Nightly cargo is sometimes slow to start

```rust
fn foo<T>() 
where T:std::marker::Sized
{
    let buf = std::mem::size_of::<T>();
}
```

fails to compile on nightly (in playground), timing out : https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=ddbfcdb9f8be5d6984f1886cc201489e

Compiles fine on stable and nightly