Incremental compiliation is ineffective when building compiler

[Aaron1011]

Rebuilding the compiler took 15 minutes after simply running touch on a file. Given that nothing has actually changed, I would expect that rebuilding would be much faster.

Steps to reproduce:

1. Clone the rust repository.
2. Set incremental = true in the rust section of config.toml.
3. Run ./x.py clean if you've previously built the compiler
4. Run ./x.py build
5. Run touch src/librustc/traits/auto_trait.rs
6. Run ./x.py build again.

The second ./x.py build invocation takes much longer than it should (~15 minutes on my machine), considering that the only thing that's changed is a file timestamp.

[pnkfelix]

It would be worth trying to figure out whether this phenomenon is due to some problem specific in the rustbuild bootstrap process, or if it something that affects other clients using incremental compilation (and therefore would be a potential blocker for turning on incremental in release mode, cc #57968)

[pnkfelix]

discussed at wg-incr-comp meeting.

We think that the issue as described here is probably largely an artifact of the build process discussed above is building all the way to stage 2, which A. imposes work that is hard to avoid (at least automatically), and B. is no longer the default in x.py for rustc development, and thus should not be affecting most rustc developers.

closing as (presumed) "fixed" or at least "not a bug for wg-incr-comp"...