bootstrap: clear miri ui-test deps when miri sysroot gets rebuilt

Author: RalfJung

src/bootstrap/src/core/build_steps/test.rs

@@ -434,7 +434,7 @@ impl Miri {
         builder: &Builder<'_>,
         compiler: Compiler,
         target: TargetSelection,
-    ) -> String {
+    ) -> PathBuf {
         let miri_sysroot = builder.out.join(compiler.host.triple).join("miri-sysroot");
         let mut cargo = builder::Cargo::new(
             builder,
@@ -467,7 +467,7 @@ impl Miri {
         // Output is "<sysroot>\n".
         let sysroot = stdout.trim_end();
         builder.verbose(|| println!("`cargo miri setup --print-sysroot` said: {sysroot:?}"));
-        sysroot.to_owned()
+        PathBuf::from(sysroot)
     }
 }
 
@@ -520,12 +520,14 @@ impl Step for Miri {
         builder.ensure(compile::Std::new(target_compiler, host));
         let host_sysroot = builder.sysroot(target_compiler);
 
-        // Miri has its own "target dir" for ui test dependencies. Make sure it gets cleared
-        // properly when rustc changes. Similar to `Builder::cargo`, we skip this in dry runs to
-        // make sure the relevant compiler has been set up properly.
+        // Miri has its own "target dir" for ui test dependencies. Make sure it gets cleared when
+        // the sysroot gets rebuilt, to avoid "found possibly newer version of crate `std`" errors.
         if !builder.config.dry_run() {
             let ui_test_dep_dir = builder.stage_out(host_compiler, Mode::ToolStd).join("miri_ui");
-            builder.clear_if_dirty(&ui_test_dep_dir, &builder.rustc(host_compiler));
+            // It currently seems to be the case that the mtime of `miri_sysroot` changes when
+            // the sysroot gets rebuilt. If that ever stops happening, we'll have to make
+            // Miri explicitly update the mtime so that the logic here keeps working.
+            builder.clear_if_dirty(&ui_test_dep_dir, &miri_sysroot);
         }
 
         // Run `cargo test`.