Error reporting bug with const_generics and const_evaluatable_checked #82959
Description
Issue three of three I'll be filing tonight.
This is the code that causes the bug:
#![feature(const_generics, const_evaluatable_checked, array_map)]
pub struct ConstCheck<const CHECK: bool>;
pub trait True {}
impl True for ConstCheck<true> {}
pub trait OrdesDec {
type Newlen;
type Output;
fn pop(self) -> (Self::Newlen, Self::Output);
}
impl<T, const N: usize> OrdesDec for [T; N]
where
ConstCheck<{N > 1}>: True,
ConstCheck<{N - 1 != 0}>: True,
{
type Newlen = [T; N - 1];
type Output = T;
fn pop(self) -> (Self::Newlen, Self::Output) {
let mut iter = IntoIter::new(self);
let end = iter.next_back().unwrap();
let new = [(); N - 1].map(move |()| iter.next().unwrap());
(new, end)
}
}I don't remember why I tried this, but I did. And the error messages I got on it were wild:
error[E0283]: type annotations needed
--> src/nightly_arr.rs:24:28
|
6 | pub trait True {}
| -------------- required by this bound in `True`
...
24 | ConstCheck<{ N > 1 }>: True,
| ^^^^ cannot infer type for struct `ConstCheck<{ N > 1 }>`
|
= note: cannot satisfy `ConstCheck<{ N > 1 }>: True`
error[E0283]: type annotations needed
--> src/nightly_arr.rs:28:5
|
24 | ConstCheck<{ N > 1 }>: True,
| ---- required by this bound in `nightly_arr::<impl OrdesDec for [T; N]>`
...
28 | type Newlen = [T; N - 1];
| ^^^^^^^^^^^^^^^^^^^^^^^^^ cannot infer type for struct `ConstCheck<{ N > 1 }>`
|
= note: cannot satisfy `ConstCheck<{ N > 1 }>: True`
help: consider specifying the type arguments in the function call
|
28 | type Newlen = [T; N - 1];::<T, N>
| ^^^^^^^^
error[E0283]: type annotations needed
--> src/nightly_arr.rs:31:5
|
31 | / fn pop(self) -> (Self::Newlen, Self::Output) {
32 | | let mut iter = IntoIter::new(self);
33 | | let end = iter.next_back().unwrap();
34 | | let new = [(); N - 1].map(move |()| iter.next().unwrap());
35 | | (new, end)
36 | | }
| |_____^ cannot infer type for struct `ConstCheck<{ N > 1 }>`
|
= note: cannot satisfy `ConstCheck<{ N > 1 }>: True`
= note: required because of the requirements on the impl of `OrdesDec` for `[T; N]`
error: aborting due to 3 previous errors
Now you're probably thinking to yourself, "Hey Auro, isn't that exactly the same code as in #82956? Oh, and #82957? And isn't it silly to pull the same dumb gag twice?" To answer the last question: yes, definitely. To answer the other two: no, not quite. Check out the constraints on the OrdesDec impl: [T; N - 1]: Sized has been swapped out for ConstCheck<{N - 1 != 0}>: True. I don't even know what's going on in those error messages, if I'm honest. They confuse me.
Meta
rustc --version --verbose:
rustc 1.52.0-nightly (35dbef235 2021-03-02)
binary: rustc
commit-hash: 35dbef235048f9a2939dc20effe083ca483c37ff
commit-date: 2021-03-02
host: x86_64-unknown-linux-gnu
release: 1.52.0-nightly
LLVM version: 11.0.1