Add a build option to specify the bootstrap cache

Setting the bootstrap cache path to an external location can help to
speed up builds in cases where the build directory is not kept between
builds, e.g. in CI or other automated build systems.

Author: lu-zero

src/bootstrap/bootstrap.py

@@ -557,7 +557,9 @@ def download_toolchain(self):
                 shutil.rmtree(bin_root)
 
             key = self.stage0_compiler.date
-            cache_dst = os.getenv('RUSTC_BOOTSTRAP_CACHE', os.path.join(self.build_dir, "cache"))
+            cache_dst = (self.get_toml('bootstrap-cache-path', 'build') or
+                os.path.join(self.build_dir, "cache"))
+
             rustc_cache = os.path.join(cache_dst, key)
             if not os.path.exists(rustc_cache):
                 os.makedirs(rustc_cache)

src/bootstrap/configure.py

@@ -149,6 +149,7 @@ def v(*args):
 # (others are conditionally saved).
 o("manage-submodules", "build.submodules", "let the build manage the git submodules")
 o("full-bootstrap", "build.full-bootstrap", "build three compilers instead of two (not recommended except for testing reproducible builds)")
+o("bootstrap-cache-path", "build.bootstrap-cache-path", "use provided path for the bootstrap cache")
 o("extended", "build.extended", "build an extended rust tool set")
 
 v("tools", None, "List of extended tools will be installed")

src/bootstrap/src/core/config/config.rs

@@ -161,6 +161,7 @@ pub struct Config {
     pub vendor: bool,
     pub target_config: HashMap<TargetSelection, Target>,
     pub full_bootstrap: bool,
+    pub bootstrap_cache_path: Option<PathBuf>,
     pub extended: bool,
     pub tools: Option<HashSet<String>>,
     pub sanitizers: bool,
@@ -827,6 +828,7 @@ define_config! {
         locked_deps: Option<bool> = "locked-deps",
         vendor: Option<bool> = "vendor",
         full_bootstrap: Option<bool> = "full-bootstrap",
+        bootstrap_cache_path: Option<PathBuf> = "bootstrap-cache-path",
         extended: Option<bool> = "extended",
         tools: Option<HashSet<String>> = "tools",
         verbose: Option<usize> = "verbose",
@@ -1389,6 +1391,7 @@ impl Config {
             locked_deps,
             vendor,
             full_bootstrap,
+            bootstrap_cache_path,
             extended,
             tools,
             verbose,
@@ -1477,6 +1480,7 @@ impl Config {
         config.reuse = reuse.map(PathBuf::from);
         config.submodules = submodules;
         config.android_ndk = android_ndk;
+        config.bootstrap_cache_path = bootstrap_cache_path;
         set(&mut config.low_priority, low_priority);
         set(&mut config.compiler_docs, compiler_docs);
         set(&mut config.library_docs_private_items, library_docs_private_items);

src/bootstrap/src/core/download.rs

@@ -578,10 +578,8 @@ impl Config {
             return;
         }
 
-        let cache_dst = match env::var_os("RUSTC_BOOTSTRAP_CACHE") {
-            Some(v) => PathBuf::from(v),
-            None => self.out.join("cache"),
-        };
+        let cache_dst =
+            self.bootstrap_cache_path.as_ref().cloned().unwrap_or_else(|| self.out.join("cache"));
 
         let cache_dir = cache_dst.join(key);
         if !cache_dir.exists() {
@@ -709,10 +707,9 @@ download-rustc = false
         let llvm_assertions = self.llvm_assertions;
 
         let cache_prefix = format!("llvm-{llvm_sha}-{llvm_assertions}");
-        let cache_dst = match env::var_os("RUSTC_BOOTSTRAP_CACHE") {
-            Some(v) => PathBuf::from(v),
-            None => self.out.join("cache"),
-        };
+        let cache_dst =
+            self.bootstrap_cache_path.as_ref().cloned().unwrap_or_else(|| self.out.join("cache"));
+
         let rustc_cache = cache_dst.join(cache_prefix);
         if !rustc_cache.exists() {
             t!(fs::create_dir_all(&rustc_cache));