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Description
I tried this code:
#![expect(incomplete_features, reason = "generic_const_exprs")]
#![feature(generic_const_exprs)]
pub fn compute_root_only<const N: u64>() -> u64
where
[(); N.ilog2() as usize]:,
{
N
}
#[test]
fn test() {
test_impl::<99>();
}
#[cfg(test)]
fn test_impl<const N: u64>()
where
[(); N.ilog2() as usize]:,
{
// Const generic is inferred, but it shouldn't be!
assert_eq!(compute_root_only(), N);
}I expected to see this happen:
error[E0284]: type annotations needed
--> src/lib.rs:22:16
|
22 | assert_eq!(compute_root_only(), N);
| ^^^^^^^^^^^^^^^^^ cannot infer the value of the const parameter `N` declared on the function `compute_root_only`
|
note: required by a const generic parameter in `compute_root_only`
--> src/lib.rs:4:26
|
4 | pub fn compute_root_only<const N: u64>() -> u64
| ^^^^^^^^^^^^ required by this const generic parameter in `compute_root_only`
help: consider specifying the generic argument
|
22 | assert_eq!(compute_root_only::<N>(), N);
| +++++
Instead, this happened: It compiles successfully 🤯
The key thing is where bound. If it is simplified to something simpler like this:
where
[(); N]:,
Then I do get expected compiler error.
This is basically a miscompilation, compiler should not pick arbitrary matching where bound from the context of the caller to infer types of the called function unless corresponding constant is used explicitly.
Meta
rustc --version --verbose:
rustc 1.89.0-nightly (cdd545be1 2025-06-07)
binary: rustc
commit-hash: cdd545be1b4f024d38360aa9f000dcb782fbc81b
commit-date: 2025-06-07
host: x86_64-unknown-linux-gnu
release: 1.89.0-nightly
LLVM version: 20.1.5