals: &SessionGlobals, f: impl FnOnce() -> R) -> R {
+    assert!(
+        !SESSION_GLOBALS.is_set(),
+        "SESSION_GLOBALS should never be overwritten! \
+         Use another thread if you need another SessionGlobals"
+    );
+    SESSION_GLOBALS.set(session_globals, f)
+}
+
+#[inline]
+pub fn create_default_session_if_not_set_then<R, F>(f: F) -> R
+where
+    F: FnOnce(&SessionGlobals) -> R,
+{
+    create_session_if_not_set_then(edition::DEFAULT_EDITION, f)
+}
+
+#[inline]
+pub fn create_session_if_not_set_then<R, F>(edition: Edition, f: F) -> R
+where
+    F: FnOnce(&SessionGlobals) -> R,
+{
+    if !SESSION_GLOBALS.is_set() {
+        let session_globals = SessionGlobals::new(edition);
+        SESSION_GLOBALS.set(&session_globals, || SESSION_GLOBALS.with(f))
+    } else {
+        SESSION_GLOBALS.with(f)
+    }
+}
+
+#[inline]
+pub fn with_session_globals<R, F>(f: F) -> R
+where
+    F: FnOnce(&SessionGlobals) -> R,
+{
+    SESSION_GLOBALS.with(f)
+}
+
+#[inline]
+pub fn create_default_session_globals_then<R>(f: impl FnOnce() -> R) -> R {
+    create_session_globals_then(edition::DEFAULT_EDITION, f)
 }
 
 // If this ever becomes non thread-local, `decode_syntax_context`
 // and `decode_expn_id` will need to be updated to handle concurrent
 // deserialization.
-scoped_tls::scoped_thread_local!(pub static SESSION_GLOBALS: SessionGlobals);
+scoped_tls::scoped_thread_local!(static SESSION_GLOBALS: SessionGlobals);
 
 // FIXME: We should use this enum or something like it to get rid of the
 // use of magic `/rust/1.x/...` paths across the board.
@@ -855,13 +901,13 @@ impl<D: Decoder> Decodable<D> for Span {
 /// the `SourceMap` provided to this function. If that is not available,
 /// we fall back to printing the raw `Span` field values.
 pub fn with_source_map<T, F: FnOnce() -> T>(source_map: Lrc<SourceMap>, f: F) -> T {
-    SESSION_GLOBALS.with(|session_globals| {
+    with_session_globals(|session_globals| {
         *session_globals.source_map.borrow_mut() = Some(source_map);
     });
     struct ClearSourceMap;
     impl Drop for ClearSourceMap {
         fn drop(&mut self) {
-            SESSION_GLOBALS.with(|session_globals| {
+            with_session_globals(|session_globals| {
                 session_globals.source_map.borrow_mut().take();
             });
         }
@@ -880,7 +926,7 @@ pub fn debug_with_source_map(
 }
 
 pub fn default_span_debug(span: Span, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-    SESSION_GLOBALS.with(|session_globals| {
+    with_session_globals(|session_globals| {
         if let Some(source_map) = &*session_globals.source_map.borrow() {
             debug_with_source_map(span, f, source_map)
         } else {
diff --git a/compiler/rustc_span/src/span_encoding.rs b/compiler/rustc_span/src/span_encoding.rs
index 5ea39b343b5c1..cb017709c6c7b 100644
--- a/compiler/rustc_span/src/span_encoding.rs
+++ b/compiler/rustc_span/src/span_encoding.rs
@@ -5,7 +5,6 @@
 // See https://internals.rust-lang.org/t/rfc-compiler-refactoring-spans/1357/28
 
 use crate::hygiene::SyntaxContext;
-use crate::SESSION_GLOBALS;
 use crate::{BytePos, SpanData};
 
 use rustc_data_structures::fx::FxIndexSet;
@@ -122,5 +121,5 @@ impl SpanInterner {
 // If an interner exists, return it. Otherwise, prepare a fresh one.
 #[inline]
 fn with_span_interner<T, F: FnOnce(&mut SpanInterner) -> T>(f: F) -> T {
-    SESSION_GLOBALS.with(|session_globals| f(&mut *session_globals.span_interner.lock()))
+    crate::with_session_globals(|session_globals| f(&mut *session_globals.span_interner.lock()))
 }
diff --git a/compiler/rustc_span/src/symbol.rs b/compiler/rustc_span/src/symbol.rs
index be4f12c6d1cb8..edb97d70517b0 100644
--- a/compiler/rustc_span/src/symbol.rs
+++ b/compiler/rustc_span/src/symbol.rs
@@ -13,7 +13,7 @@ use std::fmt;
 use std::hash::{Hash, Hasher};
 use std::str;
 
-use crate::{Edition, Span, DUMMY_SP, SESSION_GLOBALS};
+use crate::{with_session_globals, Edition, Span, DUMMY_SP};
 
 #[cfg(test)]
 mod tests;
@@ -947,7 +947,7 @@ symbols! {
         receiver,
         recursion_limit,
         reexport_test_harness_main,
-        ref_unwind_safe,
+        ref_unwind_safe_trait,
         reference,
         reflect,
         reg,
@@ -1073,7 +1073,6 @@ symbols! {
         self_in_typedefs,
         self_struct_ctor,
         semitransparent,
-        send,
         send_trait,
         shl,
         shl_assign,
@@ -1299,7 +1298,7 @@ symbols! {
         unused_qualifications,
         unwind,
         unwind_attributes,
-        unwind_safe,
+        unwind_safe_trait,
         unwrap,
         unwrap_or,
         use_extern_macros,
@@ -1790,7 +1789,7 @@ impl Ident {
 
 #[inline]
 fn with_interner<T, F: FnOnce(&mut Interner) -> T>(f: F) -> T {
-    SESSION_GLOBALS.with(|session_globals| f(&mut *session_globals.symbol_interner.lock()))
+    with_session_globals(|session_globals| f(&mut *session_globals.symbol_interner.lock()))
 }
 
 /// An alternative to [`Symbol`], useful when the chars within the symbol need to
diff --git a/compiler/rustc_span/src/symbol/tests.rs b/compiler/rustc_span/src/symbol/tests.rs
index 47da03424b770..11dea265b4e66 100644
--- a/compiler/rustc_span/src/symbol/tests.rs
+++ b/compiler/rustc_span/src/symbol/tests.rs
@@ -1,6 +1,6 @@
 use super::*;
 
-use crate::{edition, SessionGlobals};
+use crate::create_default_session_globals_then;
 
 #[test]
 fn interner_tests() {
@@ -18,7 +18,7 @@ fn interner_tests() {
 
 #[test]
 fn without_first_quote_test() {
-    SESSION_GLOBALS.set(&SessionGlobals::new(edition::DEFAULT_EDITION), || {
+    create_default_session_globals_then(|| {
         let i = Ident::from_str("'break");
         assert_eq!(i.without_first_quote().name, kw::Break);
     });
diff --git a/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs b/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs
index cca40ff1ce6a6..26a5a65ed360a 100644
--- a/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs
+++ b/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs
@@ -688,15 +688,17 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
             return false;
         }
 
-        // Blacklist traits for which it would be nonsensical to suggest borrowing.
+        // List of traits for which it would be nonsensical to suggest borrowing.
         // For instance, immutable references are always Copy, so suggesting to
         // borrow would always succeed, but it's probably not what the user wanted.
-        let blacklist: Vec<_> =
-            [LangItem::Copy, LangItem::Clone, LangItem::Unpin, LangItem::Sized, LangItem::Send]
+        let mut never_suggest_borrow: Vec<_> =
+            [LangItem::Copy, LangItem::Clone, LangItem::Unpin, LangItem::Sized]
                 .iter()
                 .filter_map(|lang_item| self.tcx.lang_items().require(*lang_item).ok())
                 .collect();
 
+        never_suggest_borrow.push(self.tcx.get_diagnostic_item(sym::send_trait).unwrap());
+
         let span = obligation.cause.span;
         let param_env = obligation.param_env;
         let trait_ref = trait_ref.skip_binder();
@@ -798,7 +800,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                 ty::TraitRef::new(trait_ref.def_id, imm_substs),
                 trait_ref,
                 false,
-                &blacklist[..],
+                &never_suggest_borrow[..],
             ) {
                 return true;
             } else {
@@ -806,7 +808,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                     ty::TraitRef::new(trait_ref.def_id, mut_substs),
                     trait_ref,
                     true,
-                    &blacklist[..],
+                    &never_suggest_borrow[..],
                 );
             }
         } else {
diff --git a/compiler/rustc_typeck/src/check/upvar.rs b/compiler/rustc_typeck/src/check/upvar.rs
index 6e6d4778b269c..52436816f77f1 100644
--- a/compiler/rustc_typeck/src/check/upvar.rs
+++ b/compiler/rustc_typeck/src/check/upvar.rs
@@ -670,7 +670,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         if self.need_2229_migrations_for_trait(
             min_captures,
             var_hir_id,
-            tcx.lang_items().send_trait(),
+            tcx.get_diagnostic_item(sym::send_trait),
         ) {
             auto_trait_reasons.insert("`Send`");
         }
@@ -686,7 +686,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         if self.need_2229_migrations_for_trait(
             min_captures,
             var_hir_id,
-            tcx.lang_items().unwind_safe_trait(),
+            tcx.get_diagnostic_item(sym::unwind_safe_trait),
         ) {
             auto_trait_reasons.insert("`UnwindSafe`");
         }
@@ -694,7 +694,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         if self.need_2229_migrations_for_trait(
             min_captures,
             var_hir_id,
-            tcx.lang_items().ref_unwind_safe_trait(),
+            tcx.get_diagnostic_item(sym::ref_unwind_safe_trait),
         ) {
             auto_trait_reasons.insert("`RefUnwindSafe`");
         }
diff --git a/library/alloc/src/collections/btree/set.rs b/library/alloc/src/collections/btree/set.rs
index 34ec27e461c75..45ea30b538691 100644
--- a/library/alloc/src/collections/btree/set.rs
+++ b/library/alloc/src/collections/btree/set.rs
@@ -940,18 +940,20 @@ impl<T> BTreeSet<T> {
         BTreeSet { map: self.map.split_off(key) }
     }
 
-    /// Creates an iterator which uses a closure to determine if a value should be removed.
+    /// Creates an iterator that visits all values in ascending order and uses a closure
+    /// to determine if a value should be removed.
     ///
-    /// If the closure returns true, then the value is removed and yielded.
-    /// If the closure returns false, the value will remain in the list and will not be yielded
-    /// by the iterator.
+    /// If the closure returns `true`, the value is removed from the set and yielded. If
+    /// the closure returns `false`, or panics, the value remains in the set and will
+    /// not be yielded.
     ///
-    /// If the iterator is only partially consumed or not consumed at all, each of the remaining
-    /// values will still be subjected to the closure and removed and dropped if it returns true.
+    /// If the iterator is only partially consumed or not consumed at all, each of the
+    /// remaining values is still subjected to the closure and removed and dropped if it
+    /// returns `true`.
     ///
-    /// It is unspecified how many more values will be subjected to the closure
-    /// if a panic occurs in the closure, or if a panic occurs while dropping a value, or if the
-    /// `DrainFilter` itself is leaked.
+    /// It is unspecified how many more values will be subjected to the closure if a
+    /// panic occurs in the closure, or if a panic occurs while dropping a value, or if
+    /// the `DrainFilter` itself is leaked.
     ///
     /// # Examples
     ///
diff --git a/library/core/src/marker.rs b/library/core/src/marker.rs
index 271565693d66e..fb957348bebd3 100644
--- a/library/core/src/marker.rs
+++ b/library/core/src/marker.rs
@@ -31,7 +31,6 @@ use crate::hash::Hasher;
 /// [ub]: ../../reference/behavior-considered-undefined.html
 #[stable(feature = "rust1", since = "1.0.0")]
 #[cfg_attr(not(test), rustc_diagnostic_item = "send_trait")]
-#[lang = "send"]
 #[rustc_on_unimplemented(
     message = "`{Self}` cannot be sent between threads safely",
     label = "`{Self}` cannot be sent between threads safely"
diff --git a/library/std/src/panic.rs b/library/std/src/panic.rs
index ee069eefd4541..7bc987db8814b 100644
--- a/library/std/src/panic.rs
+++ b/library/std/src/panic.rs
@@ -133,7 +133,7 @@ pub fn panic_any<M: 'static + Any + Send>(msg: M) -> ! {
 /// [`AssertUnwindSafe`] wrapper struct can be used to force this trait to be
 /// implemented for any closed over variables passed to `catch_unwind`.
 #[stable(feature = "catch_unwind", since = "1.9.0")]
-#[cfg_attr(not(test), lang = "unwind_safe")]
+#[cfg_attr(not(test), rustc_diagnostic_item = "unwind_safe_trait")]
 #[rustc_on_unimplemented(
     message = "the type `{Self}` may not be safely transferred across an unwind boundary",
     label = "`{Self}` may not be safely transferred across an unwind boundary"
@@ -149,7 +149,7 @@ pub auto trait UnwindSafe {}
 /// This is a "helper marker trait" used to provide impl blocks for the
 /// [`UnwindSafe`] trait, for more information see that documentation.
 #[stable(feature = "catch_unwind", since = "1.9.0")]
-#[cfg_attr(not(test), lang = "ref_unwind_safe")]
+#[cfg_attr(not(test), rustc_diagnostic_item = "ref_unwind_safe_trait")]
 #[rustc_on_unimplemented(
     message = "the type `{Self}` may contain interior mutability and a reference may not be safely \
                transferrable across a catch_unwind boundary",
diff --git a/src/bootstrap/doc.rs b/src/bootstrap/doc.rs
index 7ec3c2d738a67..d3f2c87c0d22f 100644
--- a/src/bootstrap/doc.rs
+++ b/src/bootstrap/doc.rs
@@ -600,7 +600,7 @@ impl Step for Rustc {
 }
 
 macro_rules! tool_doc {
-    ($tool: ident, $should_run: literal, $path: literal, [$($krate: literal),+ $(,)?] $(, binary=$bin:expr)?) => {
+    ($tool: ident, $should_run: literal, $path: literal, [$($krate: literal),+ $(,)?], binary=$bin:expr) => {
         #[derive(Debug, Copy, Clone, Hash, PartialEq, Eq)]
         pub struct $tool {
             stage: u32,
@@ -669,9 +669,9 @@ macro_rules! tool_doc {
                     cargo.arg("-p").arg($krate);
                 )+
 
-                $(if !$bin {
+                if !$bin {
                     cargo.rustdocflag("--document-private-items");
-                })?
+                }
                 cargo.rustdocflag("--enable-index-page");
                 cargo.rustdocflag("--show-type-layout");
                 cargo.rustdocflag("-Zunstable-options");
@@ -681,7 +681,13 @@ macro_rules! tool_doc {
     }
 }
 
-tool_doc!(Rustdoc, "rustdoc-tool", "src/tools/rustdoc", ["rustdoc", "rustdoc-json-types"]);
+tool_doc!(
+    Rustdoc,
+    "rustdoc-tool",
+    "src/tools/rustdoc",
+    ["rustdoc", "rustdoc-json-types"],
+    binary = false
+);
 tool_doc!(
     Rustfmt,
     "rustfmt-nightly",
diff --git a/src/librustdoc/clean/cfg/tests.rs b/src/librustdoc/clean/cfg/tests.rs
index 34b9cbcb6793a..275d1b3ebd938 100644
--- a/src/librustdoc/clean/cfg/tests.rs
+++ b/src/librustdoc/clean/cfg/tests.rs
@@ -2,8 +2,8 @@ use super::*;
 
 use rustc_ast::attr;
 use rustc_ast::Path;
+use rustc_span::create_default_session_globals_then;
 use rustc_span::symbol::{Ident, Symbol};
-use rustc_span::with_default_session_globals;
 use rustc_span::DUMMY_SP;
 
 fn word_cfg(s: &str) -> Cfg {
@@ -52,7 +52,7 @@ macro_rules! dummy_meta_item_list {
 
 #[test]
 fn test_cfg_not() {
-    with_default_session_globals(|| {
+    create_default_session_globals_then(|| {
         assert_eq!(!Cfg::False, Cfg::True);
         assert_eq!(!Cfg::True, Cfg::False);
         assert_eq!(!word_cfg("test"), Cfg::Not(Box::new(word_cfg("test"))));
@@ -70,7 +70,7 @@ fn test_cfg_not() {
 
 #[test]
 fn test_cfg_and() {
-    with_default_session_globals(|| {
+    create_default_session_globals_then(|| {
         let mut x = Cfg::False;
         x &= Cfg::True;
         assert_eq!(x, Cfg::False);
@@ -154,7 +154,7 @@ fn test_cfg_and() {
 
 #[test]
 fn test_cfg_or() {
-    with_default_session_globals(|| {
+    create_default_session_globals_then(|| {
         let mut x = Cfg::True;
         x |= Cfg::False;
         assert_eq!(x, Cfg::True);
@@ -238,7 +238,7 @@ fn test_cfg_or() {
 
 #[test]
 fn test_parse_ok() {
-    with_default_session_globals(|| {
+    create_default_session_globals_then(|| {
         let mi = dummy_meta_item_word("all");
         assert_eq!(Cfg::parse(&mi), Ok(word_cfg("all")));
 
@@ -271,7 +271,7 @@ fn test_parse_ok() {
 
 #[test]
 fn test_parse_err() {
-    with_default_session_globals(|| {
+    create_default_session_globals_then(|| {
         let mi = attr::mk_name_value_item(Ident::from_str("foo"), LitKind::Bool(false), DUMMY_SP);
         assert!(Cfg::parse(&mi).is_err());
 
@@ -303,7 +303,7 @@ fn test_parse_err() {
 
 #[test]
 fn test_render_short_html() {
-    with_default_session_globals(|| {
+    create_default_session_globals_then(|| {
         assert_eq!(word_cfg("unix").render_short_html(), "Unix");
         assert_eq!(name_value_cfg("target_os", "macos").render_short_html(), "macOS");
         assert_eq!(name_value_cfg("target_pointer_width", "16").render_short_html(), "16-bit");
@@ -358,7 +358,7 @@ fn test_render_short_html() {
 
 #[test]
 fn test_render_long_html() {
-    with_default_session_globals(|| {
+    create_default_session_globals_then(|| {
         assert_eq!(
             word_cfg("unix").render_long_html(),
             "This is supported on <strong>Unix</strong> only."
@@ -442,7 +442,7 @@ fn test_render_long_html() {
 fn test_simplify_with() {
     // This is a tiny subset of things that could be simplified, but it likely covers 90% of
     // real world usecases well.
-    with_default_session_globals(|| {
+    create_default_session_globals_then(|| {
         let foo = word_cfg("foo");
         let bar = word_cfg("bar");
         let baz = word_cfg("baz");
diff --git a/src/librustdoc/doctest.rs b/src/librustdoc/doctest.rs
index 1281f76fd0fdc..cd914f05e680c 100644
--- a/src/librustdoc/doctest.rs
+++ b/src/librustdoc/doctest.rs
@@ -513,7 +513,7 @@ crate fn make_test(
     // Uses librustc_ast to parse the doctest and find if there's a main fn and the extern
     // crate already is included.
     let result = rustc_driver::catch_fatal_errors(|| {
-        rustc_span::with_session_globals(edition, || {
+        rustc_span::create_session_if_not_set_then(edition, |_| {
             use rustc_errors::emitter::{Emitter, EmitterWriter};
             use rustc_errors::Handler;
             use rustc_parse::maybe_new_parser_from_source_str;
diff --git a/src/librustdoc/html/highlight/tests.rs b/src/librustdoc/html/highlight/tests.rs
index a0da2c963d167..a505865b149c4 100644
--- a/src/librustdoc/html/highlight/tests.rs
+++ b/src/librustdoc/html/highlight/tests.rs
@@ -1,8 +1,8 @@
 use super::write_code;
 use crate::html::format::Buffer;
 use expect_test::expect_file;
+use rustc_span::create_default_session_globals_then;
 use rustc_span::edition::Edition;
-use rustc_span::with_default_session_globals;
 
 const STYLE: &str = r#"
 <style>
@@ -18,7 +18,7 @@ const STYLE: &str = r#"
 
 #[test]
 fn test_html_highlighting() {
-    with_default_session_globals(|| {
+    create_default_session_globals_then(|| {
         let src = include_str!("fixtures/sample.rs");
         let html = {
             let mut out = Buffer::new();
@@ -31,7 +31,7 @@ fn test_html_highlighting() {
 
 #[test]
 fn test_dos_backline() {
-    with_default_session_globals(|| {
+    create_default_session_globals_then(|| {
         let src = "pub fn foo() {\r\n\
     println!(\"foo\");\r\n\
 }\r\n";
diff --git a/src/librustdoc/html/render/mod.rs b/src/librustdoc/html/render/mod.rs
index 97ee682c11c4d..56d50cb407926 100644
--- a/src/librustdoc/html/render/mod.rs
+++ b/src/librustdoc/html/render/mod.rs
@@ -1027,7 +1027,7 @@ fn render_assoc_items(
                 write!(
                     w,
                     "<h2 id=\"deref-methods\" class=\"small-section-header\">\
-                         Methods from {trait_}&lt;Target = {type_}&gt;\
+                         <span>Methods from {trait_}&lt;Target = {type_}&gt;</span>\
                          <a href=\"#deref-methods\" class=\"anchor\"></a>\
                      </h2>",
                     trait_ = trait_.print(cx),
diff --git a/src/librustdoc/html/static/css/rustdoc.css b/src/librustdoc/html/static/css/rustdoc.css
index 71123aa9a0e44..66dfd2fac84c6 100644
--- a/src/librustdoc/html/static/css/rustdoc.css
+++ b/src/librustdoc/html/static/css/rustdoc.css
@@ -1092,11 +1092,6 @@ a.test-arrow:hover{
 	left: -10px;
 }
 
-#main > .variant, #main > .structfield {
-	display: block;
-}
-
-
 :target > code {
 	opacity: 1;
 }
diff --git a/src/librustdoc/passes/unindent_comments/tests.rs b/src/librustdoc/passes/unindent_comments/tests.rs
index 9c9924841b948..82d1afac5eceb 100644
--- a/src/librustdoc/passes/unindent_comments/tests.rs
+++ b/src/librustdoc/passes/unindent_comments/tests.rs
@@ -1,7 +1,7 @@
 use super::*;
+use rustc_span::create_default_session_globals_then;
 use rustc_span::source_map::DUMMY_SP;
 use rustc_span::symbol::Symbol;
-use rustc_span::with_default_session_globals;
 
 fn create_doc_fragment(s: &str) -> Vec<DocFragment> {
     vec![DocFragment {
@@ -17,7 +17,7 @@ fn create_doc_fragment(s: &str) -> Vec<DocFragment> {
 
 #[track_caller]
 fn run_test(input: &str, expected: &str) {
-    with_default_session_globals(|| {
+    create_default_session_globals_then(|| {
         let mut s = create_doc_fragment(input);
         unindent_fragments(&mut s);
         assert_eq!(&s.iter().collect::<String>(), expected);
diff --git a/src/test/ui-fulldeps/mod_dir_path_canonicalized.rs b/src/test/ui-fulldeps/mod_dir_path_canonicalized.rs
index 448c57da754a0..bb246de0e572d 100644
--- a/src/test/ui-fulldeps/mod_dir_path_canonicalized.rs
+++ b/src/test/ui-fulldeps/mod_dir_path_canonicalized.rs
@@ -19,7 +19,7 @@ use std::path::Path;
 mod gravy;
 
 pub fn main() {
-    rustc_span::with_default_session_globals(|| parse());
+    rustc_span::create_default_session_globals_then(|| parse());
 
     assert_eq!(gravy::foo(), 10);
 }
diff --git a/src/test/ui-fulldeps/pprust-expr-roundtrip.rs b/src/test/ui-fulldeps/pprust-expr-roundtrip.rs
index 091c834eccf5f..36ff8b01fd454 100644
--- a/src/test/ui-fulldeps/pprust-expr-roundtrip.rs
+++ b/src/test/ui-fulldeps/pprust-expr-roundtrip.rs
@@ -202,7 +202,7 @@ impl MutVisitor for AddParens {
 }
 
 fn main() {
-    rustc_span::with_default_session_globals(|| run());
+    rustc_span::create_default_session_globals_then(|| run());
 }
 
 fn run() {
diff --git a/src/test/ui/consts/rustc-const-stability-require-const.rs b/src/test/ui/consts/rustc-const-stability-require-const.rs
new file mode 100644
index 0000000000000..4fb259b335c71
--- /dev/null
+++ b/src/test/ui/consts/rustc-const-stability-require-const.rs
@@ -0,0 +1,47 @@
+#![crate_type = "lib"]
+#![feature(staged_api)]
+#![stable(feature = "foo", since = "1.0.0")]
+
+#[stable(feature = "foo", since = "1.0.0")]
+#[rustc_const_unstable(feature = "const_foo", issue = "none")]
+pub fn foo() {}
+//~^ ERROR attributes `#[rustc_const_unstable]` and `#[rustc_const_stable]` require the function or method to be `const`
+
+#[stable(feature = "bar", since = "1.0.0")]
+#[rustc_const_stable(feature = "const_bar", since = "1.0.0")]
+pub fn bar() {}
+//~^ ERROR attributes `#[rustc_const_unstable]` and `#[rustc_const_stable]` require the function or method to be `const`
+
+#[stable(feature = "potato", since = "1.0.0")]
+pub struct Potato;
+
+impl Potato {
+    #[stable(feature = "salad", since = "1.0.0")]
+    #[rustc_const_unstable(feature = "const_salad", issue = "none")]
+    pub fn salad(&self) -> &'static str { "mmmmmm" }
+    //~^ ERROR attributes `#[rustc_const_unstable]` and `#[rustc_const_stable]` require the function or method to be `const`
+
+    #[stable(feature = "roasted", since = "1.0.0")]
+    #[rustc_const_unstable(feature = "const_roasted", issue = "none")]
+    pub fn roasted(&self) -> &'static str { "mmmmmmmmmm" }
+    //~^ ERROR attributes `#[rustc_const_unstable]` and `#[rustc_const_stable]` require the function or method to be `const`
+}
+
+#[stable(feature = "bar", since = "1.0.0")]
+#[rustc_const_stable(feature = "const_bar", since = "1.0.0")]
+pub extern "C" fn bar_c() {}
+//~^ ERROR attributes `#[rustc_const_unstable]` and `#[rustc_const_stable]` require the function or method to be `const`
+
+#[stable(feature = "foo", since = "1.0.0")]
+#[rustc_const_unstable(feature = "const_foo", issue = "none")]
+pub extern "C" fn foo_c() {}
+//~^ ERROR attributes `#[rustc_const_unstable]` and `#[rustc_const_stable]` require the function or method to be `const`
+
+
+#[stable(feature = "foobar", since = "1.0.0")]
+#[rustc_const_unstable(feature = "foobar_const", issue = "none")]
+pub const fn foobar() {}
+
+#[stable(feature = "barfoo", since = "1.0.0")]
+#[rustc_const_stable(feature = "barfoo_const", since = "1.0.0")]
+pub const fn barfoo() {}
diff --git a/src/test/ui/consts/rustc-const-stability-require-const.stderr b/src/test/ui/consts/rustc-const-stability-require-const.stderr
new file mode 100644
index 0000000000000..1027b9311b7a6
--- /dev/null
+++ b/src/test/ui/consts/rustc-const-stability-require-const.stderr
@@ -0,0 +1,86 @@
+error: attributes `#[rustc_const_unstable]` and `#[rustc_const_stable]` require the function or method to be `const`
+  --> $DIR/rustc-const-stability-require-const.rs:7:1
+   |
+LL | #[rustc_const_unstable(feature = "const_foo", issue = "none")]
+   | -------------------------------------------------------------- attribute specified here
+LL | pub fn foo() {}
+   | ^^^^^^^^^^^^
+   |
+help: make the function or method const
+  --> $DIR/rustc-const-stability-require-const.rs:7:1
+   |
+LL | pub fn foo() {}
+   | ^^^^^^^^^^^^
+
+error: attributes `#[rustc_const_unstable]` and `#[rustc_const_stable]` require the function or method to be `const`
+  --> $DIR/rustc-const-stability-require-const.rs:12:1
+   |
+LL | #[rustc_const_stable(feature = "const_bar", since = "1.0.0")]
+   | ------------------------------------------------------------- attribute specified here
+LL | pub fn bar() {}
+   | ^^^^^^^^^^^^
+   |
+help: make the function or method const
+  --> $DIR/rustc-const-stability-require-const.rs:12:1
+   |
+LL | pub fn bar() {}
+   | ^^^^^^^^^^^^
+
+error: attributes `#[rustc_const_unstable]` and `#[rustc_const_stable]` require the function or method to be `const`
+  --> $DIR/rustc-const-stability-require-const.rs:21:5
+   |
+LL |     #[rustc_const_unstable(feature = "const_salad", issue = "none")]
+   |     ---------------------------------------------------------------- attribute specified here
+LL |     pub fn salad(&self) -> &'static str { "mmmmmm" }
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+help: make the function or method const
+  --> $DIR/rustc-const-stability-require-const.rs:21:5
+   |
+LL |     pub fn salad(&self) -> &'static str { "mmmmmm" }
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: attributes `#[rustc_const_unstable]` and `#[rustc_const_stable]` require the function or method to be `const`
+  --> $DIR/rustc-const-stability-require-const.rs:26:5
+   |
+LL |     #[rustc_const_unstable(feature = "const_roasted", issue = "none")]
+   |     ------------------------------------------------------------------ attribute specified here
+LL |     pub fn roasted(&self) -> &'static str { "mmmmmmmmmm" }
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+help: make the function or method const
+  --> $DIR/rustc-const-stability-require-const.rs:26:5
+   |
+LL |     pub fn roasted(&self) -> &'static str { "mmmmmmmmmm" }
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: attributes `#[rustc_const_unstable]` and `#[rustc_const_stable]` require the function or method to be `const`
+  --> $DIR/rustc-const-stability-require-const.rs:32:1
+   |
+LL | #[rustc_const_stable(feature = "const_bar", since = "1.0.0")]
+   | ------------------------------------------------------------- attribute specified here
+LL | pub extern "C" fn bar_c() {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+help: make the function or method const
+  --> $DIR/rustc-const-stability-require-const.rs:32:1
+   |
+LL | pub extern "C" fn bar_c() {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: attributes `#[rustc_const_unstable]` and `#[rustc_const_stable]` require the function or method to be `const`
+  --> $DIR/rustc-const-stability-require-const.rs:37:1
+   |
+LL | #[rustc_const_unstable(feature = "const_foo", issue = "none")]
+   | -------------------------------------------------------------- attribute specified here
+LL | pub extern "C" fn foo_c() {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+help: make the function or method const
+  --> $DIR/rustc-const-stability-require-const.rs:37:1
+   |
+LL | pub extern "C" fn foo_c() {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 6 previous errors
+
diff --git a/src/test/ui/consts/rustc-impl-const-stability.rs b/src/test/ui/consts/rustc-impl-const-stability.rs
new file mode 100644
index 0000000000000..01daa1c9cb42f
--- /dev/null
+++ b/src/test/ui/consts/rustc-impl-const-stability.rs
@@ -0,0 +1,21 @@
+// build-pass
+
+#![crate_type = "lib"]
+#![allow(incomplete_features)]
+#![feature(staged_api)]
+#![feature(const_trait_impl)]
+#![stable(feature = "foo", since = "1.0.0")]
+
+
+#[stable(feature = "potato", since = "1.27.0")]
+pub struct Data {
+    _data: u128
+}
+
+#[stable(feature = "potato", since = "1.27.0")]
+impl const Default for Data {
+    #[rustc_const_unstable(feature = "data_foo", issue = "none")]
+    fn default() -> Data {
+        Data { _data: 42 }
+    }
+}
diff --git a/src/tools/clippy/clippy_lints/src/doc.rs b/src/tools/clippy/clippy_lints/src/doc.rs
index 4e164d33a0512..d6e3e92fc5eb6 100644
--- a/src/tools/clippy/clippy_lints/src/doc.rs
+++ b/src/tools/clippy/clippy_lints/src/doc.rs
@@ -26,6 +26,7 @@ use rustc_span::source_map::{BytePos, FilePathMapping, MultiSpan, SourceMap, Spa
 use rustc_span::{sym, FileName, Pos};
 use std::io;
 use std::ops::Range;
+use std::thread;
 use url::Url;
 
 declare_clippy_lint! {
@@ -584,10 +585,10 @@ fn get_current_span(spans: &[(usize, Span)], idx: usize) -> (usize, Span) {
 }
 
 fn check_code(cx: &LateContext<'_>, text: &str, edition: Edition, span: Span) {
-    fn has_needless_main(code: &str, edition: Edition) -> bool {
+    fn has_needless_main(code: String, edition: Edition) -> bool {
         rustc_driver::catch_fatal_errors(|| {
-            rustc_span::with_session_globals(edition, || {
-                let filename = FileName::anon_source_code(code);
+            rustc_span::create_session_globals_then(edition, || {
+                let filename = FileName::anon_source_code(&code);
 
                 let sm = Lrc::new(SourceMap::new(FilePathMapping::empty()));
                 let emitter = EmitterWriter::new(box io::sink(), None, false, false, false, None, false);
@@ -649,7 +650,10 @@ fn check_code(cx: &LateContext<'_>, text: &str, edition: Edition, span: Span) {
         .unwrap_or_default()
     }
 
-    if has_needless_main(text, edition) {
+    // Because of the global session, we need to create a new session in a different thread with
+    // the edition we need.
+    let text = text.to_owned();
+    if thread::spawn(move || has_needless_main(text, edition)).join().expect("thread::spawn failed") {
         span_lint(cx, NEEDLESS_DOCTEST_MAIN, span, "needless `fn main` in doctest");
     }
 }
diff --git a/src/tools/error_index_generator/main.rs b/src/tools/error_index_generator/main.rs
index e3700136aca8a..01a3fc812b208 100644
--- a/src/tools/error_index_generator/main.rs
+++ b/src/tools/error_index_generator/main.rs
@@ -284,7 +284,8 @@ fn parse_args() -> (OutputFormat, PathBuf) {
 fn main() {
     rustc_driver::init_env_logger("RUST_LOG");
     let (format, dst) = parse_args();
-    let result = rustc_span::with_default_session_globals(move || main_with_result(format, &dst));
+    let result =
+        rustc_span::create_default_session_globals_then(move || main_with_result(format, &dst));
     if let Err(e) = result {
         panic!("{}", e.to_string());
     }
diff --git a/src/tools/rustfmt/src/formatting.rs b/src/tools/rustfmt/src/formatting.rs
index b69ecdc5cb8ae..e0403574eebc1 100644
--- a/src/tools/rustfmt/src/formatting.rs
+++ b/src/tools/rustfmt/src/formatting.rs
@@ -34,7 +34,7 @@ impl<'b, T: Write + 'b> Session<'b, T> {
             return Err(ErrorKind::VersionMismatch);
         }
 
-        rustc_span::with_session_globals(self.config.edition().into(), || {
+        rustc_span::create_session_if_not_set_then(self.config.edition().into(), |_| {
             if self.config.disable_all_formatting() {
                 // When the input is from stdin, echo back the input.
                 if let Input::Text(ref buf) = input {