travis: Don't set `RUST_LOG` globally

I have a suspicion that this caused a large regression in cycle times by forcing
the compiler to perform more checks on every `debug!` statement, so let's test
this out by removing the `RUST_LOG` env var globally.

This regression in cycle time was witnessed between [two] [builds] where the
[PR] in question didn't do much suspicious. Judging by how the stage0 times
*also* regressed though then this is my best guess.

[two]: https://travis-ci.org/rust-lang/rust/builds/210149932
[builds]: https://travis-ci.org/rust-lang/rust/builds/210179995
[PR]: https://github.com/rust-lang/rust/pull/40446

1 files changed, 5 insertions, 0 deletions

diff --git a/src/bootstrap/native.rs b/src/bootstrap/native.rs
index dea1a607255..62eed8be3cc 100644
--- a/src/bootstrap/native.rs
+++ b/src/bootstrap/native.rs
@@ -18,6 +18,7 @@
 //! LLVM and compiler-rt are essentially just wired up to everything else to
 //! ensure that they're always in place if needed.
 
+use std::env;
 use std::fs::{self, File};
 use std::io::{Read, Write};
 use std::path::Path;
@@ -145,6 +146,10 @@ pub fn llvm(build: &Build, target: &str) {
         cfg.define("CMAKE_CXX_FLAGS", build.cflags(target).join(" "));
     }
 
+    if env::var_os("SCCACHE_ERROR_LOG").is_some() {
+        cfg.env("RUST_LOG", "sccache=debug");
+    }
+
     // FIXME: we don't actually need to build all LLVM tools and all LLVM
     //        libraries here, e.g. we just want a few components and a few
     //        tools. Figure out how to filter them down and only build the right