Fix the issue of wrong diagnosis for extern pub fn

Author: chenyukang

compiler/rustc_parse/src/parser/item.rs

@@ -2182,7 +2182,11 @@ impl<'a> Parser<'a> {
             // `extern ABI fn`
             || self.check_keyword_case(kw::Extern, case)
                 && self.look_ahead(1, |t| t.can_begin_literal_maybe_minus())
-                && self.look_ahead(2, |t| t.is_keyword_case(kw::Fn, case))
+                && (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
+                    (self.may_recover()
+                        && self.look_ahead(2, |t| t.is_keyword(kw::Pub))
+                        && self.look_ahead(3, |t| t.is_keyword_case(kw::Fn, case))))
     }
 
     /// Parses all the "front matter" (or "qualifiers") for a `fn` declaration,

tests/ui/parser/issue-113342.rs

@@ -0,0 +1,9 @@
+#[link(name = "my_c_library")]
+extern "C" {
+    fn my_c_function(x: i32) -> bool;
+}
+
+#[no_mangle]
+extern "C" pub fn id(x: i32) -> i32 { x } //~ ERROR expected `fn`, found keyword `pub`
+
+fn main() {}

tests/ui/parser/issue-113342.stderr

@@ -0,0 +1,11 @@
+error: expected `fn`, found keyword `pub`
+  --> $DIR/issue-113342.rs:7:12
+   |
+LL | extern "C" pub fn id(x: i32) -> i32 { x }
+   | -----------^^^
+   | |          |
+   | |          expected `fn`
+   | help: visibility `pub` must come before `extern "C"`: `pub extern "C"`
+
+error: aborting due to previous error
+