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.

config.example.toml

@@ -300,6 +300,10 @@
 # This is only useful for verifying that rustc generates reproducible builds.
 #full-bootstrap = false
 
+# Set the bootstrap/download cache path. It is useful when building rust
+# repeatedly in a CI invironment.
+# bootstrap-cache-path = /shared/cache
+
 # Enable a build of the extended Rust tool set which is not only the compiler
 # but also tools such as Cargo. This will also produce "combined installers"
 # which are used to install Rust and Cargo together.

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));

src/bootstrap/src/utils/change_tracker.rs

@@ -136,4 +136,9 @@ pub const CONFIG_CHANGE_HISTORY: &[ChangeInfo] = &[
         severity: ChangeSeverity::Info,
         summary: "`x install` now skips providing tarball sources (under 'build/dist' path) to speed up the installation process.",
     },
+    ChangeInfo {
+        change_id: 121976,
+        severity: ChangeSeverity::Info,
+        summary: "A new `boostrap-cache-path` option has been introduced. Set it in your config.toml to use a different path for the download cache.",
+    },
 ];