Improve error message for extern "C" unsafe fn()

This was handled correctly already for `extern unsafe fn()`.

Co-authored-by: Folkert <folkert@folkertdev.nl>

compiler/rustc_parse/src/parser/item.rs

@@ -2483,12 +2483,15 @@ impl<'a> Parser<'a> {
     /// `check_pub` adds additional `pub` to the checks in case users place it
     /// wrongly, can be used to ensure `pub` never comes after `default`.
     pub(super) fn check_fn_front_matter(&mut self, check_pub: bool, case: Case) -> bool {
+        const ALL_QUALS: &[Symbol] =
+            &[kw::Pub, kw::Gen, kw::Const, kw::Async, kw::Unsafe, kw::Safe, kw::Extern];
+
         // We use an over-approximation here.
         // `const const`, `fn const` won't parse, but we're not stepping over other syntax either.
         // `pub` is added in case users got confused with the ordering like `async pub fn`,
         // only if it wasn't preceded by `default` as `default pub` is invalid.
         let quals: &[Symbol] = if check_pub {
-            &[kw::Pub, kw::Gen, kw::Const, kw::Async, kw::Unsafe, kw::Safe, kw::Extern]
+            ALL_QUALS
         } else {
             &[kw::Gen, kw::Const, kw::Async, kw::Unsafe, kw::Safe, kw::Extern]
         };
@@ -2518,9 +2521,9 @@ impl<'a> Parser<'a> {
             || self.check_keyword_case(kw::Extern, case)
                 && self.look_ahead(1, |t| t.can_begin_string_literal())
                 && (self.look_ahead(2, |t| t.is_keyword_case(kw::Fn, case)) ||
-                    // this branch is only for better diagnostic in later, `pub` is not allowed here
+                    // this branch is only for better diagnostics; `pub`, `unsafe`, etc. are not allowed here
                     (self.may_recover()
-                        && self.look_ahead(2, |t| t.is_keyword(kw::Pub))
+                        && self.look_ahead(2, |t| ALL_QUALS.iter().any(|&kw| t.is_keyword(kw)))
                         && self.look_ahead(3, |t| t.is_keyword_case(kw::Fn, case))))
     }
 

tests/ui/parser/issues/issue-19398.rs

@@ -1,6 +1,10 @@
 trait T {
     extern "Rust" unsafe fn foo();
-    //~^ ERROR expected `{`, found keyword `unsafe`
+    //~^ ERROR expected `fn`, found keyword `unsafe`
+    //~| NOTE expected `fn`
+    //~| HELP `unsafe` must come before `extern "Rust"`
+    //~| SUGGESTION unsafe extern "Rust"
+    //~| NOTE keyword order for functions declaration is `pub`, `default`, `const`, `async`, `unsafe`, `extern`
 }
 
 fn main() {}

tests/ui/parser/issues/issue-19398.stderr

@@ -1,13 +1,13 @@
-error: expected `{`, found keyword `unsafe`
+error: expected `fn`, found keyword `unsafe`
   --> $DIR/issue-19398.rs:2:19
    |
-LL | trait T {
-   |         - while parsing this item list starting here
 LL |     extern "Rust" unsafe fn foo();
-   |                   ^^^^^^ expected `{`
+   |     --------------^^^^^^
-LL |
+   |     |             |
-LL | }
+   |     |             expected `fn`
-   | - the item list ends here
+   |     help: `unsafe` must come before `extern "Rust"`: `unsafe extern "Rust"`
+   |
+   = note: keyword order for functions declaration is `pub`, `default`, `const`, `async`, `unsafe`, `extern`
 
 error: aborting due to 1 previous error
 

tests/ui/parser/issues/issue-87217-keyword-order/wrong-unsafe-abi.rs

@@ -7,6 +7,10 @@
 // Visibilities are tested elsewhere.
 
 extern "C" unsafe fn test() {}
-//~^ ERROR
+//~^ ERROR expected `fn`, found keyword `unsafe`
+//~| NOTE expected `fn`
+//~| HELP `unsafe` must come before `extern "C"`
+//~| SUGGESTION unsafe extern "C"
+//~| NOTE keyword order for functions declaration is `pub`, `default`, `const`, `async`, `unsafe`, `extern`
 
 fn main() {}

tests/ui/parser/issues/issue-87217-keyword-order/wrong-unsafe-abi.stderr

@@ -1,8 +1,13 @@
-error: expected `{`, found keyword `unsafe`
+error: expected `fn`, found keyword `unsafe`
   --> $DIR/wrong-unsafe-abi.rs:9:12
    |
 LL | extern "C" unsafe fn test() {}
-   |            ^^^^^^ expected `{`
+   | -----------^^^^^^
+   | |          |
+   | |          expected `fn`
+   | help: `unsafe` must come before `extern "C"`: `unsafe extern "C"`
+   |
+   = note: keyword order for functions declaration is `pub`, `default`, `const`, `async`, `unsafe`, `extern`
 
 error: aborting due to 1 previous error