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

@@ -712,20 +712,25 @@ impl<'a> AstValidator<'a> {
     }
 
     fn maybe_lint_missing_abi(&mut self, span: Span, id: NodeId) {
-        // 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
-            .sess
-            .source_map()
-            .span_to_snippet(span)
-            .is_ok_and(|snippet| !snippet.starts_with("#["))
-        {
-            self.lint_buffer.buffer_lint(
-                MISSING_ABI,
-                id,
-                span,
-                BuiltinLintDiag::MissingAbi(span, abi::Abi::FALLBACK),
-            )
+        if span.edition().at_least_rust_2024() {
+            self.dcx()
+                .emit_err(errors::MissingAbi { span, default_abi: abi::Abi::FALLBACK.name() });
+        } else {
+            // 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
+                .sess
+                .source_map()
+                .span_to_snippet(span)
+                .is_ok_and(|snippet| !snippet.starts_with("#["))
+            {
+                self.lint_buffer.buffer_lint(
+                    MISSING_ABI,
+                    id,
+                    span,
+                    BuiltinLintDiag::MissingAbi(span, abi::Abi::FALLBACK),
+                )
+            }
         }
     }
 }

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,
+}