improve the way bootstrap handles rustlib components

When CI rustc is enabled, bootstrap tries to symlink the rust source (project root)
into target sysroot right before copying it from the CI rustc's sysroot. This becomes
a problem in CI builders (which we currently don't see because they don't use CI rustc)
because the copying part will fail as they run on read-only mode.

This change fixes the problem by copying `rustc-src` from the CI rustc sysroot and only symlinking
`rustc-src` from the rust source when download-rustc is not enabled.

Signed-off-by: onur-ozkan <work@onurozkan.dev>

src/bootstrap/src/core/build_steps/compile.rs

@@ -161,9 +161,10 @@ impl Step for Std {
             // This check is specific to testing std itself; see `test::Std` for more details.
             && !self.force_recompile
         {
+            let sysroot = builder.ensure(Sysroot { compiler, force_recompile: false });
             cp_rustc_component_to_ci_sysroot(
                 builder,
-                compiler,
+                &sysroot,
                 builder.config.ci_rust_std_contents(),
             );
             return;
@@ -797,12 +798,7 @@ impl Step for StartupObjects {
     }
 }
 
-fn cp_rustc_component_to_ci_sysroot(
-    builder: &Builder<'_>,
-    compiler: Compiler,
-    contents: Vec<String>,
-) {
-    let sysroot = builder.ensure(Sysroot { compiler, force_recompile: false });
+fn cp_rustc_component_to_ci_sysroot(builder: &Builder<'_>, sysroot: &Path, contents: Vec<String>) {
     let ci_rustc_dir = builder.config.ci_rustc_dir();
 
     for file in contents {
@@ -881,13 +877,7 @@ impl Step for Rustc {
         // NOTE: the ABI of the beta compiler is different from the ABI of the downloaded compiler,
         // so its artifacts can't be reused.
         if builder.download_rustc() && compiler.stage != 0 {
-            // Copy the existing artifacts instead of rebuilding them.
-            // NOTE: this path is only taken for tools linking to rustc-dev (including ui-fulldeps tests).
-            cp_rustc_component_to_ci_sysroot(
-                builder,
-                compiler,
-                builder.config.ci_rustc_dev_contents(),
-            );
+            builder.ensure(Sysroot { compiler, force_recompile: false });
             return compiler.stage;
         }
 
@@ -1646,19 +1636,29 @@ impl Step for Sysroot {
                 );
             }
         }
-        // Same for the rustc-src component.
-        let sysroot_lib_rustlib_rustcsrc = sysroot.join("lib/rustlib/rustc-src");
-        t!(fs::create_dir_all(&sysroot_lib_rustlib_rustcsrc));
-        let sysroot_lib_rustlib_rustcsrc_rust = sysroot_lib_rustlib_rustcsrc.join("rust");
-        if let Err(e) =
-            symlink_dir(&builder.config, &builder.src, &sysroot_lib_rustlib_rustcsrc_rust)
-        {
-            eprintln!(
-                "WARNING: creating symbolic link `{}` to `{}` failed with {}",
-                sysroot_lib_rustlib_rustcsrc_rust.display(),
-                builder.src.display(),
-                e,
+
+        // Unlike rust-src component, we have to handle rustc-src a bit differently.
+        // When using CI rustc, we copy rustc-src component from its sysroot,
+        // otherwise we handle it in a similar way what we do for rust-src above.
+        if builder.download_rustc() {
+            cp_rustc_component_to_ci_sysroot(
+                builder,
+                &sysroot,
+                builder.config.ci_rustc_dev_contents(),
             );
+        } else {
+            let sysroot_lib_rustlib_rustcsrc = sysroot.join("lib/rustlib/rustc-src");
+            t!(fs::create_dir_all(&sysroot_lib_rustlib_rustcsrc));
+            let sysroot_lib_rustlib_rustcsrc_rust = sysroot_lib_rustlib_rustcsrc.join("rust");
+            if let Err(e) =
+                symlink_dir(&builder.config, &builder.src, &sysroot_lib_rustlib_rustcsrc_rust)
+            {
+                eprintln!(
+                "WARNING: creating symbolic link `{}` to `{}` failed with {}",
+                    sysroot_lib_rustlib_rustcsrc_rust.display(),
+                    builder.src.display(),
+                    e,
+                );
         }
 
         sysroot