Auto merge of #43059 - Mark-Simulacrum:rustbuild-2.0, r=alexcrichton

Rework Rustbuild to an eagerly compiling approach

This introduces a new dependency on `serde`; I don't believe that's a problem since bootstrap is compiled with nightly/beta always so proc macros are available. Compile times are slightly longer -- about 2-3x (30 seconds vs. 10 seconds). I don't think this is too big a problem, especially since recompiling bootstrap is somewhat rare. I think we can remove the dependency on Serde if necessary, though, so let me know.

r? @alexcrichton

Author: bors

src/Cargo.lock

@@ -33,8 +33,11 @@ build_helper = { path = "../build_helper" }
 cmake = "0.1.23"
 filetime = "0.1"
 num_cpus = "1.0"
-toml = "0.1"
 getopts = "0.2"
-rustc-serialize = "0.3"
 gcc = "0.3.50"
 libc = "0.2"
+serde = "1.0.8"
+serde_derive = "1.0.8"
+serde_json = "1.0.2"
+toml = "0.4"
+lazy_static = "0.2"