Optimize the diagnostic generation for extern unsafe

compiler/rustc_parse/src/parser/item.rs

@@ -997,35 +997,24 @@ impl<'a> Parser<'a> {
     fn parse_item_foreign_mod(
         &mut self,
         attrs: &mut Vec<Attribute>,
-        unsafety: Unsafe,
+        mut unsafety: Unsafe,
     ) -> PResult<'a, ItemInfo> {
-        let sp_start = self.prev_token.span;
         let abi = self.parse_abi(); // ABI?
-        match self.parse_item_list(attrs, |p| p.parse_foreign_item(ForceCollect::No)) {
-            Ok(items) => {
-                let module = ast::ForeignMod { unsafety, abi, items };
-                Ok((Ident::empty(), ItemKind::ForeignMod(module)))
-            }
-            Err(mut err) => {
-                let current_qual_sp = self.prev_token.span;
-                let current_qual_sp = current_qual_sp.to(sp_start);
-                if let Ok(current_qual) = self.span_to_snippet(current_qual_sp) {
-                    // FIXME(davidtwco): avoid depending on the error message text
-                    if err.message[0].0.expect_str() == "expected `{`, found keyword `unsafe`" {
-                        let invalid_qual_sp = self.token.uninterpolated_span();
-                        let invalid_qual = self.span_to_snippet(invalid_qual_sp).unwrap();
-
-                        err.span_suggestion(
-                                current_qual_sp.to(invalid_qual_sp),
-                                &format!("`{}` must come before `{}`", invalid_qual, current_qual),
-                                format!("{} {}", invalid_qual, current_qual),
-                                Applicability::MachineApplicable,
-                            ).note("keyword order for functions declaration is `pub`, `default`, `const`, `async`, `unsafe`, `extern`");
-                    }
-                }
-                Err(err)
-            }
+        if unsafety == Unsafe::No
+            && self.token.is_keyword(kw::Unsafe)
+            && self.look_ahead(1, |t| t.kind == token::OpenDelim(Delimiter::Brace))
+        {
+            let mut err = self.expect(&token::OpenDelim(Delimiter::Brace)).unwrap_err();
+            err.emit();
+            unsafety = Unsafe::Yes(self.token.span);
+            self.eat_keyword(kw::Unsafe);
         }
+        let module = ast::ForeignMod {
+            unsafety,
+            abi,
+            items: self.parse_item_list(attrs, |p| p.parse_foreign_item(ForceCollect::No))?,
+        };
+        Ok((Ident::empty(), ItemKind::ForeignMod(module)))
     }
 
     /// Parses a foreign item (one in an `extern { ... }` block).

src/test/ui/parser/issues/issue-19398.stderr

@@ -4,15 +4,10 @@ error: expected `{`, found keyword `unsafe`
 LL | trait T {
    |         - while parsing this item list starting here
 LL |     extern "Rust" unsafe fn foo();
-   |     --------------^^^^^^
-   |     |             |
-   |     |             expected `{`
-   |     help: `unsafe` must come before `extern "Rust"`: `unsafe extern "Rust"`
+   |                   ^^^^^^ expected `{`
 LL |
 LL | }
    | - the item list ends here
-   |
-   = note: keyword order for functions declaration is `pub`, `default`, `const`, `async`, `unsafe`, `extern`
 
 error: aborting due to previous error
 

src/test/ui/parser/unsafe-foreign-mod-2.rs

@@ -0,0 +1,8 @@
+extern "C" unsafe {
+               //~^ ERROR expected `{`, found keyword `unsafe`
+               //~| ERROR extern block cannot be declared unsafe
+    unsafe fn foo();
+        //~^ ERROR functions in `extern` blocks cannot have qualifiers
+}
+
+fn main() {}

src/test/ui/parser/unsafe-foreign-mod-2.stderr

@@ -0,0 +1,28 @@
+error: expected `{`, found keyword `unsafe`
+  --> $DIR/unsafe-foreign-mod-2.rs:1:12
+   |
+LL | extern "C" unsafe {
+   |            ^^^^^^ expected `{`
+
+error: extern block cannot be declared unsafe
+  --> $DIR/unsafe-foreign-mod-2.rs:1:12
+   |
+LL | extern "C" unsafe {
+   |            ^^^^^^
+
+error: functions in `extern` blocks cannot have qualifiers
+  --> $DIR/unsafe-foreign-mod-2.rs:4:15
+   |
+LL | extern "C" unsafe {
+   | ----------------- in this `extern` block
+...
+LL |     unsafe fn foo();
+   |               ^^^
+   |
+help: remove the qualifiers
+   |
+LL |     fn foo();
+   |     ~~
+
+error: aborting due to 3 previous errors
+