Consider eliding long macro backtraces of recursive macros

[huonw]

A recursive macro can easily cause long useless error messages:

#![feature(macro_rules)]

macro_rules! go {
    () => { fn main() { undefined } };
    ($_x:tt $($rest: tt)*) => {
        go!($($rest)*)
    }
}

go!(1 2 3 4 5 6 7 8 9 10)

<anon>:4:25: 4:34 error: unresolved name `undefined`.
<anon>:4     () => { fn main() { undefined } };
                                 ^~~~~~~~~
<anon>:3:1: 8:2 note: in expansion of go!
<anon>:6:9: 6:23 note: expansion site
<anon>:3:1: 8:2 note: in expansion of go!
<anon>:6:9: 6:23 note: expansion site
<anon>:3:1: 8:2 note: in expansion of go!
<anon>:6:9: 6:23 note: expansion site
<anon>:3:1: 8:2 note: in expansion of go!
<anon>:6:9: 6:23 note: expansion site
<anon>:3:1: 8:2 note: in expansion of go!
<anon>:6:9: 6:23 note: expansion site
<anon>:3:1: 8:2 note: in expansion of go!
<anon>:6:9: 6:23 note: expansion site
<anon>:3:1: 8:2 note: in expansion of go!
<anon>:6:9: 6:23 note: expansion site
<anon>:3:1: 8:2 note: in expansion of go!
<anon>:6:9: 6:23 note: expansion site
<anon>:3:1: 8:2 note: in expansion of go!
<anon>:6:9: 6:23 note: expansion site
<anon>:3:1: 8:2 note: in expansion of go!
<anon>:6:9: 6:23 note: expansion site
<anon>:3:1: 8:2 note: in expansion of go!
<anon>:10:1: 10:26 note: expansion site
error: aborting due to previous error

We could detect the repeating backtrace and replace it with:

<anon>:4:25: 4:34 error: unresolved name `undefined`.
<anon>:4     () => { fn main() { undefined } };
                                 ^~~~~~~~~
<anon>:3:1: 8:2 note: in expansion of go!
<anon>:6:9: 6:23 note: expansion site
[... 8 repeats ...]
<anon>:3:1: 8:2 note: in expansion of go!
<anon>:6:9: 6:23 note: expansion site
<anon>:3:1: 8:2 note: in expansion of go!
<anon>:10:1: 10:26 note: expansion site
error: aborting due to previous error

In the best case, this would detect sequences of mutually recursive macro invocations of any length, but 1 is almost certainly the most common case, and if not that, then 2. It should also only elide the identical frames.

[steveklabnik]

Updated macro:

macro_rules! go {
    () => { fn main() { undefined } };
    ($_x:tt $($rest: tt)*) => {
        go!($($rest)*);
    }
}

go!(1 2 3 4 5 6 7 8 9 10);

No change otherwise.

[jonas-schievink]

This specific case is now fixed:

<anon>:2:25: 2:34 error: unresolved name `undefined` [E0425]
<anon>:2     () => { fn main() { undefined } };
                                 ^~~~~~~~~
<anon>:4:9: 4:24 note: in this expansion of go! (defined in <anon>)
<anon>:8:1: 8:27 note: in this expansion of go! (defined in <anon>)
<anon>:2:25: 2:34 help: see the detailed explanation for E0425
error: aborting due to previous error

Self-recursive macro invocations are no longer printed. However, if the macro calls another macro (or invokes another arm of itself, such that the backtrace shows 2 alternating spans), the backtrace is still repeated.

I would consider this to be an E-easy candidate.

[Mark-Simulacrum]

Closing as fixed.

macro_rules! go {
    () => { fn main() { undefined } };
    (a $_x:tt $($rest: tt)*) => {
        go!(b $($rest)*);
    };
    (b $_x:tt $($rest: tt)*) => {
        go!(a $($rest)*);
    };
    ($_x:tt) => {
        go!();
    };
}

go!(a 1 2 3 4 5 6 7 8 9 10);

error[E0425]: cannot find value `undefined` in this scope
  --> test.rs:2:25
   |
2  |     () => { fn main() { undefined } };
   |                         ^^^^^^^^^ not found in this scope
...
14 | go!(a 1 2 3 4 5 6 7 8 9 10);
   | ---------------------------- in this macro invocation

error: aborting due to previous error(s)