Auto merge of #4115 - ehuss:fix-compiletest, r=Manishearth
Auto merge of #4115 - ehuss:fix-compiletest, r=Manishearth

Fix compile-test from forcing a rebuild.

If the `compile-test` test was run, then running a cargo build command immediately after caused everything to be rebuilt. This is because the `compile-test` test was deleting all `.rmeta` files in the target directory. Cargo recently switched to always generating `.rmeta` files (https://github.com/rust-lang/cargo/pull/6883), and when the files are deleted, it thinks it needs to be rebuilt.

I am not very familiar with compiletest or clippy, so please take a close look and test this out (with the most recent nightly). In particular, make sure it doesn't revert the fixes from #3380 (it seems to work for me). Also @oli-obk mentioned something related in https://github.com/rust-lang/rust/pull/60190#issuecomment-493617188, and I want to make sure that is addressed as well.

Fixes #4114

aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust