Turn on fast_submodules unconditionally

I don't know why anyone would turn this off; doing so makes builds much slower (nearly a 60x slowdown according to #49057).
Remove the option to do so, which makes bootstrap a little easier to maintain.

Bootstrap continues to allow you to manage submodules manually by setting `submodules = false`.

config.toml.example

@@ -240,10 +240,6 @@ changelog-seen = 2
 # Indicate whether git submodules are managed and updated automatically.
 #submodules = true
 
-# Update git submodules only when the checked out commit in the submodules differs
-# from what is committed in the main rustc repo.
-#fast-submodules = true
-
 # The path to (or name of) the GDB executable to use. This is only used for
 # executing the debuginfo test suite.
 #gdb = "gdb"

src/bootstrap/CHANGELOG.md

@@ -11,6 +11,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 - The options `infodir`, `localstatedir`, and `gpg-password-file` are no longer allowed in config.toml. Previously, they were ignored without warning. Note that `infodir` and `localstatedir` are still accepted by `./configure`, with a warning. [#82451](https://github.com/rust-lang/rust/pull/82451)
 - Add options for enabling overflow checks, one for std (`overflow-checks-std`) and one for everything else (`overflow-checks`). Both default to false.
 - Change the names for `dist` commands to match the component they generate. [#90684](https://github.com/rust-lang/rust/pull/90684)
+- The `build.fast-submodules` option has been removed. Fast submodule checkouts are enabled unconditionally. Automatic submodule handling can still be disabled with `build.submodules = false`.
 
 ### Non-breaking changes
 

src/bootstrap/bootstrap.py

@@ -926,19 +926,15 @@ class RustBuild(object):
             return config
         return default_build_triple(self.verbose)
 
-    def check_submodule(self, module, slow_submodules):
-        if not slow_submodules:
+    def check_submodule(self, module):
         checked_out = subprocess.Popen(["git", "rev-parse", "HEAD"],
                                         cwd=os.path.join(self.rust_root, module),
                                         stdout=subprocess.PIPE)
         return checked_out
-        else:
-            return None
 
     def update_submodule(self, module, checked_out, recorded_submodules):
         module_path = os.path.join(self.rust_root, module)
 
-        if checked_out is not None:
         default_encoding = sys.getdefaultencoding()
         checked_out = checked_out.communicate()[0].decode(default_encoding).strip()
         if recorded_submodules[module] == checked_out:
@@ -991,11 +987,7 @@ class RustBuild(object):
         git_version_str = require(['git', '--version']).split()[2].decode(default_encoding)
         self.git_version = distutils.version.LooseVersion(git_version_str)
 
-        slow_submodules = self.get_toml('fast-submodules') == "false"
         start_time = time()
-        if slow_submodules:
-            print('Unconditionally updating submodules')
-        else:
         print('Updating only changed submodules')
         default_encoding = sys.getdefaultencoding()
         # Only update submodules that are needed to build bootstrap.  These are needed because Cargo
@@ -1022,7 +1014,7 @@ class RustBuild(object):
         filtered_submodules = []
         submodules_names = []
         for module in submodules:
-            check = self.check_submodule(module, slow_submodules)
+            check = self.check_submodule(module)
             filtered_submodules.append((module, check))
             submodules_names.append(module)
         recorded = subprocess.Popen(["git", "ls-tree", "HEAD"] + submodules_names,

src/bootstrap/config.rs

@@ -50,7 +50,6 @@ pub struct Config {
     pub ninja_in_file: bool,
     pub verbose: usize,
     pub submodules: Option<bool>,
-    pub fast_submodules: bool,
     pub compiler_docs: bool,
     pub docs_minification: bool,
     pub docs: bool,
@@ -517,7 +516,6 @@ define_config! {
         compiler_docs: Option<bool> = "compiler-docs",
         docs_minification: Option<bool> = "docs-minification",
         submodules: Option<bool> = "submodules",
-        fast_submodules: Option<bool> = "fast-submodules",
         gdb: Option<String> = "gdb",
         nodejs: Option<String> = "nodejs",
         npm: Option<String> = "npm",
@@ -705,7 +703,6 @@ impl Config {
         config.rust_optimize = true;
         config.rust_optimize_tests = true;
         config.submodules = None;
-        config.fast_submodules = true;
         config.docs = true;
         config.docs_minification = true;
         config.rust_rpath = true;
@@ -847,7 +844,6 @@ impl Config {
         set(&mut config.compiler_docs, build.compiler_docs);
         set(&mut config.docs_minification, build.docs_minification);
         set(&mut config.docs, build.docs);
-        set(&mut config.fast_submodules, build.fast_submodules);
         set(&mut config.locked_deps, build.locked_deps);
         set(&mut config.vendor, build.vendor);
         set(&mut config.full_bootstrap, build.full_bootstrap);

src/bootstrap/lib.rs

@@ -556,10 +556,8 @@ impl Build {
         }
 
         // check_submodule
-        if self.config.fast_submodules {
-            let checked_out_hash = output(
-                Command::new("git").args(&["rev-parse", "HEAD"]).current_dir(&absolute_path),
-            );
+        let checked_out_hash =
+            output(Command::new("git").args(&["rev-parse", "HEAD"]).current_dir(&absolute_path));
         // update_submodules
         let recorded = output(
             Command::new("git")
@@ -577,7 +575,6 @@ impl Build {
             // already checked out
             return;
         }
-        }
 
         println!("Updating submodule {}", relative_path.display());
         self.run(