Pointer dereferencing and `let _ = …` inconsistency?

I thought that a statement

SOME_EXPRESSION;

and

let _ = SOME_EXPRESSION;

are equivalent, and the only difference is in warnings about unused values. However, now I noticed that

fn main() {
    let x = std::ptr::null::<u8>();
    let _ = *x;
}

compiles, while

fn main() {
    let x = std::ptr::null::<u8>();
    *x;
}

doesn’t (dereference of raw pointer is unsafe and requires unsafe function or block). Does anyone know if that’s intentional? Is it because there is a difference between the two versions in terms or run-time behavior?

4 Likes

That seems funky. My guess is that it has something to do with place expressions and the fact that _ is a pattern.

7 Likes

And since it's not obvious from that bug:

Compiler folks are working on moving the unsafety checking to THIR, rather than MIR, so it'll be more consistent about stuff like this.

(MIR is better for flow-sensitive stuff -- that's why NLL happens there -- but for things that are based on syntactic blocks it's better in something that still has that structure.)

Yeah, I’ve seen that… however I’m still wondering why let _ = EXPR; and EXPR; generate different MIR in the first place. (They do, right? Otherwise an unsafety-check on MIR would return consistent results.) If let _ = EXPR; and EXPR; really are semantically equivalent, then generating less MIR for one of them seems like a potential missed optimization on the other. Sure LLVM can probably handle it most of the times, but still…

let _ = EXPR; is matching EXPR as a place, but then doesn't actually need to read anything from it to match the pattern, so there's no read in the MIR for unsafetyck to notice.

Whereas EXPR; is an expression statement, so it evaluates the expression -- which requires the read and thus unsafetyck sees it. It's like drop(EXPR);.

2 Likes

Fascinating... for some reason I thought ident; wouldn't drop the variable in ident.

I don't know if there's a strong reason for it to be this way, but that's definitely how it works today.

Maybe it's so that behaviour of the expression doesn't change when you add or remove a ; from the last line of a block?

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