Tidy: show todo reason when lint fails (and fix the lint's tidy allow statement which was weird and broken...)

src/tools/tidy/Readme.md

@@ -103,7 +103,8 @@ fn eee() {}
 fn z() {}
 // tidy-alphabetical-end
 ```
-<!--ignore-tidy-todo-->While not exactly a tidy directive, // TODO will fail tidy and make sure you can't merge a PR with unfinished work.
+<!-- ignore-tidy-todo -->
+While not exactly a tidy directive, // TODO will fail tidy and make sure you can't merge a PR with unfinished work.
 
 ### Test Specific Directives
 

src/tools/tidy/src/style.rs

@@ -226,9 +226,9 @@ fn long_line_is_ok(extension: &str, is_error_code: bool, max_columns: usize, lin
 }
 
 macro_rules! suppressible_tidy_err {
-    ($err:ident, $skip:expr, $msg:literal) => {
+    ($err:ident, $skip:expr, $msg:literal $($args: tt)*) => {
         if let Err(()) = $skip.check() {
-            $err(&format!($msg));
+            $err(&format!($msg $($args)*));
         }
     };
 }
@@ -455,10 +455,22 @@ fn check_file_style(base_path: &Path, check: &mut RunningCheck, file: &Path, con
             })
             && filename != "tests.rs"
         {
+            let without_macro_call =
+                trimmed.split_once("todo!").expect("todo in line because of previous check").1;
+            let without_start =
+                without_macro_call.split_once("(").map_or(without_macro_call, |(_, s)| s);
+            let without_end =
+                without_start.rsplit_once(")").map_or(without_start, |(s, _)| s).trim();
+            let message = if without_end.is_empty() {
+                format_args!("")
+            } else {
+                format_args!("\n >> TODO: {}", without_end)
+            };
+
             suppressible_tidy_err!(
                 err,
                 ignore.todo,
-                "the `todo!` macro is used for tasks that should be done before merging a PR. If you want to panic here, use `panic!`, `unimplemented!`, `unreachable!`, `rustc_middle::bug!` or an assertion"
+                "the `todo!` macro is used for tasks that should be done before merging a PR.\nIf you want to panic here, use `panic!`, `unimplemented!`, `unreachable!`, `rustc_middle::bug!` or an assertion{message}"
             )
         }
 
@@ -508,11 +520,20 @@ fn check_file_style(base_path: &Path, check: &mut RunningCheck, file: &Path, con
             if contains_potential_directive && (!has_recognized_directive) {
                 err("Unrecognized tidy directive")
             }
-            // Allow using TODO in diagnostic suggestions by marking the
-            // relevant line with `ignore-tidy-todo`.
-            if trimmed.contains("TODO") && !trimmed.contains("ignore-tidy-todo") {
-                err(
-                    "TODO is used for tasks that should be done before merging a PR; If you want to leave a message in the codebase use FIXME",
+            if trimmed.contains("TODO") {
+                let without_todo =
+                    trimmed.split_once("TODO").expect("TODO in line because of previous check").1;
+                let without_colon = without_todo.trim().trim_start_matches(":").trim();
+
+                let message = if without_colon.is_empty() {
+                    format_args!("")
+                } else {
+                    format_args!("\n >> TODO: {}", without_colon)
+                };
+                suppressible_tidy_err!(
+                    err,
+                    ignore.todo,
+                    "TODO is used for tasks that should be done before merging a PR;\nIf you want to leave a message in the codebase use FIXME{message}",
                 )
             }
             if trimmed.contains("//") && trimmed.contains(" XXX") {

src/tools/tidy/src/style/tests.rs

@@ -79,7 +79,7 @@ todo!()
         ),
         vec![
             "/test.rs:1:  ignoring todo usage unnecessarily".to_string(),
-            "/test.rs:3: the `todo!` macro is used for tasks that should be done before merging a PR. If you want to panic here, use `panic!`, `unimplemented!`, `unreachable!`, `rustc_middle::bug!` or an assertion".to_string()
+            "/test.rs:3: the `todo!` macro is used for tasks that should be done before merging a PR.\nIf you want to panic here, use `panic!`, `unimplemented!`, `unreachable!`, `rustc_middle::bug!` or an assertion".to_string()
         ]
     );
 }