Make bootstrap command caching opt-in

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

@@ -2022,6 +2022,7 @@ impl Step for Assemble {
 
                 let host_llvm_bin_dir = command(&host_llvm_config)
                     .arg("--bindir")
+                    .cached()
                     .run_capture_stdout(builder)
                     .stdout()
                     .trim()

src/bootstrap/src/core/build_steps/dist.rs

@@ -2269,6 +2269,7 @@ fn maybe_install_llvm(
     {
         trace!("LLVM already built, installing LLVM files");
         let mut cmd = command(host_llvm_config);
+        cmd.cached();
         cmd.arg("--libfiles");
         builder.verbose(|| println!("running {cmd:?}"));
         let files = cmd.run_capture_stdout(builder).stdout();

src/bootstrap/src/core/build_steps/llvm.rs

@@ -486,8 +486,11 @@ impl Step for Llvm {
             let LlvmResult { host_llvm_config, .. } =
                 builder.ensure(Llvm { target: builder.config.host_target });
             if !builder.config.dry_run() {
-                let llvm_bindir =
-                    command(&host_llvm_config).arg("--bindir").run_capture_stdout(builder).stdout();
+                let llvm_bindir = command(&host_llvm_config)
+                    .arg("--bindir")
+                    .cached()
+                    .run_capture_stdout(builder)
+                    .stdout();
                 let host_bin = Path::new(llvm_bindir.trim());
                 cfg.define(
                     "LLVM_TABLEGEN",
@@ -593,7 +596,13 @@ impl Step for Llvm {
 }
 
 pub fn get_llvm_version(builder: &Builder<'_>, llvm_config: &Path) -> String {
-    command(llvm_config).arg("--version").run_capture_stdout(builder).stdout().trim().to_owned()
+    command(llvm_config)
+        .arg("--version")
+        .cached()
+        .run_capture_stdout(builder)
+        .stdout()
+        .trim()
+        .to_owned()
 }
 
 pub fn get_llvm_version_major(builder: &Builder<'_>, llvm_config: &Path) -> u8 {

src/bootstrap/src/core/build_steps/test.rs

@@ -2043,6 +2043,7 @@ HELP: You can add it into `bootstrap.toml` in `rust.codegen-backends = [{name:?}
             if !builder.config.dry_run() {
                 let llvm_version = get_llvm_version(builder, &host_llvm_config);
                 let llvm_components = command(&host_llvm_config)
+                    .cached()
                     .arg("--components")
                     .run_capture_stdout(builder)
                     .stdout();
@@ -2062,8 +2063,11 @@ HELP: You can add it into `bootstrap.toml` in `rust.codegen-backends = [{name:?}
             // separate compilations. We can add LLVM's library path to the
             // rustc args as a workaround.
             if !builder.config.dry_run() && suite.ends_with("fulldeps") {
-                let llvm_libdir =
-                    command(&host_llvm_config).arg("--libdir").run_capture_stdout(builder).stdout();
+                let llvm_libdir = command(&host_llvm_config)
+                    .cached()
+                    .arg("--libdir")
+                    .run_capture_stdout(builder)
+                    .stdout();
                 let link_llvm = if target.is_msvc() {
                     format!("-Clink-arg=-LIBPATH:{llvm_libdir}")
                 } else {

src/bootstrap/src/core/build_steps/tool.rs

@@ -1581,10 +1581,6 @@ impl Builder<'_> {
     /// `host`.
     pub fn tool_cmd(&self, tool: Tool) -> BootstrapCommand {
         let mut cmd = command(self.tool_exe(tool));
-
-        // Do not cache tool invocations, as they can have side effects
-        cmd.do_not_cache();
-
         let compiler = self.compiler(0, self.config.host_target);
         let host = &compiler.host;
         // Prepares the `cmd` provided to be able to run the `compiler` provided.

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

@@ -132,10 +132,7 @@ impl Cargo {
     }
 
     pub fn into_cmd(self) -> BootstrapCommand {
-        let mut cmd: BootstrapCommand = self.into();
-        // Disable caching for commands originating from Cargo-related operations.
-        cmd.do_not_cache();
-        cmd
+        self.into()
     }
 
     /// Same as [`Cargo::new`] except this one doesn't configure the linker with
@@ -1085,7 +1082,7 @@ impl Builder<'_> {
             && let Some(llvm_config) = self.llvm_config(target)
         {
             let llvm_libdir =
-                command(llvm_config).arg("--libdir").run_capture_stdout(self).stdout();
+                command(llvm_config).cached().arg("--libdir").run_capture_stdout(self).stdout();
             if target.is_msvc() {
                 rustflags.arg(&format!("-Clink-arg=-LIBPATH:{llvm_libdir}"));
             } else {

src/bootstrap/src/utils/exec.rs

@@ -264,8 +264,11 @@ impl<'a> BootstrapCommand {
         self
     }
 
-    pub fn do_not_cache(&mut self) -> &mut Self {
-        self.should_cache = false;
+    /// Cache the command. If it will be executed multiple times with the exact same arguments
+    /// and environment variables in the same bootstrap invocation, the previous result will be
+    /// loaded from memory.
+    pub fn cached(&mut self) -> &mut Self {
+        self.should_cache = true;
         self
     }
 
@@ -425,7 +428,7 @@ impl From<Command> for BootstrapCommand {
     fn from(command: Command) -> Self {
         let program = command.get_program().to_owned();
         Self {
-            should_cache: true,
+            should_cache: false,
             command,
             failure_behavior: BehaviorOnFailure::Exit,
             run_in_dry_run: false,

src/bootstrap/src/utils/helpers.rs

@@ -510,6 +510,8 @@ pub fn check_cfg_arg(name: &str, values: Option<&[&str]>) -> String {
 #[track_caller]
 pub fn git(source_dir: Option<&Path>) -> BootstrapCommand {
     let mut git = command("git");
+    // git commands are almost always read-only, so cache them by default
+    git.cached();
 
     if let Some(source_dir) = source_dir {
         git.current_dir(source_dir);