From 9196844f0d86242ee3d6a446fe681b6b2d2d674c Mon Sep 17 00:00:00 2001
From: Folkert de Vries <folkert@folkertdev.nl>
Date: Mon, 8 Sep 2025 19:27:07 +0200
Subject: c-variadic: reject functions with unsupported extern ABI

---
 compiler/rustc_ast/src/ast.rs                   | 5 +++++
 compiler/rustc_ast_passes/messages.ftl          | 7 +++++--
 compiler/rustc_ast_passes/src/ast_validation.rs | 7 +++++--
 compiler/rustc_ast_passes/src/errors.rs         | 9 +++++++--
 4 files changed, 22 insertions(+), 6 deletions(-)

(limited to 'compiler')

diff --git a/compiler/rustc_ast/src/ast.rs b/compiler/rustc_ast/src/ast.rs
index 915bae9e5f5..3e8fddd9954 100644
--- a/compiler/rustc_ast/src/ast.rs
+++ b/compiler/rustc_ast/src/ast.rs
@@ -2325,6 +2325,11 @@ impl FnSig {
             }
         }
     }
+
+    /// The span of the header's extern, or where to insert it if empty.
+    pub fn extern_span(&self) -> Span {
+        self.header.ext.span().unwrap_or(self.safety_span().shrink_to_hi())
+    }
 }
 
 /// A constraint on an associated item.
diff --git a/compiler/rustc_ast_passes/messages.ftl b/compiler/rustc_ast_passes/messages.ftl
index 1aa38a1beda..8dcf3e3aa38 100644
--- a/compiler/rustc_ast_passes/messages.ftl
+++ b/compiler/rustc_ast_passes/messages.ftl
@@ -57,8 +57,6 @@ ast_passes_auto_super_lifetime = auto traits cannot have super traits or lifetim
     .label = {ast_passes_auto_super_lifetime}
     .suggestion = remove the super traits or lifetime bounds
 
-ast_passes_bad_c_variadic = defining functions with C-variadic arguments is only allowed for free functions with the "C" or "C-unwind" calling convention
-
 ast_passes_body_in_extern = incorrect `{$kind}` inside `extern` block
     .cannot_have = cannot have a body
     .invalid = the invalid body
@@ -68,11 +66,16 @@ ast_passes_bound_in_context = bounds on `type`s in {$ctx} have no effect
 
 ast_passes_c_variadic_associated_function = associated functions cannot have a C variable argument list
 
+ast_passes_c_variadic_bad_extern = `...` is not supported for `extern "{$abi}"` functions
+    .label = `extern "{$abi}"` because of this
+    .help = only `extern "C"` and `extern "C-unwind"` functions may have a C variable argument list
+
 ast_passes_c_variadic_must_be_unsafe =
     functions with a C variable argument list must be unsafe
     .suggestion = add the `unsafe` keyword to this definition
 
 ast_passes_c_variadic_no_extern = `...` is not supported for non-extern functions
+    .help = only `extern "C"` and `extern "C-unwind"` functions may have a C variable argument list
 
 ast_passes_const_and_c_variadic = functions cannot be both `const` and C-variadic
     .const = `const` because of this
diff --git a/compiler/rustc_ast_passes/src/ast_validation.rs b/compiler/rustc_ast_passes/src/ast_validation.rs
index 7b32778ddf0..a6ef89b553d 100644
--- a/compiler/rustc_ast_passes/src/ast_validation.rs
+++ b/compiler/rustc_ast_passes/src/ast_validation.rs
@@ -698,7 +698,6 @@ impl<'a> AstValidator<'a> {
             FnCtxt::Foreign => return,
             FnCtxt::Free => match sig.header.ext {
                 Extern::Implicit(_) => {
-                    // Implicitly defaults to C.
                     if !matches!(sig.header.safety, Safety::Unsafe(_)) {
                         self.dcx().emit_err(errors::CVariadicMustBeUnsafe {
                             span: variadic_param.span,
@@ -708,7 +707,11 @@ impl<'a> AstValidator<'a> {
                 }
                 Extern::Explicit(StrLit { symbol_unescaped, .. }, _) => {
                     if !matches!(symbol_unescaped, sym::C | sym::C_dash_unwind) {
-                        self.dcx().emit_err(errors::BadCVariadic { span: variadic_param.span });
+                        self.dcx().emit_err(errors::CVariadicBadExtern {
+                            span: variadic_param.span,
+                            abi: symbol_unescaped,
+                            extern_span: sig.extern_span(),
+                        });
                     }
 
                     if !matches!(sig.header.safety, Safety::Unsafe(_)) {
diff --git a/compiler/rustc_ast_passes/src/errors.rs b/compiler/rustc_ast_passes/src/errors.rs
index 946a22d2e75..ae805042c54 100644
--- a/compiler/rustc_ast_passes/src/errors.rs
+++ b/compiler/rustc_ast_passes/src/errors.rs
@@ -327,6 +327,7 @@ pub(crate) struct CVariadicAssociatedFunction {
 
 #[derive(Diagnostic)]
 #[diag(ast_passes_c_variadic_no_extern)]
+#[help]
 pub(crate) struct CVariadicNoExtern {
     #[primary_span]
     pub span: Span,
@@ -348,10 +349,14 @@ pub(crate) struct CVariadicMustBeUnsafe {
 }
 
 #[derive(Diagnostic)]
-#[diag(ast_passes_bad_c_variadic)]
-pub(crate) struct BadCVariadic {
+#[diag(ast_passes_c_variadic_bad_extern)]
+#[help]
+pub(crate) struct CVariadicBadExtern {
     #[primary_span]
     pub span: Span,
+    pub abi: Symbol,
+    #[label]
+    pub extern_span: Span,
 }
 
 #[derive(Diagnostic)]
-- 
cgit 1.4.1-3-g733a5