Const functions sometimes don't do overflow checks in release mode

[Y0ba]

I tried this code in release mode:

const fn foo(arg: i32) -> i32 {
    arg + i32::MAX
}
const x: i32 = foo(1);

I expected to see this happen: error: this arithmetic operation will overflow

Instead, code successfully compiled.

If I replace arg with 1 in function than compilation failing with an overflow error.

[tesuji]

This is consistent with overflow check. If you run the code at debug mode, rustc will error:

error: any use of this value will cause an error
 --> src/lib.rs:2:5
  |
2 |     arg + i32::MAX
  |     ^^^^^^^^^^^^^^
  |     |
  |     attempt to add with overflow
  |     inside `foo` at src/lib.rs:2:5
  |     inside `x` at src/lib.rs:4:16
3 | }
4 | const x: i32 = foo(1);
  | ----------------------
  |
  = note: `#[deny(const_err)]` on by default

[Y0ba]

But inconsistent with other const computations (like 1 + i32::MAX which fails in release mode too). Overflow checking is disabled in release mode because of performance penalty (#47739) but does it matter in const functions?

[tesuji]

Probably it relates to const propagation here, which is unlike 1+ i32::MAX case.

[matu3ba]

But inconsistent with other const computations (like 1 + i32::MAX which fails in release mode too). Overflow checking is disabled in release mode because of performance penalty (#47739) but does it matter in const functions?

Compile-time const unwrapping soon gets implemented. This should fix the code.
It matters, because you want all checks on in compile-time evaluations/execution.