Force deterministic rustc versioning

This changes our shim such that `rustc -vV` lies to Cargo, giving it canonical
output independent of the toolchain we're using.

Notably, this output feeds into the -Cmetadata hash and ultimately that affects
both incremental hashing (and as such what query order) and codegen unit
partitioning. This should make both more deterministic.

We may also need to rebuild std with this metadata hash.

Author: Mark-Simulacrum

collector/src/bin/rustc-fake.rs

@@ -5,6 +5,25 @@ use std::time::{Duration, Instant};
 
 fn main() {
     let mut args = env::args_os().skip(1).collect::<Vec<_>>();
+
+    if args.iter().any(|arg| arg == "-vV") {
+        // Normally, we'd just run rustc here and return its output. But, in
+        // practice, that output affects `-Cmetadata` and that causes
+        // non-determinism as it feeds into the incremental hash state.
+        //
+        // So we return a canonical preset output.
+        println!(
+            "rustc 1.44.0-nightly (75208942f 2020-03-27)
+binary: rustc
+commit-hash: 75208942f6144daac669e8e382029fc33bdce841
+commit-date: 2020-03-27
+host: x86_64-unknown-linux-gnu
+release: 1.44.0-nightly
+LLVM version: 9.0"
+        );
+        return;
+    }
+
     let rustc = env::var_os("RUSTC_REAL").unwrap();
 
     if let Some(count) = env::var("RUSTC_THREAD_COUNT")