nix: only use rust-src in devShell toolchain

See the included comment. This helps avoid a large dependency on the rustc
toolchain and helps reduce closure size for Nix users.

Partial revert of 749481e.

Signed-off-by: Austin Seipp <[email protected]>

flake.nix

@@ -34,7 +34,7 @@
  pkgs.lib.all (re: builtins.match re relPath == null) regexes;
  };
 
- ourRustVersion = pkgs.rust-bin.selectLatestNightlyWith (toolchain: toolchain.complete);
+ ourRustVersion = pkgs.rust-bin.selectLatestNightlyWith (toolchain: toolchain.default);
 
  ourRustPlatform = pkgs.makeRustPlatform {
  rustc = ourRustVersion;
@@ -147,7 +147,16 @@
 
  devShells.default = pkgs.mkShell {
  buildInputs = with pkgs; [
- ourRustVersion
+ # NOTE (aseipp): explicitly add rust-src to the rustc compiler only in
+ # devShell. this in turn causes a dependency on the rust compiler src,
+ # which bloats the closure size by several GiB. but doing this here
+ # and not by default avoids the default flake install from including
+ # that dependency, so it's worth it
+ #
+ # relevant PR: https://github.com/rust-lang/rust/pull/129687
+ (ourRustVersion.override {
+ extensions = [ "rust-src" ];
+ })
 
  # Foreign dependencies
  openssl zstd libgit2 libssh2