Incomplete warning min_const_fn for unstable trait bounds other than Sized

[gnzlbg]

#![feature(min_const_fn)]
use std::marker::PhantomData;
pub struct Foo<T>(PhantomData<T>);
pub trait Bar {}
impl<T: Bar> Foo<T> {
    pub const fn foo() -> Self {
        Foo(PhantomData)
    }
}
fn main() {}

(Playground)

Errors:

   Compiling playground v0.0.1 (/playground)
error: trait bounds other than `Sized` on const fn parameters are unstable
 --> src/main.rs:5:6
  |
5 | impl<T: Bar> Foo<T> {
  |      ^

error: aborting due to previous error

error: Could not compile `playground`.

To learn more, run the command again with --verbose.

The error should mention which feature has to be enabled to make use of this. The feature is const_fn.

cc @oli-obk @eddyb

[eddyb]

Also, it should point at the Bar, I would think?

[oli-obk]

Pointing at the Bar is hard. It might be multiple bound spread out through where bounds and generics.

We can change the message to "this generic parameter has nontrivial bounds which is not permitted in const fn"

[gnzlbg]

FWIW I think that the bigger issue is that the message says that the feature is unstable, but doesn't mention how the feature is actually called. I guessed, and happened to guess just fine, but if min_const_fn becomes stable at some point, the error should properly mention which features users have to enable for this on nightly.