poor error message when casting from fat raw pointer to usize

[durka]

playpen

trait Num {}
impl Num for i32 {}

fn main() {
    &0i32 as *const Num as usize;
}

This compiles on stable, but beta and nightly require an extra as *const () in the middle to cast away the fat. The error message says to use a "raw pointer" which is misleading as *const Num is already a raw pointer, it's just a fat one.

Since this code compiles on stable, I'm not sure if it's also a regression or if I missed an RFC (to be fair, I miss a lot of RFCs).

[bluss]

I'm sure it went in as a bug fix. cc @arielb1

[arielb1]

This is indeed intentional: the spec at https://github.com/rust-lang/rfcs/blob/master/text/0401-coercions.md has this relevant part: e has type *T and U is a numeric type, while T: Sized; ptr-addr-cast. IIRC @nrc made that decision.

[arielb1]

The old (1.1) cast code was a badly specified mess of if-s. It is rather badly behaved.

[durka]

Noted. Is this still a (minor) diagnostics issue, or I just don't
understand the terminology? The compiler's advice is to cast through a "raw
pointer", but the actual fix is to cast through a thin pointer.
On Aug 1, 2015 12:28 PM, "arielb1" [email protected] wrote:

The old (1.1) cast code was a badly specified mess of if-s. Do not report
bugs.

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Reply to this email directly or view it on GitHub
#27450 (comment).

[arielb1]

This may be a diagnostic issue. cc @steveklabnik