coverage: Give each coverage test mode a separate output directory

When multiple test modes share the same test directory, this will allow them to
avoid interfering with each others' files and timestamps.

src/tools/compiletest/src/common.rs

@@ -78,14 +78,23 @@ impl Default for Mode {
 }
 
 impl Mode {
-    pub fn disambiguator(self) -> &'static str {
+    pub fn aux_dir_disambiguator(self) -> &'static str {
         // Pretty-printing tests could run concurrently, and if they do,
         // they need to keep their output segregated.
         match self {
             Pretty => ".pretty",
             _ => "",
         }
     }
+
+    pub fn output_dir_disambiguator(self) -> &'static str {
+        // Coverage tests use the same test files for multiple test modes,
+        // so each mode should have a separate output directory.
+        match self {
+            CoverageMap | RunCoverage => self.to_str(),
+            _ => "",
+        }
+    }
 }
 
 string_enum! {
@@ -699,6 +708,7 @@ pub fn output_testname_unique(
     let mode = config.compare_mode.as_ref().map_or("", |m| m.to_str());
     let debugger = config.debugger.as_ref().map_or("", |m| m.to_str());
     PathBuf::from(&testpaths.file.file_stem().unwrap())
+        .with_extra_extension(config.mode.output_dir_disambiguator())
         .with_extra_extension(revision.unwrap_or(""))
         .with_extra_extension(mode)
         .with_extra_extension(debugger)

src/tools/compiletest/src/runtest.rs

@@ -2720,7 +2720,7 @@ impl<'test> TestCx<'test> {
     fn aux_output_dir_name(&self) -> PathBuf {
         self.output_base_dir()
             .join("auxiliary")
-            .with_extra_extension(self.config.mode.disambiguator())
+            .with_extra_extension(self.config.mode.aux_dir_disambiguator())
     }
 
     /// Generates a unique name for the test, such as `testname.revision.mode`.