Make implicit "C" abi an error in Rust 2024.

Author: m-ou-se

compiler/rustc_ast_passes/messages.ftl

@@ -79,6 +79,10 @@ ast_passes_extern_types_cannot = `type`s inside `extern` blocks cannot have {$de
     .suggestion = remove the {$remove_descr}
     .label = `extern` block begins here
 
+ast_passes_extern_without_abi = extern declaration without an ABI
+    .label = ABI should be specified here
+    .suggestion = explicitly specify an ABI
+
 ast_passes_feature_on_non_nightly = `#![feature]` may not be used on the {$channel} release channel
     .suggestion = remove the attribute
     .stable_since = the feature `{$name}` has been stable since `{$since}` and no longer requires an attribute to enable

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -712,6 +712,10 @@ impl<'a> AstValidator<'a> {
     }
 
     fn maybe_lint_missing_abi(&mut self, span: Span, id: NodeId) {
+        if span.edition().at_least_rust_2024() {
+            self.dcx()
+                .emit_err(errors::MissingAbi { span, default_abi: abi::Abi::FALLBACK.name() });
+        }
         // FIXME(davidtwco): This is a hack to detect macros which produce spans of the
         // call site which do not have a macro backtrace. See #61963.
         if self

compiler/rustc_ast_passes/src/errors.rs

@@ -833,3 +833,12 @@ pub(crate) struct DuplicatePreciseCapturing {
     #[label]
     pub bound2: Span,
 }
+
+#[derive(Diagnostic)]
+#[diag(ast_passes_extern_without_abi)]
+pub(crate) struct MissingAbi {
+    #[primary_span]
+    #[suggestion(code = "extern \"{default_abi}\"", applicability = "machine-applicable")]
+    pub span: Span,
+    pub default_abi: &'static str,
+}