missing-const-for-fn might want to skip macros

[matthiaskrgr]

Summary

.

Reproducer

I tried this code:

#![recursion_limit = "10"]
macro_rules! link {
    ($outer:ident, $inner:ident) => {
        struct $outer($inner);
        impl $outer {
            fn new() -> $outer {
                $outer($inner::new())
            }
        }
        impl std::ops::Deref for $outer {
            type Target = $inner;
            fn deref(&self) -> &$inner {
                &self.0
            }
        }
    };
}

struct Bottom;

impl Bottom {
  const  fn new() -> Bottom {
        Bottom
    }
}


link!(A, B);
link!(B, C);
link!(C, D);
link!(D, E);
link!(E, F);
link!(F, G);
link!(G, H);
link!(H, I);
link!(I, J);
link!(J, K);
link!(K, Bottom);

clippy sugttests touching the macro :s

warning: this could be a `const fn`
  --> a.rs:6:13
   |
6  | /             fn new() -> $outer {
7  | |                 $outer($inner::new())
8  | |             }
   | |_____________^
...
38 |   link!(K, Bottom);
   |   ---------------- in this macro invocation
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#missing_const_for_fn
   = note: requested on the command line with `-W clippy::missing-const-for-fn`
   = note: this warning originates in the macro `link` (in Nightly builds, run with -Z macro-backtrace for more info)
help: make the function `const`
   |
6  |             const fn new() -> $outer {
   |             +++++

the resulting code causes overflows:

error[E0275]: overflow evaluating the requirement `J: std::marker::Freeze`
   |
   = help: consider increasing the recursion limit by adding a `#![recursion_limit = "20"]` attribute to your crate (`a`)
note: required because it appears within the type `I`
  --> a.rs:36:7
   |
36 | link!(I, J);
   |       ^
note: required because it appears within the type `H`
  --> a.rs:35:7
   |
35 | link!(H, I);
   |       ^
note: required because it appears within the type `G`
  --> a.rs:34:7
   |
34 | link!(G, H);
   |       ^
note: required because it appears within the type `F`
  --> a.rs:33:7
   |
33 | link!(F, G);
   |       ^
note: required because it appears within the type `E`
  --> a.rs:32:7
   |
32 | link!(E, F);
   |       ^
note: required because it appears within the type `D`
  --> a.rs:31:7
   |
31 | link!(D, E);
   |       ^
note: required because it appears within the type `C`
  --> a.rs:30:7
   |
30 | link!(C, D);
   |       ^
note: required because it appears within the type `B`
  --> a.rs:29:7
   |
29 | link!(B, C);
   |       ^
note: required because it appears within the type `A`
  --> a.rs:28:7
   |
28 | link!(A, B);
   |       ^

error[E0275]: overflow evaluating the requirement `K: std::marker::Freeze`
   |
   = help: consider increasing the recursion limit by adding a `#![recursion_limit = "20"]` attribute to your crate (`a`)
note: required because it appears within the type `J`
  --> a.rs:37:7
   |
37 | link!(J, K);
   |       ^
note: required because it appears within the type `I`
  --> a.rs:36:7
   |
36 | link!(I, J);
   |       ^
note: required because it appears within the type `H`
  --> a.rs:35:7
   |
35 | link!(H, I);
   |       ^
note: required because it appears within the type `G`
  --> a.rs:34:7
   |
34 | link!(G, H);
   |       ^
note: required because it appears within the type `F`
  --> a.rs:33:7
   |
33 | link!(F, G);
   |       ^
note: required because it appears within the type `E`
  --> a.rs:32:7
   |
32 | link!(E, F);
   |       ^
note: required because it appears within the type `D`
  --> a.rs:31:7
   |
31 | link!(D, E);
   |       ^
note: required because it appears within the type `C`
  --> a.rs:30:7
   |
30 | link!(C, D);
   |       ^
note: required because it appears within the type `B`
  --> a.rs:29:7
   |
29 | link!(B, C);
   |       ^

error: aborting due to 2 previous errors

Version

rustc 1.81.0-nightly (9c3bc805d 2024-06-27)
binary: rustc
commit-hash: 9c3bc805dd9cb84019c124b9a50fdff1e62a7ec9
commit-date: 2024-06-27
host: x86_64-unknown-linux-gnu
release: 1.81.0-nightly
LLVM version: 18.1.7

Additional Labels

No response

[DaniPopes]

I'm guessing this would still happen even without the macro; idk what the precedent is for lints with recursion_limit, but otherwise this seems correct to me