Incremental Compilation needs to rebuild everything when feature gates are changed

[oberien]

Given this example:

main.rs

#![feature(nll)]

mod foo;

fn main() {
    foo::foo();
}

foo.rs

pub fn foo() -> Vec<usize> {
    let mut vec = Vec::new();
    vec.push(vec.len());
    vec
}

Without the feature gate nll, foo::foo won't compile, because vec is borrowed by vec.push which doesn't allow vec.len to borrow it again. With nll, this is solved and the code compiles.
With the annotation in main.rs, cargo build will build everything just fine.
After a successful build if we remove #[feature(nll)] and execute cargo build again, everything still compiles, even though it shouldn.

I think that this is due to incremental compilation. Cargo reuses the old module foo instead of recompiling it. If it would recompile it (e.g. after adding a newline to the end of foo.rs), the build breaks as expected.

[alexcrichton]

cc @michaelwoerister

@oberien thanks for the report! I think this is a rustc issue because Cargo is reinvoking the compiler, but it looks like stale results may be getting used by incremental compilation?

[nikomatsakis]

We should probably include the features in the hash of command line options that we give, I guess.

[nikomatsakis]

triage: P-medium

[nikomatsakis]

Well, I'll triage as P-high just to make sure @michaelwoerister sees this.

triage: P-high

[michaelwoerister]

This is a duplicate of #47003.

[nikomatsakis]

Shall we close it then, @michaelwoerister ?

[michaelwoerister]

Yes, closing as duplicate. Moving over the P-high tag to the original.