Review feedback

* Use a match statement.
* Clarify why we can't use `file_stem()`.
* Error if the `:` is missing for Tidy error codes, rather than no-oping.

compiler/rustc_mir/src/transform/coverage/debug.rs

@@ -152,13 +152,15 @@ impl DebugOptions {
                     None => (setting_str, None),
                     Some((k, v)) => (k, Some(v)),
                 };
-                if option == "allow_unused_expressions" {
+                match option {
+                    "allow_unused_expressions" => {
                         allow_unused_expressions = bool_option_val(option, value);
                         debug!(
                             "{} env option `allow_unused_expressions` is set to {}",
                             RUSTC_COVERAGE_DEBUG_OPTIONS, allow_unused_expressions
                         );
-                } else if option == "counter_format" {
+                    }
+                    "counter_format" => {
                         match value {
                             None => {
                                 bug!(
@@ -177,13 +179,15 @@ impl DebugOptions {
                                 );
                             }
                         };
-                } else {
+                    }
+                    _ => {
                         bug!(
                             "Unsupported setting `{}` in environment variable {}",
                             option,
                             RUSTC_COVERAGE_DEBUG_OPTIONS
                         )
                     }
+                };
             }
         }
 

src/tools/tidy/src/error_codes_check.rs

@@ -85,10 +85,16 @@ fn extract_error_codes(
     for line in f.lines() {
         let s = line.trim();
         if !reached_no_explanation && s.starts_with('E') && s.contains("include_str!(\"") {
-            let err_code = match s.split_once(':') {
-                None => continue,
-                Some((err_code, _)) => err_code.to_owned(),
-            };
+            let err_code = s
+                .split_once(':')
+                .expect(
+                    format!(
+                        "Expected a line with the format `E0xxx: include_str!(\"..\")`, but got {} without a `:` delimiter",
+                        s,
+                    ).as_str()
+                )
+                .0
+                .to_owned();
             if !error_codes.contains_key(&err_code) {
                 error_codes.insert(err_code.clone(), false);
             }

src/tools/tidy/src/ui_tests.rs

@@ -19,6 +19,9 @@ pub fn check(path: &Path, bad: &mut bool) {
                     //
                     // For now, just make sure that there is a corresponding
                     // `$testname.rs` file.
+                    //
+                    // NB: We do not use file_stem() as some file names have multiple `.`s and we
+                    // must strip all of them.
                     let testname =
                         file_path.file_name().unwrap().to_str().unwrap().split_once('.').unwrap().0;
                     if !file_path.with_file_name(testname).with_extension("rs").exists() {