Better diagnostic for fn items in variadic functions

compiler/rustc_hir_typeck/messages.ftl

@@ -79,6 +79,11 @@ hir_typeck_field_multiply_specified_in_initializer =
     .label = used more than once
     .previous_use_label = first use of `{$ident}`
 
+hir_typeck_fn_item_to_variadic_function = can't pass a function item to a variadic function
+    .suggestion = use a function pointer instead
+    .help = a function item is zero-sized and needs to be cast into a function pointer to be used in FFI
+    .note = for more information on function items, visit https://doc.rust-lang.org/reference/types/function-item.html
+
 hir_typeck_fru_expr = this expression does not end in a comma...
 hir_typeck_fru_expr2 = ... so this is interpreted as a `..` range expression, instead of functional record update syntax
 hir_typeck_fru_note = this expression may have been misinterpreted as a `..` range expression

compiler/rustc_hir_typeck/src/errors.rs

@@ -797,3 +797,15 @@ pub(crate) struct PassToVariadicFunction<'a, 'tcx> {
     #[note(hir_typeck_teach_help)]
     pub(crate) teach: bool,
 }
+
+#[derive(Diagnostic)]
+#[diag(hir_typeck_fn_item_to_variadic_function, code = E0617)]
+#[help]
+#[note]
+pub(crate) struct PassFnItemToVariadicFunction {
+    #[primary_span]
+    pub span: Span,
+    #[suggestion(code = " as {replace}", applicability = "machine-applicable", style = "verbose")]
+    pub sugg_span: Span,
+    pub replace: String,
+}

compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs

@@ -472,9 +472,15 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                         variadic_error(tcx.sess, arg.span, arg_ty, "c_uint");
                     }
                     ty::FnDef(..) => {
-                        let ptr_ty = Ty::new_fn_ptr(self.tcx, arg_ty.fn_sig(self.tcx));
-                        let ptr_ty = self.resolve_vars_if_possible(ptr_ty);
-                        variadic_error(tcx.sess, arg.span, arg_ty, &ptr_ty.to_string());
+                        let fn_ptr = Ty::new_fn_ptr(self.tcx, arg_ty.fn_sig(self.tcx));
+                        let fn_ptr = self.resolve_vars_if_possible(fn_ptr).to_string();
+
+                        let fn_item_spa = arg.span;
+                        tcx.sess.dcx().emit_err(errors::PassFnItemToVariadicFunction {
+                            span: fn_item_spa,
+                            sugg_span: fn_item_spa.shrink_to_hi(),
+                            replace: fn_ptr,
+                        });
                     }
                     _ => {}
                 }

tests/ui/c-variadic/fn-item-diagnostic-issue-69232.rs

@@ -0,0 +1,13 @@
+// https://github.com/rust-lang/rust/issues/69232
+
+extern "C" {
+    fn foo(x: usize, ...);
+}
+
+fn test() -> u8 {
+    127
+}
+
+fn main() {
+    unsafe { foo(1, test) }; //~ ERROR can't pass a function item to a variadic function
+}

tests/ui/c-variadic/fn-item-diagnostic-issue-69232.stderr

@@ -0,0 +1,16 @@
+error[E0617]: can't pass a function item to a variadic function
+  --> $DIR/fn-item-diagnostic-issue-69232.rs:12:21
+   |
+LL |     unsafe { foo(1, test) };
+   |                     ^^^^
+   |
+   = help: a function item is zero-sized and needs to be cast into a function pointer to be used in FFI
+   = note: for more information on function items, visit https://doc.rust-lang.org/reference/types/function-item.html
+help: use a function pointer instead
+   |
+LL |     unsafe { foo(1, test as fn() -> u8) };
+   |                          +++++++++++++
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0617`.

tests/ui/c-variadic/issue-32201.rs

@@ -7,7 +7,8 @@ fn bar(_: *const u8) {}
 fn main() {
     unsafe {
         foo(0, bar);
-        //~^ ERROR can't pass `fn(*const u8) {bar}` to variadic function
-        //~| HELP cast the value to `fn(*const u8)`
+        //~^ ERROR can't pass a function item to a variadic function
+        //~| HELP a function item is zero-sized and needs to be cast into a function pointer to be used in FFI
+        ////~| HELP use a function pointer instead
     }
 }

tests/ui/c-variadic/issue-32201.stderr

@@ -1,8 +1,15 @@
-error[E0617]: can't pass `fn(*const u8) {bar}` to variadic function
+error[E0617]: can't pass a function item to a variadic function
   --> $DIR/issue-32201.rs:9:16
    |
 LL |         foo(0, bar);
-   |                ^^^ help: cast the value to `fn(*const u8)`: `bar as fn(*const u8)`
+   |                ^^^
+   |
+   = help: a function item is zero-sized and needs to be cast into a function pointer to be used in FFI
+   = note: for more information on function items, visit https://doc.rust-lang.org/reference/types/function-item.html
+help: use a function pointer instead
+   |
+LL |         foo(0, bar as fn(*const u8));
+   |                    ++++++++++++++++
 
 error: aborting due to 1 previous error
 

tests/ui/error-codes/E0617.rs

@@ -20,7 +20,8 @@ fn main() {
         //~^ ERROR can't pass `u16` to variadic function
         //~| HELP cast the value to `c_uint`
         printf(::std::ptr::null(), printf);
-        //~^ ERROR can't pass `unsafe extern "C" fn(*const i8, ...) {printf}` to variadic function
-        //~| HELP cast the value to `unsafe extern "C" fn(*const i8, ...)`
+        //~^ ERROR can't pass a function item to a variadic function
+        //~| HELP a function item is zero-sized and needs to be cast into a function pointer to be used in FFI
+        //~| HELP use a function pointer instead
     }
 }

tests/ui/error-codes/E0617.stderr

@@ -28,16 +28,18 @@ error[E0617]: can't pass `u16` to variadic function
 LL |         printf(::std::ptr::null(), 0u16);
    |                                    ^^^^ help: cast the value to `c_uint`: `0u16 as c_uint`
 
-error[E0617]: can't pass `unsafe extern "C" fn(*const i8, ...) {printf}` to variadic function
+error[E0617]: can't pass a function item to a variadic function
   --> $DIR/E0617.rs:22:36
    |
 LL |         printf(::std::ptr::null(), printf);
    |                                    ^^^^^^
    |
-help: cast the value to `unsafe extern "C" fn(*const i8, ...)`
+   = help: a function item is zero-sized and needs to be cast into a function pointer to be used in FFI
+   = note: for more information on function items, visit https://doc.rust-lang.org/reference/types/function-item.html
+help: use a function pointer instead
    |
 LL |         printf(::std::ptr::null(), printf as unsafe extern "C" fn(*const i8, ...));
-   |                                    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+   |                                           +++++++++++++++++++++++++++++++++++++++
 
 error: aborting due to 6 previous errors