normalize download-rustc's prefix when running compiletests

Author: pietroalbini

src/bootstrap/config.rs

@@ -1506,19 +1506,25 @@ impl Config {
 
     /// Return whether we will use a downloaded, pre-compiled version of rustc, or just build from source.
     pub(crate) fn download_rustc(&self) -> bool {
-        static DOWNLOAD_RUSTC: OnceCell<bool> = OnceCell::new();
+        self.download_rustc_commit().is_some()
+    }
+
+    pub(crate) fn download_rustc_commit(&self) -> Option<&'static str> {
+        static DOWNLOAD_RUSTC: OnceCell<Option<String>> = OnceCell::new();
         if self.dry_run() && DOWNLOAD_RUSTC.get().is_none() {
             // avoid trying to actually download the commit
-            return false;
+            return None;
         }
 
-        *DOWNLOAD_RUSTC.get_or_init(|| match &self.download_rustc_commit {
-            None => false,
-            Some(commit) => {
-                self.download_ci_rustc(commit);
-                true
-            }
-        })
+        DOWNLOAD_RUSTC
+            .get_or_init(|| match &self.download_rustc_commit {
+                None => None,
+                Some(commit) => {
+                    self.download_ci_rustc(commit);
+                    Some(commit.clone())
+                }
+            })
+            .as_deref()
     }
 
     pub(crate) fn initial_rustfmt(&self) -> Option<PathBuf> {

src/bootstrap/test.rs

@@ -1671,6 +1671,10 @@ note: if you're sure you want to do this, please open an issue as to why. In the
 
         cmd.arg("--channel").arg(&builder.config.channel);
 
+        if let Some(commit) = builder.config.download_rustc_commit() {
+            cmd.env("FAKE_DOWNLOAD_RUSTC_PREFIX", format!("/rustc/{commit}"));
+        }
+
         builder.ci_env.force_coloring_in_ci(&mut cmd);
 
         builder.info(&format!(

src/tools/compiletest/src/runtest.rs

@@ -3536,6 +3536,8 @@ impl<'test> TestCx<'test> {
             Some(self.config.sysroot_base.join("lib").join("rustlib").join("src").join("rust")),
             // Virtual `/rustc/$sha` remapped paths (if `remap-debuginfo` is enabled):
             option_env!("CFG_VIRTUAL_RUST_SOURCE_BASE_DIR").map(PathBuf::from),
+            // Virtual `/rustc/$sha` coming from download-rustc:
+            std::env::var_os("FAKE_DOWNLOAD_RUSTC_PREFIX").map(PathBuf::from),
         ];
         for base_dir in source_bases {
             if let Some(base_dir) = base_dir {