Specific error message when trying to exhaustively match on pointer types

[SergioBenitez]

As the title suggests, exhaustive pattern checking for usize and isize fails when checks for similar patterns for {i,u}{n} succeed. Consider the following program (playground link):

// Okay.
fn g(len: u8) {
    match len {
        0 => { }
        1 => { }
        2 => { }
        3 ..= std::u8::MAX => { }
    }
}

// Fails.
fn f(len: usize) {
    match len {
        0 => {},
        1 => {},
        2 => {},
        3 ..= std::usize::MAX => {},
    }
}

// Okay.
fn j(len: i32) {
    match len {
        std::i32::MIN ..= -1 => {},
        0 => { }
        1 => { }
        2 => { }
        3 ..= std::i32::MAX => { }
    }
}

// Fails.
fn h(len: isize) {
    match len {
        std::isize::MIN ..= -1 => {},
        0 => { }
        1 => { }
        2 => { }
        3 ..= std::isize::MAX => { }
    }
}

While checking succeeds for g (u8) and j (i32), it fails for f (usize) and h (isize).

cc @varkor

---

This is intentional (exhaustive matching on pointers is tracked at #56354), but where we would exhaustively match, then we could suggest using #![feature(precise_pointer_size_matching)] and explain why the pattern wasn't exhaustively matched.

[varkor]

Exhaustive pattern matching is disabled by default for usize and isize because the width differs depending on the platform. You can enable it using #![feature(precise_pointer_size_matching)]. There's a tracking issue at #56354.

[SergioBenitez]

@varkor If that's the case, the error message should allude to this.

[varkor]

That's a good point — we could emit better diagnostics here. Let's repurpose this issue for that.

[Nadrieril]

The error is now:

error[E0004]: non-exhaustive patterns: `_` not covered
 --> src/main.rs:2:11
  |
2 |     match len {
  |           ^^^ pattern `_` not covered
  |
  = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
  = note: the matched value is of type `usize`
  = note: `usize` does not have a fixed maximum value, so a wildcard `_` is necessary to match exhaustively

So we can close this

[varkor]

And there's a test for it already, perfect.

rust/src/test/ui/pattern/usefulness/non-exhaustive-pattern-pointer-size-int.stderr

Lines 1 to 10 in 85bc953

	error[E0004]: non-exhaustive patterns: `_` not covered
	--> $DIR/non-exhaustive-pattern-pointer-size-int.rs:4:11
	|
	LL | match 0usize {
	| ^^^^^^ pattern `_` not covered
	|
	= help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
	= note: the matched value is of type `usize`
	= note: `usize` does not have a fixed maximum value, so a wildcard `_` is necessary to match exhaustively
	= help: add `#![feature(precise_pointer_size_matching)]` to the crate attributes to enable precise `usize` matching

Thanks!