Experiment: try out removing opaque types from typeck expectations

[oli-obk]

a721052 made type-alias-impl-trait different from return-position-impl-trait.

type Foo = impl std::fmt::Debug;

fn foo(b: bool) -> Foo {
    if b {
        vec![42_i32]
    } else {
        std::iter::empty().collect()
        //~^ ERROR `Foo` cannot be built from an iterator over elements of type `_`
    }
}

does not compile while the equivalent return-position-impl-trait program compiles:

fn foo(b: bool) -> impl std::fmt::Debug{
    if b {
        vec![42_i32]
    } else {
        std::iter::empty().collect()
    }
}

We should figure out what we actually want here and whether we can go even further and just remove opaque types from Expectation entirely. Things to try:

• fn foo() -> for<'a> impl FnOnce(&'a str) -> usize { |s| s.len() }
• fn foo() -> impl FnOnce(&'static str) -> usize { |s| s.len() }

[oli-obk]

At present

#![feature(type_alias_impl_trait)]

type Foo = impl for<'a> FnOnce(&'a str) -> usize;
type Bar = impl FnOnce(&'static str) -> usize;

// these two work
fn foo() -> Foo { if true { |s| s.len() } else { panic!() } }
fn bar() -> Bar { if true { |s| s.len() } else { panic!() } }

// these two don't work
fn foo2() -> impl for<'a> FnOnce(&'a str) -> usize { if true { |s| s.len() } else { panic!() } }
fn bar2() -> impl FnOnce(&'static str) -> usize { if true { |s| s.len() } else { panic!() } }

[oli-obk]

cc @lcnr