Allow building serde and serde_derive in parallel

This reduces build time of bootstrap by ~6s

src/bootstrap/Cargo.lock

@@ -50,6 +50,7 @@ dependencies = [
  "opener",
  "pretty_assertions",
  "serde",
+ "serde_derive",
  "serde_json",
  "sha2",
  "sysinfo",
@@ -564,9 +565,6 @@ name = "serde"
 version = "1.0.137"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "61ea8d54c77f8315140a05f4c7237403bf38b72704d031543aa1d16abbf517d1"
-dependencies = [
- "serde_derive",
-]
 
 [[package]]
 name = "serde_derive"

src/bootstrap/Cargo.toml

@@ -39,7 +39,10 @@ cc = "1.0.69"
 libc = "0.2"
 hex = "0.4"
 object = { version = "0.29.0", default-features = false, features = ["archive", "coff", "read_core", "unaligned"] }
-serde = { version = "1.0.8", features = ["derive"] }
+serde = "1.0.137"
+# Directly use serde_derive rather than through the derive feature of serde to allow building both
+# in parallel and to allow serde_json and toml to start building as soon as serde has been built.
+serde_derive = "1.0.137"
 serde_json = "1.0.2"
 sha2 = "0.10"
 tar = "0.4"

src/bootstrap/compile.rs

@@ -16,7 +16,7 @@ use std::path::{Path, PathBuf};
 use std::process::{Command, Stdio};
 use std::str;
 
-use serde::Deserialize;
+use serde_derive::Deserialize;
 
 use crate::builder::crate_description;
 use crate::builder::Cargo;

src/bootstrap/config.rs

@@ -25,6 +25,7 @@ use crate::flags::{Color, Flags};
 use crate::util::{exe, output, t};
 use once_cell::sync::OnceCell;
 use serde::{Deserialize, Deserializer};
+use serde_derive::Deserialize;
 
 macro_rules! check_ci_llvm {
     ($name:expr) => {

src/bootstrap/format.rs

@@ -87,7 +87,7 @@ fn get_modified_rs_files(build: &Builder<'_>) -> Result<Option<Vec<String>>, Str
     get_git_modified_files(Some(&build.config.src), &vec!["rs"])
 }
 
-#[derive(serde::Deserialize)]
+#[derive(serde_derive::Deserialize)]
 struct RustfmtConfig {
     ignore: Vec<String>,
 }

src/bootstrap/metadata.rs

@@ -1,7 +1,7 @@
 use std::path::PathBuf;
 use std::process::Command;
 
-use serde::Deserialize;
+use serde_derive::Deserialize;
 
 use crate::cache::INTERNER;
 use crate::util::output;

src/bootstrap/metrics.rs

@@ -7,7 +7,7 @@
 use crate::builder::Step;
 use crate::util::t;
 use crate::Build;
-use serde::{Deserialize, Serialize};
+use serde_derive::{Deserialize, Serialize};
 use std::cell::RefCell;
 use std::fs::File;
 use std::io::BufWriter;

src/bootstrap/toolstate.rs

@@ -1,6 +1,6 @@
 use crate::builder::{Builder, RunConfig, ShouldRun, Step};
 use crate::util::t;
-use serde::{Deserialize, Serialize};
+use serde_derive::{Deserialize, Serialize};
 use std::collections::HashMap;
 use std::env;
 use std::fmt;