Make bootstrap command caching opt-in

It was too dangerous to cache by default, and was it causing real bugs.

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

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);