make a help text for 'cannot find macro' actionable

Author: togami2864

compiler/rustc_resolve/src/diagnostics.rs

@@ -2441,10 +2441,9 @@ fn show_candidates(
 
             for candidate in accessible_path_strings {
                 msg.push('\n');
-                msg.push_str(&candidate.0);
+                msg.push_str(&format!("use {};", &candidate.0));
             }
-
-            err.note(&msg);
+            err.help(&msg);
         }
     } else if !matches!(mode, DiagnosticMode::Import) {
         assert!(!inaccessible_path_strings.is_empty());

src/test/ui/macros/issue-102601.rs

@@ -0,0 +1,13 @@
+mod hey {
+    pub use std::println as bitflags;
+}
+
+fn main() {
+    bitflags! { //~ ERROR cannot find macro `bitflags` in this scope
+        struct Flags: u32 {
+            const A = 0b00000001;
+        }
+    }
+}
+//~ HELP consider importing this macro:
+//       use hey::bitflags;

src/test/ui/macros/issue-102601.stderr

@@ -0,0 +1,11 @@
+error: cannot find macro `bitflags` in this scope
+  --> $DIR/issue-102601.rs:6:5
+   |
+LL |     bitflags! {
+   |     ^^^^^^^^
+   |
+   = help: consider importing this macro:
+           use hey::bitflags;
+
+error: aborting due to previous error
+