perf(rust): avoid calling rustup in more conditions (starship#4174)

Author: davidkna

src/modules/rust.rs

@@ -55,9 +55,10 @@ impl RustToolingEnvironmentInfo {
         // To display appropriate versions preventing `rustc` from downloading toolchains, we have to
         // check
         // 1. `$RUSTUP_TOOLCHAIN`
-        // 2. `rustup override list`
+        // 2. The override list from ~/.rustup/settings.toml (like `rustup override list`)
         // 3. `rust-toolchain` or `rust-toolchain.toml` in `.` or parent directories
-        // 4. `rustup default`
+        // 4. The `default_toolchain` from ~/.rustup/settings.toml (like `rustup default`)
+        // 5. `rustup default` (in addition to the above, this also looks at global fallback config files)
         // as `rustup` does.
         // https://github.com/rust-lang/rustup.rs/tree/eb694fcada7becc5d9d160bf7c623abe84f8971d#override-precedence
         //
@@ -74,6 +75,11 @@ impl RustToolingEnvironmentInfo {
                             .lookup_override(context.current_dir.as_path())
                     })
                     .or_else(|| find_rust_toolchain_file(context))
+                    .or_else(|| {
+                        self.get_rustup_settings(context)
+                            .default_toolchain()
+                            .map(std::string::ToString::to_string)
+                    })
                     .or_else(|| execute_rustup_default(context));
 
                 log::debug!("Environmental toolchain override is {:?}", out);
@@ -87,12 +93,32 @@ impl RustToolingEnvironmentInfo {
     fn get_rustup_rustc_version(&self, context: &Context) -> &RustupRunRustcVersionOutcome {
         self.rustup_rustc_output.get_or_init(|| {
             let out = if let Some(toolchain) = self.get_env_toolchain_override(context) {
-                create_command("rustup")
-                    .and_then(|mut cmd| {
-                        cmd.args(&["run", toolchain, "rustc", "--version"])
-                            .current_dir(&context.current_dir)
-                            .output()
+                // First try runnig ~/.rustup/toolchains/<toolchain>/bin/rustc --version
+                rustup_home()
+                    .map(|rustup_folder| {
+                        rustup_folder
+                            .join("toolchains")
+                            .join(toolchain)
+                            .join("bin")
+                            .join("rustc")
+                    })
+                    .and_then(|rustc| {
+                        log::trace!("Running rustc --version directly with {:?}", rustc);
+                        create_command(rustc).map(|mut cmd| {
+                            cmd.arg("--version");
+                            cmd
+                        })
+                    })
+                    .or_else(|_| {
+                        // If that fails, try running rustup rustup run <toolchain> rustc --version
+                        // Depending on the source of the toolchain override, it might not have been a full toolchain name ("stable" or "nightly").
+                        log::trace!("Running rustup {toolchain} rustc --version");
+                        create_command("rustup").map(|mut cmd| {
+                            cmd.args(&["run", toolchain, "rustc", "--version"]);
+                            cmd
+                        })
                     })
+                    .and_then(|mut cmd| cmd.current_dir(&context.current_dir).output())
                     .map(extract_toolchain_from_rustup_run_rustc_version)
                     .unwrap_or(RustupRunRustcVersionOutcome::RustupNotWorking)
             } else {