const_trait_impl does not keep track of closures

[fmease]

The feature effects doesn't track the constness of closures.
This currently type-checks but it shouldn't:
Update: We (project-const-traits) have decided that we do want the code below to compile. It still holds true that effects doesn't track the constness of closures yet but we now plan on removing the need for const || {} (and remove the whole feature const_closures at some point) and want to rely on effect inference to make it work. Please see this Zulip discussion.

#![feature(effects)]
const fn f() { (|| {})() }

However, the following definitely shouldn't compile:

#![feature(effects)]
const fn f() { (|| println!())() }

Without effects, they get rejected early with error[E0015]: cannot call non-const closure in constant functions.

See also this Zulip discussion.

[fmease]

cc this other Zulip discussion.

[fmease]

@obeis Have you read this Zulip discussion already? The plan has slightly changed. See also the updated issue description.

[fee1-dead]

cc @rust-lang/project-const-traits

[fmease]

#![feature(const_trait_impl)]
const fn f() { (|| println!())() }

Gets correctly rejected.

Closing in favor of rust-lang/project-const-traits#10