resolve: Scale back unloading of speculatively loaded crates

compiler/rustc_metadata/src/creader.rs

@@ -1070,16 +1070,6 @@ impl<'a, 'tcx> CrateLoader<'a, 'tcx> {
     pub fn maybe_process_path_extern(&mut self, name: Symbol) -> Option<CrateNum> {
         self.maybe_resolve_crate(name, CrateDepKind::Explicit, None).ok()
     }
-
-    pub fn unload_unused_crates(&mut self) {
-        for opt_cdata in &mut self.cstore.metas {
-            if let Some(cdata) = opt_cdata
-                && !cdata.used()
-            {
-                *opt_cdata = None;
-            }
-        }
-    }
 }
 
 fn global_allocator_spans(krate: &ast::Crate) -> Vec<Span> {

compiler/rustc_metadata/src/rmeta/decoder/cstore_impl.rs

@@ -519,6 +519,16 @@ pub(in crate::rmeta) fn provide(providers: &mut Providers) {
             tcx.untracked().cstore.freeze();
             tcx.arena.alloc_from_iter(CStore::from_tcx(tcx).iter_crate_data().map(|(cnum, _)| cnum))
         },
+        used_crates: |tcx, ()| {
+            // The list of loaded crates is now frozen in query cache,
+            // so make sure cstore is not mutably accessed from here on.
+            tcx.untracked().cstore.freeze();
+            tcx.arena.alloc_from_iter(
+                CStore::from_tcx(tcx)
+                    .iter_crate_data()
+                    .filter_map(|(cnum, data)| data.used().then_some(cnum)),
+            )
+        },
         ..providers.queries
     };
     provide_extern(&mut providers.extern_queries);

compiler/rustc_middle/src/query/mod.rs

@@ -1872,6 +1872,13 @@ rustc_queries! {
         eval_always
         desc { "fetching all foreign CrateNum instances" }
     }
+    // Crates that are loaded non-speculatively (not for diagnostics or doc links).
+    // FIXME: This is currently only used for collecting lang items, but should be used instead of
+    // `crates` in most other cases too.
+    query used_crates(_: ()) -> &'tcx [CrateNum] {
+        eval_always
+        desc { "fetching `CrateNum`s for all crates loaded non-speculatively" }
+    }
 
     /// A list of all traits in a crate, used by rustdoc and error reporting.
     query traits(_: CrateNum) -> &'tcx [DefId] {

compiler/rustc_passes/src/lang_items.rs

@@ -250,7 +250,7 @@ fn get_lang_items(tcx: TyCtxt<'_>, (): ()) -> LanguageItems {
     let mut collector = LanguageItemCollector::new(tcx, resolver);
 
     // Collect lang items in other crates.
-    for &cnum in tcx.crates(()).iter() {
+    for &cnum in tcx.used_crates(()).iter() {
         for &(def_id, lang_item) in tcx.defined_lang_items(cnum).iter() {
             collector.collect_item(lang_item, def_id, None);
         }

compiler/rustc_resolve/src/late.rs

@@ -23,7 +23,6 @@ use rustc_hir::def::Namespace::{self, *};
 use rustc_hir::def::{self, CtorKind, DefKind, LifetimeRes, NonMacroAttrKind, PartialRes, PerNS};
 use rustc_hir::def_id::{DefId, LocalDefId, CRATE_DEF_ID, LOCAL_CRATE};
 use rustc_hir::{BindingAnnotation, PrimTy, TraitCandidate};
-use rustc_metadata::creader::CStore;
 use rustc_middle::middle::resolve_bound_vars::Set1;
 use rustc_middle::{bug, span_bug};
 use rustc_session::config::{CrateType, ResolveDocLinks};
@@ -4557,10 +4556,6 @@ impl<'a: 'ast, 'b, 'ast, 'tcx> LateResolutionVisitor<'a, 'b, 'ast, 'tcx> {
                         // Encoding foreign def ids in proc macro crate metadata will ICE.
                         return None;
                     }
-                    // Doc paths should be resolved speculatively and should not produce any
-                    // diagnostics, but if they are indeed resolved, then we need to keep the
-                    // corresponding crate alive.
-                    CStore::from_tcx_mut(self.r.tcx).set_used_recursively(def_id.krate);
                 }
                 res
             });

compiler/rustc_resolve/src/lib.rs

@@ -1634,7 +1634,6 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
             self.tcx
                 .sess
                 .time("resolve_postprocess", || self.crate_loader(|c| c.postprocess(krate)));
-            self.crate_loader(|c| c.unload_unused_crates());
         });
 
         // Make sure we don't mutate the cstore from here on.

tests/ui/extern-flag/empty-extern-arg.stderr

@@ -1,6 +1,13 @@
 error: extern location for std does not exist: 
 
+error: `#[panic_handler]` function required, but not found
+
+error: unwinding panics are not supported without std
+   |
+   = help: using nightly cargo, use -Zbuild-std with panic="abort" to avoid unwinding
+   = note: since the core library is usually precompiled with panic="unwind", rebuilding your crate with panic="abort" may not be enough to fix the problem
+
 error: requires `sized` lang_item
 
-error: aborting due to 2 previous errors
+error: aborting due to 4 previous errors