Use `cfg_attr` AST placeholder AST `cfg_attr_trace` for diagnostics

PR 138515, we insert a placeholder attribute so that checks for attributes can still know about the placement of `cfg` attributes. When we suggest removing items with `cfg_attr`s (fix Issue 56328) and make them verbose. 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 +
   |
```

5 files changed, 9 insertions, 4 deletions

diff --git a/compiler/rustc_lint/messages.ftl b/compiler/rustc_lint/messages.ftl
index 7fdf26bf3af..ea4d263dfa5 100644
--- a/compiler/rustc_lint/messages.ftl
+++ b/compiler/rustc_lint/messages.ftl
@@ -953,7 +953,8 @@ lint_unused_doc_comment = unused doc comment
     .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
+    .label = unused
+    .suggestion = remove the unused `extern crate`
 
 lint_unused_import_braces = braces around {$node} is unnecessary
 
diff --git a/compiler/rustc_lint/src/early/diagnostics.rs b/compiler/rustc_lint/src/early/diagnostics.rs
index 40ca9e05d95..279f452a305 100644
--- a/compiler/rustc_lint/src/early/diagnostics.rs
+++ b/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);
diff --git a/compiler/rustc_lint/src/lints.rs b/compiler/rustc_lint/src/lints.rs
index af8fa8ffa1f..16f11c45eb9 100644
--- a/compiler/rustc_lint/src/lints.rs
+++ b/compiler/rustc_lint/src/lints.rs
@@ -3051,7 +3051,9 @@ 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,
 }
 
diff --git a/compiler/rustc_lint_defs/src/lib.rs b/compiler/rustc_lint_defs/src/lib.rs
index b4069b317bf..dc30532ba1a 100644
--- a/compiler/rustc_lint_defs/src/lib.rs
+++ b/compiler/rustc_lint_defs/src/lib.rs
@@ -736,6 +736,7 @@ pub enum BuiltinLintDiag {
         ty: String,
     },
     UnusedExternCrate {
+        span: Span,
         removal_span: Span,
     },
     ExternCrateNotIdiomatic {
diff --git a/compiler/rustc_resolve/src/check_unused.rs b/compiler/rustc_resolve/src/check_unused.rs
index e97233e97ce..0579e91c0d6 100644
--- a/compiler/rustc_resolve/src/check_unused.rs
+++ b/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,
                         },
                     );