Don't default to `submodules = true` unless the rust repo has a .git directory

src/bootstrap/config.rs

@@ -13,6 +13,7 @@
 use std::str::FromStr;
 
 use crate::cache::{Interned, INTERNER};
+use crate::channel::GitInfo;
 pub use crate::flags::Subcommand;
 use crate::flags::{Color, Flags};
 use crate::util::exe;
@@ -48,7 +49,7 @@ pub struct Config {
     /// Call Build::ninja() instead of this.
     pub ninja_in_file: bool,
     pub verbose: usize,
-    pub submodules: bool,
+    pub submodules: Option<bool>,
     pub fast_submodules: bool,
     pub compiler_docs: bool,
     pub docs_minification: bool,
@@ -552,7 +553,7 @@ pub fn default_opts() -> Config {
         config.backtrace = true;
         config.rust_optimize = true;
         config.rust_optimize_tests = true;
-        config.submodules = true;
+        config.submodules = None;
         config.fast_submodules = true;
         config.docs = true;
         config.docs_minification = true;
@@ -658,11 +659,11 @@ pub fn parse(args: &[String]) -> Config {
         config.npm = build.npm.map(PathBuf::from);
         config.gdb = build.gdb.map(PathBuf::from);
         config.python = build.python.map(PathBuf::from);
+        config.submodules = build.submodules;
         set(&mut config.low_priority, build.low_priority);
         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.submodules, build.submodules);
         set(&mut config.fast_submodules, build.fast_submodules);
         set(&mut config.locked_deps, build.locked_deps);
         set(&mut config.vendor, build.vendor);
@@ -1075,6 +1076,10 @@ pub fn any_profiler_enabled(&self) -> bool {
     pub fn llvm_enabled(&self) -> bool {
         self.rust_codegen_backends.contains(&INTERNER.intern_str("llvm"))
     }
+
+    pub fn submodules(&self, rust_info: &GitInfo) -> bool {
+        self.submodules.unwrap_or(rust_info.is_git())
+    }
 }
 
 fn set<T>(field: &mut T, val: Option<T>) {

src/bootstrap/lib.rs

@@ -486,7 +486,7 @@ fn dir_is_empty(dir: &Path) -> bool {
             t!(std::fs::read_dir(dir)).next().is_none()
         }
 
-        if !self.config.submodules {
+        if !self.config.submodules(&self.rust_info) {
             return;
         }
 
@@ -562,7 +562,7 @@ pub fn maybe_update_submodules(&self) {
             "library/stdarch",
         ];
         // Avoid running git when there isn't a git checkout.
-        if !self.config.submodules {
+        if !self.config.submodules(&self.rust_info) {
             return;
         }
         let output = output(