./x test miri: fix cleaning the miri_ui directory

Author: RalfJung

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

@@ -556,8 +556,13 @@ impl Step for Miri {
         // 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(miri.build_compiler, Mode::ToolStd).join("miri_ui");
+            // This has to match `CARGO_TARGET_TMPDIR` in Miri's `ui.rs`.
+            // This means we need `host` here as that's the target `ui.rs` is built for.
+            let ui_test_dep_dir = builder
+                .stage_out(miri.build_compiler, Mode::ToolStd)
+                .join(host)
+                .join("tmp")
+                .join("miri_ui");
             // The mtime of `miri_sysroot` changes when the sysroot gets rebuilt (also see
             // <https://github.com/RalfJung/rustc-build-sysroot/commit/10ebcf60b80fe2c3dc765af0ff19fdc0da4b7466>).
             // We can hence use that directly as a signal to clear the ui test dir.

src/tools/miri/tests/ui.rs

@@ -97,6 +97,8 @@ fn miri_config(
     let mut config = Config {
         target: Some(target.to_owned()),
         program,
+        // When changing this, remember to also adjust the logic in bootstrap, in Miri's test step,
+        // that deletes the `miri_ui` dir when it needs a rebuild.
         out_dir: PathBuf::from(env!("CARGO_TARGET_TMPDIR")).join("miri_ui"),
         threads: std::env::var("MIRI_TEST_THREADS")
             .ok()