Macro hygienics don't prevent const cycles about rust

[fmease]

A more minimal repo would be:

Even more minimal:

macro_rules! cycle {
    ($expr:expr) => { const X: () = $expr; }
}

fn main() { cycle!(X); } //~ ERROR cycle detected

---

macro_rules!-macros aka decl macros 1.2 are only partially hygienic because only local variables follow macro hygiene but not items. When you define constant X via a 1.2 macro, you can access it from other syntax contexts. That's what happens in the example above. Another example would be

macro_rules! def { () => { const X: () = (); } }

fn main() {
    def!();
    let _ = X; // successfully resolves `X` to the const item
}

Decl macros 2.0 (#![feature(decl_macro)]) on the other hand are fully hygienic: Both locals and items are hygienic. For example, the following code succeeds compilation:

#![feature(decl_macro)]

macro cycle {
    ($expr:expr) => { const X: () = $expr; }
}

fn main() {
    const X: () = ();
    cycle!(X);
}

[fmease]

TL;DR: Your example from the issue description passes compilation if you use decl macros 2.0 instead of 1.2 (which requires nightly): Playground.

[ChrisDenton]

A more minimal repo would be:

macro_rules! cycle {
    ($expr:expr) => {{
        const UTF8: &str = $expr;
    }}
}
#[allow(unused)]
fn main() {
    const UTF8: &str = "hello";
    cycle!(UTF8);
}

[Urgau]

Not sure if it's relevant to this issue but the same error happens without the macro_rules!:

#[allow(dead_code)]
fn main() {
    const UTF8: &str = "hello";
    {
        const UTF8: &str = UTF8;
    }
}

[fmease]

This works as intended. [typing explainer...]

[fmease]

Closing as works as intended :)

[workingjubilee]

I had a feeling this might get a "working as intended" but I'm glad to hear it gets fixed by macros 2.0!