Incorrect constant evaluation warning on correct code

[mooman219]

It looks like I'm incorrectly being warned of a constant evaluation error on valid code. The code runs just fine and is not violating the constant function requirements.

Offending code:

#![feature(const_fn)]

pub struct Data<T> {
    function: fn() -> T,
}

impl<T> Data<T> {
    pub const fn new(function: fn() -> T) -> Data<T> {
        Data {
            function: function,
        }
    }
}

pub static DATA: Data<i32> = Data::new(|| {
    413i32
});

fn main() {
    print!("{:?}", (DATA.function)());
}

Incorrect error:

warning: constant evaluation error: unsupported constant expr
  --> src/main.rs:15:30
   |
15 |   pub static DATA: Data<i32> = Data::new(|| {
   |  ______________________________^
16 | |     413i32
17 | | });
   | |__^
   |
   = note: #[warn(const_err)] on by default

Example of the issue on the playground in nightly.

Ping #24111

[oli-obk]

This looks very much like the AST const evaluator simply doesn't know about closures under some conditions. I'd wager it has to do with the generics.

I suggest papering over the issue by placing unsupported constant expr into the list of ignored const eval errors. This will simply work once we get miri, so there's no point in putting more effort into it.

[AgustinCB]

I'd be happy to take the task. Where such a list of const ignored errors?

[oli-obk]

@AgustinCB great!

The list is

rust/src/librustc_passes/consts.rs

Lines 134 to 138 in 24840da

	UnimplementedConstVal(_) => {}
	IndexOpFeatureGated => {}
	ErroneousReferencedConstant(_) => {}
	TypeckError => {}
	_ => {

You probably just need to ignore

rust/src/librustc/middle/const_val.rs

Line 164 in 2f84fb5

MiscCatchAll => simple!("unsupported constant expr"),

And fixup any tests that were affected