Suggest using block for extern "abi" fn with no body

aDotInTheVoid · aDotInTheVoid · commit 02fb345964ce · 2022-07-06T13:27:53.000+01:00
diff --git a/compiler/rustc_ast_passes/src/ast_validation.rs b/compiler/rustc_ast_passes/src/ast_validation.rs
@@ -13,7 +13,9 @@ use rustc_ast::walk_list;
 use rustc_ast::*;
 use rustc_ast_pretty::pprust::{self, State};
 use rustc_data_structures::fx::FxHashMap;
-use rustc_errors::{error_code, pluralize, struct_span_err, Applicability};
+use rustc_errors::{
+    error_code, pluralize, struct_span_err, Applicability, DiagnosticBuilder, ErrorGuaranteed,
+};
 use rustc_parse::validate_attr;
 use rustc_session::lint::builtin::{
     DEPRECATED_WHERE_CLAUSE_LOCATION, MISSING_ABI, PATTERNS_IN_FNS_WITHOUT_BODY,
@@ -476,22 +478,33 @@ impl<'a> AstValidator<'a> {
     }
 
     fn error_item_without_body(&self, sp: Span, ctx: &str, msg: &str, sugg: &str) {
+        self.error_item_without_body_with_help(sp, ctx, msg, sugg, |_| ());
+    }
+
+    fn error_item_without_body_with_help(
+        &self,
+        sp: Span,
+        ctx: &str,
+        msg: &str,
+        sugg: &str,
+        help: impl FnOnce(&mut DiagnosticBuilder<'_, ErrorGuaranteed>),
+    ) {
         let source_map = self.session.source_map();
         let end = source_map.end_point(sp);
         let replace_span = if source_map.span_to_snippet(end).map(|s| s == ";").unwrap_or(false) {
             end
         } else {
             sp.shrink_to_hi()
         };
-        self.err_handler()
-            .struct_span_err(sp, msg)
-            .span_suggestion(
-                replace_span,
-                &format!("provide a definition for the {}", ctx),
-                sugg,
-                Applicability::HasPlaceholders,
-            )
-            .emit();
+        let mut err = self.err_handler().struct_span_err(sp, msg);
+        err.span_suggestion(
+            replace_span,
+            &format!("provide a definition for the {}", ctx),
+            sugg,
+            Applicability::HasPlaceholders,
+        );
+        help(&mut err);
+        err.emit();
     }
 
     fn check_impl_item_provided<T>(&self, sp: Span, body: &Option<T>, ctx: &str, sugg: &str) {
@@ -1191,8 +1204,38 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
 
                 if body.is_none() {
                     let msg = "free function without a body";
-                    self.error_item_without_body(item.span, "function", msg, " { <body> }");
+                    let ext = sig.header.ext;
+
+                    let f = |e: &mut DiagnosticBuilder<'_, _>| {
+                        if let Extern::Implicit(start_span) | Extern::Explicit(_, start_span) = &ext
+                        {
+                            let start_suggestion = if let Extern::Explicit(abi, _) = ext {
+                                format!("extern \"{}\" {{", abi.symbol_unescaped)
+                            } else {
+                                "extern {".to_owned()
+                            };
+
+                            let end_suggestion = " }".to_owned();
+                            let end_span = item.span.shrink_to_hi();
+
+                            e
+                            .multipart_suggestion(
+                                "if you meant to declare an externally defined function, use an `extern` block",
+                                vec![(*start_span, start_suggestion), (end_span, end_suggestion)],
+                                Applicability::MaybeIncorrect,
+                             );
+                        }
+                    };
+
+                    self.error_item_without_body_with_help(
+                        item.span,
+                        "function",
+                        msg,
+                        " { <body> }",
+                        f,
+                    );
                 }
+
                 self.visit_vis(&item.vis);
                 self.visit_ident(item.ident);
                 let kind =
diff --git a/src/test/ui/extern/not-in-block.rs b/src/test/ui/extern/not-in-block.rs
@@ -0,0 +1,6 @@
+#![crate_type = "lib"]
+
+extern fn none_fn(x: bool) -> i32;
+//~^ ERROR free function without a body
+extern "C" fn c_fn(x: bool) -> i32;
+//~^ ERROR free function without a body
diff --git a/src/test/ui/extern/not-in-block.stderr b/src/test/ui/extern/not-in-block.stderr
@@ -0,0 +1,32 @@
+error: free function without a body
+  --> $DIR/not-in-block.rs:3:1
+   |
+LL | extern fn none_fn(x: bool) -> i32;
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+help: provide a definition for the function
+   |
+LL | extern fn none_fn(x: bool) -> i32 { <body> }
+   |                                   ~~~~~~~~~~
+help: if you meant to declare an externally defined function, use an `extern` block
+   |
+LL | extern { fn none_fn(x: bool) -> i32; }
+   | ~~~~~~~~                             +
+
+error: free function without a body
+  --> $DIR/not-in-block.rs:5:1
+   |
+LL | extern "C" fn c_fn(x: bool) -> i32;
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+help: provide a definition for the function
+   |
+LL | extern "C" fn c_fn(x: bool) -> i32 { <body> }
+   |                                    ~~~~~~~~~~
+help: if you meant to declare an externally defined function, use an `extern` block
+   |
+LL | extern "C" { fn c_fn(x: bool) -> i32; }
+   | ~~~~~~~~~~~~                          +
+
+error: aborting due to 2 previous errors
+
