Add feature diagnostic for unsafe_extern_blocks

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -1088,7 +1088,15 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                             }
                         }
                     } else if let &Safety::Unsafe(span) = safety {
-                        this.dcx().emit_err(errors::UnsafeItem { span, kind: "extern block" });
+                        let mut diag = this
+                            .dcx()
+                            .create_err(errors::UnsafeItem { span, kind: "extern block" });
+                        rustc_session::parse::add_feature_diagnostics(
+                            &mut diag,
+                            self.session,
+                            sym::unsafe_extern_blocks,
+                        );
+                        diag.emit();
                     }
 
                     if abi.is_none() {

tests/ui/feature-gates/feature-gate-unsafe-extern-blocks.stderr

@@ -3,6 +3,10 @@ error: extern block cannot be declared unsafe
    |
 LL | unsafe extern "C" {
    | ^^^^^^
+   |
+   = note: see issue #123743 <https://github.com/rust-lang/rust/issues/123743> for more information
+   = help: add `#![feature(unsafe_extern_blocks)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
 
 error[E0658]: `unsafe extern {}` blocks and `safe` keyword are experimental
   --> $DIR/feature-gate-unsafe-extern-blocks.rs:9:5

tests/ui/parser/unsafe-foreign-mod-2.stderr

@@ -9,6 +9,10 @@ error: extern block cannot be declared unsafe
    |
 LL | extern "C" unsafe {
    |            ^^^^^^
+   |
+   = note: see issue #123743 <https://github.com/rust-lang/rust/issues/123743> for more information
+   = help: add `#![feature(unsafe_extern_blocks)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
 
 error: items in unadorned `extern` blocks cannot have safety qualifiers
   --> $DIR/unsafe-foreign-mod-2.rs:4:5

tests/ui/parser/unsafe-foreign-mod.stderr

@@ -3,6 +3,10 @@ error: extern block cannot be declared unsafe
    |
 LL | unsafe extern "C" {
    | ^^^^^^
+   |
+   = note: see issue #123743 <https://github.com/rust-lang/rust/issues/123743> for more information
+   = help: add `#![feature(unsafe_extern_blocks)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
 
 error: aborting due to 1 previous error