Improve comments

src/bootstrap/metadata.rs

@@ -74,6 +74,8 @@ fn workspace_members(build: &Build) -> impl Iterator<Item = Package> {
     let collect_metadata = |manifest_path| {
         let mut cargo = Command::new(&build.initial_cargo);
         cargo
+            // Will read the libstd Cargo.toml
+            // which uses the unstable `public-dependency` feature.
             .env("RUSTC_BOOTSTRAP", "1")
             .arg("metadata")
             .arg("--format-version")

src/tools/rust-installer/combine-installers.sh

@@ -11,6 +11,8 @@ abs_path() {
     (unset CDPATH && cd "$path" > /dev/null && pwd)
 }
 
+# Running cargo will read the libstd Cargo.toml
+# which uses the unstable `public-dependency` feature.
 export RUSTC_BOOTSTRAP=1
 
 src_dir="$(abs_path $(dirname "$0"))"

src/tools/rust-installer/gen-installer.sh

@@ -11,6 +11,8 @@ abs_path() {
     (unset CDPATH && cd "$path" > /dev/null && pwd)
 }
 
+# Running cargo will read the libstd Cargo.toml
+# which uses the unstable `public-dependency` feature.
 export RUSTC_BOOTSTRAP=1
 
 src_dir="$(abs_path $(dirname "$0"))"

src/tools/rust-installer/make-tarballs.sh

@@ -11,6 +11,8 @@ abs_path() {
     (unset CDPATH && cd "$path" > /dev/null && pwd)
 }
 
+# Running cargo will read the libstd Cargo.toml
+# which uses the unstable `public-dependency` feature.
 export RUSTC_BOOTSTRAP=1
 
 src_dir="$(abs_path $(dirname "$0"))"

src/tools/tidy/src/main.rs

@@ -16,7 +16,9 @@ use std::sync::atomic::{AtomicBool, Ordering};
 use std::thread::{self, scope, ScopedJoinHandle};
 
 fn main() {
-    // Allow using unstable cargo features in the standard library.
+    // Running Cargo will read the libstd Cargo.toml
+    // which uses the unstable `public-dependency` feature.
+    //
     // `setenv` might not be thread safe, so run it before using multiple threads.
     env::set_var("RUSTC_BOOTSTRAP", "1");