Fix bad-c-variadic error being emitted multiple times

If a function incorrectly contains multiple `...` args, and is also not
foreign or `unsafe extern "C"`, only emit the latter error once.

Author: nicholasbishop

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -485,6 +485,16 @@ impl<'a> AstValidator<'a> {
     /// Reject C-variadic type unless the function is foreign,
     /// or free and `unsafe extern "C"` semantically.
     fn check_c_variadic_type(&self, fk: FnKind<'a>) {
+        let Some(variadic_span) = fk
+            .decl()
+            .inputs
+            .iter()
+            .find(|arg| matches!(arg.ty.kind, TyKind::CVarArgs))
+            .map(|arg| arg.span)
+        else {
+            return;
+        };
+
         match (fk.ctxt(), fk.header()) {
             (Some(FnCtxt::Foreign), _) => return,
             (Some(FnCtxt::Free), Some(header)) => match header.ext {
@@ -499,11 +509,7 @@ impl<'a> AstValidator<'a> {
             _ => {}
         };
 
-        for Param { ty, span, .. } in &fk.decl().inputs {
-            if let TyKind::CVarArgs = ty.kind {
-                self.err_handler().emit_err(errors::BadCVariadic { span: *span });
-            }
-        }
+        self.err_handler().emit_err(errors::BadCVariadic { span: variadic_span });
     }
 
     fn check_item_named(&self, ident: Ident, kind: &str) {

tests/ui/c-variadic/issue-86053-1.stderr

@@ -52,12 +52,6 @@ error: only foreign or `unsafe extern "C"` functions may be C-variadic
 LL |     self , ... ,   self ,   self , ... ) where F : FnOnce ( & 'a & 'b usize ) {
    |            ^^^
 
-error: only foreign or `unsafe extern "C"` functions may be C-variadic
-  --> $DIR/issue-86053-1.rs:11:36
-   |
-LL |     self , ... ,   self ,   self , ... ) where F : FnOnce ( & 'a & 'b usize ) {
-   |                                    ^^^
-
 error[E0412]: cannot find type `F` in this scope
   --> $DIR/issue-86053-1.rs:11:48
    |
@@ -76,6 +70,6 @@ help: you might be missing a type parameter
 LL | fn ordering4 < 'a , 'b, F     > ( a :            ,   self , self ,   self ,
    |                       +++
 
-error: aborting due to 11 previous errors
+error: aborting due to 10 previous errors
 
 For more information about this error, try `rustc --explain E0412`.

tests/ui/parser/variadic-ffi-semantic-restrictions.rs

@@ -49,11 +49,9 @@ impl X {
     //~| ERROR C-variadic function must be declared with at least one named argument
     fn i_f3(..., x: isize, ...) {}
     //~^ ERROR only foreign or `unsafe extern "C"` functions may be C-variadic
-    //~| ERROR only foreign or `unsafe extern "C"` functions may be C-variadic
     //~| ERROR `...` must be the last argument of a C-variadic function
     fn i_f4(..., x: isize, ...) {}
     //~^ ERROR only foreign or `unsafe extern "C"` functions may be C-variadic
-    //~| ERROR only foreign or `unsafe extern "C"` functions may be C-variadic
     //~| ERROR `...` must be the last argument of a C-variadic function
 }
 

tests/ui/parser/variadic-ffi-semantic-restrictions.stderr

@@ -118,89 +118,77 @@ error: only foreign or `unsafe extern "C"` functions may be C-variadic
 LL |     fn i_f3(..., x: isize, ...) {}
    |             ^^^
 
-error: only foreign or `unsafe extern "C"` functions may be C-variadic
-  --> $DIR/variadic-ffi-semantic-restrictions.rs:50:28
-   |
-LL |     fn i_f3(..., x: isize, ...) {}
-   |                            ^^^
-
 error: `...` must be the last argument of a C-variadic function
-  --> $DIR/variadic-ffi-semantic-restrictions.rs:54:13
+  --> $DIR/variadic-ffi-semantic-restrictions.rs:53:13
    |
 LL |     fn i_f4(..., x: isize, ...) {}
    |             ^^^
 
 error: only foreign or `unsafe extern "C"` functions may be C-variadic
-  --> $DIR/variadic-ffi-semantic-restrictions.rs:54:13
+  --> $DIR/variadic-ffi-semantic-restrictions.rs:53:13
    |
 LL |     fn i_f4(..., x: isize, ...) {}
    |             ^^^
 
 error: only foreign or `unsafe extern "C"` functions may be C-variadic
-  --> $DIR/variadic-ffi-semantic-restrictions.rs:54:28
-   |
-LL |     fn i_f4(..., x: isize, ...) {}
-   |                            ^^^
-
-error: only foreign or `unsafe extern "C"` functions may be C-variadic
-  --> $DIR/variadic-ffi-semantic-restrictions.rs:61:23
+  --> $DIR/variadic-ffi-semantic-restrictions.rs:59:23
    |
 LL |     fn t_f1(x: isize, ...) {}
    |                       ^^^
 
 error: only foreign or `unsafe extern "C"` functions may be C-variadic
-  --> $DIR/variadic-ffi-semantic-restrictions.rs:63:23
+  --> $DIR/variadic-ffi-semantic-restrictions.rs:61:23
    |
 LL |     fn t_f2(x: isize, ...);
    |                       ^^^
 
 error: C-variadic function must be declared with at least one named argument
-  --> $DIR/variadic-ffi-semantic-restrictions.rs:65:13
+  --> $DIR/variadic-ffi-semantic-restrictions.rs:63:13
    |
 LL |     fn t_f3(...) {}
    |             ^^^
 
 error: only foreign or `unsafe extern "C"` functions may be C-variadic
-  --> $DIR/variadic-ffi-semantic-restrictions.rs:65:13
+  --> $DIR/variadic-ffi-semantic-restrictions.rs:63:13
    |
 LL |     fn t_f3(...) {}
    |             ^^^
 
 error: C-variadic function must be declared with at least one named argument
-  --> $DIR/variadic-ffi-semantic-restrictions.rs:68:13
+  --> $DIR/variadic-ffi-semantic-restrictions.rs:66:13
    |
 LL |     fn t_f4(...);
    |             ^^^
 
 error: only foreign or `unsafe extern "C"` functions may be C-variadic
-  --> $DIR/variadic-ffi-semantic-restrictions.rs:68:13
+  --> $DIR/variadic-ffi-semantic-restrictions.rs:66:13
    |
 LL |     fn t_f4(...);
    |             ^^^
 
 error: `...` must be the last argument of a C-variadic function
-  --> $DIR/variadic-ffi-semantic-restrictions.rs:71:13
+  --> $DIR/variadic-ffi-semantic-restrictions.rs:69:13
    |
 LL |     fn t_f5(..., x: isize) {}
    |             ^^^
 
 error: only foreign or `unsafe extern "C"` functions may be C-variadic
-  --> $DIR/variadic-ffi-semantic-restrictions.rs:71:13
+  --> $DIR/variadic-ffi-semantic-restrictions.rs:69:13
    |
 LL |     fn t_f5(..., x: isize) {}
    |             ^^^
 
 error: `...` must be the last argument of a C-variadic function
-  --> $DIR/variadic-ffi-semantic-restrictions.rs:74:13
+  --> $DIR/variadic-ffi-semantic-restrictions.rs:72:13
    |
 LL |     fn t_f6(..., x: isize);
    |             ^^^
 
 error: only foreign or `unsafe extern "C"` functions may be C-variadic
-  --> $DIR/variadic-ffi-semantic-restrictions.rs:74:13
+  --> $DIR/variadic-ffi-semantic-restrictions.rs:72:13
    |
 LL |     fn t_f6(..., x: isize);
    |             ^^^
 
-error: aborting due to 34 previous errors
+error: aborting due to 32 previous errors