Rollup merge of #131550 - compiler-errors:extern-diags, r=spastorino

Make some tweaks to extern block diagnostics

Self-explanatory. See the diagnostic changes; I hope they make them a bit more descriptive.

r? spastorino

compiler/rustc_ast_passes/messages.ftl

@@ -62,12 +62,12 @@ ast_passes_equality_in_where = equality constraints are not yet supported in `wh
 
 ast_passes_extern_block_suggestion = if you meant to declare an externally defined function, use an `extern` block
 
-ast_passes_extern_fn_qualifiers = functions in `extern` blocks cannot have qualifiers
+ast_passes_extern_fn_qualifiers = functions in `extern` blocks cannot have `{$kw}` qualifier
     .label = in this `extern` block
-    .suggestion = remove this qualifier
+    .suggestion = remove the `{$kw}` qualifier
 
-ast_passes_extern_invalid_safety = items in unadorned `extern` blocks cannot have safety qualifiers
-    .suggestion = add unsafe to this `extern` block
+ast_passes_extern_invalid_safety = items in `extern` blocks without an `unsafe` qualifier cannot have safety qualifiers
+    .suggestion = add `unsafe` to this `extern` block
 
 ast_passes_extern_item_ascii = items in `extern` blocks cannot use non-ascii identifiers
     .label = in this `extern` block

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -524,21 +524,24 @@ impl<'a> AstValidator<'a> {
         // Deconstruct to ensure exhaustiveness
         FnHeader { safety: _, coroutine_kind, constness, ext }: FnHeader,
     ) {
-        let report_err = |span| {
-            self.dcx()
-                .emit_err(errors::FnQualifierInExtern { span, block: self.current_extern_span() });
+        let report_err = |span, kw| {
+            self.dcx().emit_err(errors::FnQualifierInExtern {
+                span,
+                kw,
+                block: self.current_extern_span(),
+            });
         };
         match coroutine_kind {
-            Some(knd) => report_err(knd.span()),
+            Some(kind) => report_err(kind.span(), kind.as_str()),
             None => (),
         }
         match constness {
-            Const::Yes(span) => report_err(span),
+            Const::Yes(span) => report_err(span, "const"),
             Const::No => (),
         }
         match ext {
             Extern::None => (),
-            Extern::Implicit(span) | Extern::Explicit(_, span) => report_err(span),
+            Extern::Implicit(span) | Extern::Explicit(_, span) => report_err(span, "extern"),
         }
     }
 

compiler/rustc_ast_passes/src/errors.rs

@@ -295,6 +295,7 @@ pub(crate) struct FnQualifierInExtern {
     pub span: Span,
     #[label]
     pub block: Span,
+    pub kw: &'static str,
 }
 
 #[derive(Diagnostic)]