Generic (lifetime, type, const) parameters are unhygenic

[lolbinarycat]

I tried this code:

macro_rules! my_type {
    () => (&'_magic_lifetime T);
}

fn foo<'_magic_lifetime, T>(_x: my_type!()) {}

I expected to see this happen: compile error saying 'magic_lifetime and T are not defined, with a help message about macro hygiene, or at the very least a warning.

Instead, this happened: code compiles successfully with no warning.

Meta

rustc --version --verbose:

rustc 1.88.0 (6b00bc388 2025-06-23)
binary: rustc
commit-hash: 6b00bc3880198600130e1cf62b8f8a93494488cc
commit-date: 2025-06-23
host: x86_64-unknown-linux-gnu
release: 1.88.0
LLVM version: 20.1.5

[fmease]

This works as intended.

Macro-rules also known as decl macros 1.2 have so-called mixed-site hygiene where only locals (i.e., function params and bindings defined via let) are hygienic (not sure if I'm forgetting other things but I don't think so) / have so-called def-site hygiene (as opposed to the so-called call-site hygiene used for item names, generic parameter names, etc.).

Decl macros 2.0 on the other hand (feature decl_macro) employ full def-site hygiene where every name is hygienic (item names, generic parameter names, etc.). Switch out the macro_rules! with macro, the ; with , and enable the feature decl_macro to see the effect on your code.

Retroactively "fixing" decl macros 1.2 is not possible without an edition … and we sure don't want to do that because decl macros 2.0 is the planned successor to decl macros 1.2 (admittedly, its progress has kind of stalled).

Closing in favor of the tracking issue of decl macros 2.0 (#39412).

[fmease]

You can watch https://www.youtube.com/watch?v=r0H8BUiekcM to learn more about the blockers / status of decl macros 2.0.