Use cfg_attr AST placeholder AST cfg_attr_trace for diagnostics

Previously, when evaluating a `#[cfg_attr(..)]` to false, the entire attribute was removed from the AST. Afterwards in rust-lang#138515, we insert in its place a placeholder attribute so that checks for attributes can still know about their placement. This is particularly relevant when we suggest removing items with `cfg_attr`s (fix rust-lang#56328). We use the placeholder as it is an ident that can't be written by the end user to begin with. We tweak the wording of the existing "unused `extern crate`" lint.

```
warning: unused `extern crate`
  --> $DIR/removing-extern-crate.rs:9:1
   |
LL | extern crate removing_extern_crate as foo;
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ unused
   |
note: the lint level is defined here
  --> $DIR/removing-extern-crate.rs:6:9
   |
LL | #![warn(rust_2018_idioms)]
   |         ^^^^^^^^^^^^^^^^
   = note: `#[warn(unused_extern_crates)]` implied by `#[warn(rust_2018_idioms)]`
help: remove the unused `extern crate`
   |
LL - #[cfg_attr(test, macro_use)]
LL - extern crate removing_extern_crate as foo;
LL +
   |
```

Author: estebank

compiler/rustc_lint/messages.ftl

@@ -951,8 +951,9 @@ lint_unused_doc_comment = unused doc comment
     .label = rustdoc does not generate documentation for macro invocations
     .help = to document an item produced by a macro, the macro must produce the documentation as part of its expansion
 
-lint_unused_extern_crate = unused extern crate
-    .suggestion = remove it
+lint_unused_extern_crate = unused `extern crate`
+    .label = unused
+    .suggestion = remove the unused `extern crate`
 
 lint_unused_import_braces = braces around {$node} is unnecessary
 

compiler/rustc_lint/src/early/diagnostics.rs

@@ -292,8 +292,8 @@ pub(super) fn decorate_lint(
         BuiltinLintDiag::ByteSliceInPackedStructWithDerive { ty } => {
             lints::ByteSliceInPackedStructWithDerive { ty }.decorate_lint(diag);
         }
-        BuiltinLintDiag::UnusedExternCrate { removal_span } => {
-            lints::UnusedExternCrate { removal_span }.decorate_lint(diag);
+        BuiltinLintDiag::UnusedExternCrate { span, removal_span } => {
+            lints::UnusedExternCrate { span, removal_span }.decorate_lint(diag);
         }
         BuiltinLintDiag::ExternCrateNotIdiomatic { vis_span, ident_span } => {
             let suggestion_span = vis_span.between(ident_span);

compiler/rustc_lint/src/lints.rs

@@ -3041,7 +3041,9 @@ pub(crate) struct ByteSliceInPackedStructWithDerive {
 #[derive(LintDiagnostic)]
 #[diag(lint_unused_extern_crate)]
 pub(crate) struct UnusedExternCrate {
-    #[suggestion(code = "", applicability = "machine-applicable")]
+    #[label]
+    pub span: Span,
+    #[suggestion(code = "", applicability = "machine-applicable", style = "verbose")]
     pub removal_span: Span,
 }
 

compiler/rustc_lint_defs/src/lib.rs

@@ -736,6 +736,7 @@ pub enum BuiltinLintDiag {
         ty: String,
     },
     UnusedExternCrate {
+        span: Span,
         removal_span: Span,
     },
     ExternCrateNotIdiomatic {

compiler/rustc_resolve/src/check_unused.rs

@@ -154,6 +154,7 @@ impl<'a, 'ra, 'tcx> UnusedImportCheckVisitor<'a, 'ra, 'tcx> {
                         extern_crate.id,
                         span,
                         BuiltinLintDiag::UnusedExternCrate {
+                            span: extern_crate.span,
                             removal_span: extern_crate.span_with_attributes,
                         },
                     );

tests/ui/editions/edition-extern-crate-allowed.rs

@@ -5,6 +5,6 @@
 #![warn(rust_2018_idioms)]
 
 extern crate edition_extern_crate_allowed;
-//~^ WARNING unused extern crate
+//~^ WARNING unused `extern crate`
 
 fn main() {}

tests/ui/editions/edition-extern-crate-allowed.stderr

@@ -1,15 +1,19 @@
-warning: unused extern crate
+warning: unused `extern crate`
   --> $DIR/edition-extern-crate-allowed.rs:7:1
    |
 LL | extern crate edition_extern_crate_allowed;
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: remove it
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ unused
    |
 note: the lint level is defined here
   --> $DIR/edition-extern-crate-allowed.rs:5:9
    |
 LL | #![warn(rust_2018_idioms)]
    |         ^^^^^^^^^^^^^^^^
    = note: `#[warn(unused_extern_crates)]` implied by `#[warn(rust_2018_idioms)]`
+help: remove the unused `extern crate`
+   |
+LL - extern crate edition_extern_crate_allowed;
+   |
 
 warning: 1 warning emitted
 

tests/ui/imports/extern-crate-used.rs

@@ -15,7 +15,7 @@ extern crate core as iso3;
 extern crate core as iso4;
 
 // Doesn't introduce its extern prelude entry, so it's still considered unused.
-extern crate core; //~ ERROR unused extern crate
+extern crate core; //~ ERROR unused `extern crate`
 
 mod m {
     use iso1::any as are_you_okay1;

tests/ui/imports/extern-crate-used.stderr

@@ -1,14 +1,19 @@
-error: unused extern crate
+error: unused `extern crate`
   --> $DIR/extern-crate-used.rs:18:1
    |
 LL | extern crate core;
-   | ^^^^^^^^^^^^^^^^^^ help: remove it
+   | ^^^^^^^^^^^^^^^^^^ unused
    |
 note: the lint level is defined here
   --> $DIR/extern-crate-used.rs:6:9
    |
 LL | #![deny(unused_extern_crates)]
    |         ^^^^^^^^^^^^^^^^^^^^
+help: remove the unused `extern crate`
+   |
+LL - extern crate core;
+LL +
+   |
 
 error: aborting due to 1 previous error
 

tests/ui/lint/unnecessary-extern-crate.rs

@@ -4,10 +4,10 @@
 #![feature(test)]
 
 extern crate core;
-//~^ ERROR unused extern crate
+//~^ ERROR unused `extern crate`
 //~| HELP remove
 extern crate core as x;
-//~^ ERROR unused extern crate
+//~^ ERROR unused `extern crate`
 //~| HELP remove
 
 extern crate proc_macro;
@@ -29,11 +29,11 @@ mod foo {
     pub(super) extern crate alloc as d;
 
     extern crate core;
-    //~^ ERROR unused extern crate
+    //~^ ERROR unused `extern crate`
     //~| HELP remove
 
     extern crate core as x;
-    //~^ ERROR unused extern crate
+    //~^ ERROR unused `extern crate`
     //~| HELP remove
 
     pub extern crate test;
@@ -42,11 +42,11 @@ mod foo {
 
     mod bar {
         extern crate core;
-        //~^ ERROR unused extern crate
+        //~^ ERROR unused `extern crate`
         //~| HELP remove
 
         extern crate core as x;
-        //~^ ERROR unused extern crate
+        //~^ ERROR unused `extern crate`
         //~| HELP remove
 
         pub(in crate::foo::bar) extern crate alloc as e;