Compile regression "cannot infer an appropriate lifetime for lifetime parameter"

[gz]

On linux, the following works:

git clone https://github.com/AltSysrq/proptest.git
cd proptest
rustup update nightly-2020-04-06
rustup default nightly-2020-04-06
source ~/.cargo/env
rustc --version
cargo build

(rustc version is rustc 1.44.0-nightly (b543afc 2020-04-05))

The following does not work:

rustup update nightly
rustup default nightly
source ~/.cargo/env
rustc --version
cargo build

Build fails with:

error[E0495]: cannot infer an appropriate lifetime for lifetime parameter `'a` due to conflicting requirements
  --> proptest/src/arbitrary/_core/iter.rs:47:23
   |
47 |         base.prop_map(Iterator::cloned).boxed()
   |                       ^^^^^^^^^^^^^^^^
   |
note: first, the lifetime cannot outlive the lifetime `'a` as defined on the impl at 36:9...

(rustc version is: rustc 1.44.0-nightly (42abbd8 2020-04-07))

[Centril]

Reproducer:

fn main() {}

pub trait Strategy {
    type Value;
}

fn prop_map<O, F: Fn(S::Value) -> O, S: Strategy>(_: S, _: F) -> Map<S, F> {
    loop {}
}

fn boxed<S: 'static + Strategy>(_: S) -> *const dyn Strategy<Value = S::Value> {
    loop {}
}

pub struct Map<S, F> {
    x: std::marker::PhantomData<(S, F)>,
}

impl<S: Strategy, O, F: Fn(S::Value) -> O> Strategy for Map<S, F> {
    type Value = O;
}

fn cloned<'a, T: 'a, I: Iterator<Item = &'a T>>(x: I) -> I {
    x
}

fn lift1_with<'a, T, A, S>(base: S) -> *const dyn Strategy<Value = A>
where
    T: 'static,
    A: 'static + Iterator<Item = &'a T>,
    S: Strategy<Value = A> + 'static,
{
    boxed(prop_map(base, cloned))
}

[Centril]

cargo-bisect-rustc \
    --test-dir=proptest-regression \
    --end=42abbd8878d3b67238f3611b0587c704ba94f39c \
    --start=f6fe99c798cb65280a9a56f442b371adcb7b8aa2

reports that the regression happened in 209b2be, i.e. #70164. cc @eddyb

[eddyb]

Reduced (playground/stable -> playground/nightly):

fn assert_static<T: 'static>(_: T) {}

fn capture_type<T>() {}
fn capture_lifetime<'a: 'a>() {}

// Errors on stable & nightly.
fn test_type<'a>() {
    assert_static(capture_type::<&'a ()>);
}

// Errors *only* on nightly.
fn test_lifetime<'a>() {
    assert_static(capture_lifetime::<'a>);
}

@nikomatsakis Are we supposed to ignore only lifetime parameters in ty::FnDefs? If the reasoning is that the only thing you can do with a ty::FnDef is call it, why aren't all generics ignored?

cc @rust-lang/lang @matthewjasper

[eddyb]

I should say, I didn't expect to fix a lifetime vs type inconsistency, the goal was to fix type vs const inconsistencies. Somehow we have a behavior with no tests, I'm not sure how intentional it was.

[Centril]

(I'm perfectly happy to issue a point release, 0.9.6, to proptest btw if we decide the new compiler behavior is correct and proptest is in the wrong.)