Auto merge of #124482 - spastorino:unsafe-extern-blocks, r=oli-obk

Unsafe extern blocks

This implements RFC 3484.

Tracking issue #123743 and RFC https://github.com/rust-lang/rfcs/pull/3484

This is better reviewed commit by commit.

compiler/rustc_lint/src/lints.rs

@@ -2730,6 +2730,13 @@ pub enum DeprecatedWhereClauseLocationSugg {
     },
 }
 
+#[derive(LintDiagnostic)]
+#[diag(lint_missing_unsafe_on_extern)]
+pub struct MissingUnsafeOnExtern {
+    #[suggestion(code = "unsafe ", applicability = "machine-applicable")]
+    pub suggestion: Span,
+}
+
 #[derive(LintDiagnostic)]
 #[diag(lint_single_use_lifetime)]
 pub struct SingleUseLifetime {