Cannot use Self to construct a tuple struct in a macro

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.

@alexreg

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

@varkor

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?

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).

@varkor

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

Fixed in #57560

@petrochenkov

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

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Cannot use `Self` to construct a tuple struct in a macro #57523

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

Cannot use Self to construct a tuple struct in a macro #57523

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varkor commented on Jan 11, 2019

alexreg commented on Jan 11, 2019

petrochenkov commented on Jan 12, 2019

petrochenkov commented on Jan 12, 2019

petrochenkov commented on Jan 12, 2019

alexreg commented on Jan 12, 2019

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Cannot use `Self` to construct a tuple struct in a macro #57523