Cannot use `Self` to construct a tuple struct in a macro

[kennytm]

Reproduced on both on beta and nightly (not tested on stable since #51994 wasn't stabilized in 1.31).

#![warn(clippy::use_self)]

pub struct S(pub u16);

impl From<u8> for S {
    fn from(a: u8) -> Self {
        Self(a.into())            // <-- this is fine
    }
}

macro_rules! a {
    () => {
        impl From<u16> for S {
            fn from(a: u16) -> Self {
                // S(a)            // <-- this is fine but triggers the `clippy::use_self` warning
                Self(a)            // <-- ERROR E0423
                // Self { 0: a }   // <-- this is fine but ugly
            }
        }
    }
}

a!();

fn main() {}

This is particularly troublesome on nightly since clippy::use_self is recently upgraded to emit the lint in a local macro thanks to rust-lang/rust-clippy#3627, but the suggestion failed to compile.

[varkor]

This also applies to type aliases.

struct T;

type B = T;

macro_rules! a {
    () => {
        impl T {
            fn b() -> B {
                B()
            }
        }
    }
}

a!();

I suspect this is to do with

rust/src/librustc_resolve/lib.rs

Lines 3557 to 3558 in b439861

	// FIXME: can't resolve paths in macro namespace yet, macros are
	// processed by the little special hack below.

cc @alexreg

[alexreg]

Yes, I think @varkor is right.

@petrochenkov Maybe you could elaborate on this FIXME? What's stopping us from resolving paths in the macro NS here?

[petrochenkov]

That FIXME is unrelated to the issue.
The issue is that Self in value namespace accidentally uses "let variable hygiene" rather than "item hygiene", so macro_rules "hides" it from the outside world like it would do for a let variable.

Regarding type aliases, B defined by type B = T; doesn't exist in value namespace, so B() doesn't resolve (with or without macros).

[petrochenkov]

@varkor
Looks like const generic parameters in #53645 have the same issue.

[petrochenkov]

Fixed in #57560

[alexreg]

@petrochenkov Ahh, that makes perfect sense. Feel free (but not obliged) to r? me for that PR.