bootstrap: show diagnostics relative to rustc src dir

src/bootstrap/bootstrap.py

@@ -1029,7 +1029,8 @@ def build_bootstrap_cmd(self, env):
             raise Exception("no cargo executable found at `{}`".format(
                 self.cargo()))
         args = [self.cargo(), "build", "--manifest-path",
-                os.path.join(self.rust_root, "src/bootstrap/Cargo.toml")]
+                os.path.join(self.rust_root, "src/bootstrap/Cargo.toml"),
+                "-Zroot-dir="+self.rust_root]
         args.extend("--verbose" for _ in range(self.verbose))
         if self.use_locked_deps:
             args.append("--locked")

src/bootstrap/src/core/builder/cargo.rs

@@ -703,6 +703,9 @@ impl Builder<'_> {
 
         cargo.arg("-j").arg(self.jobs().to_string());
 
+        // Make cargo emit diagnostics relative to the rustc src dir.
+        cargo.arg(format!("-Zroot-dir={}", self.src.display()));
+
         // FIXME: Temporary fix for https://github.com/rust-lang/cargo/issues/3005
         // Force cargo to output binaries with disambiguating hashes in the name
         let mut metadata = if compiler.stage == 0 {