Make macro expansion backtraces shorter

[jonas-schievink]

The second commit in this PR will stop printing the macro definition site in backtraces, which cuts their length in half and increases readability (the definition site was only correct for local macros).

The third commit will not print an invocation if the last one printed occurred at the same place (span). This will make backtraces caused by a self-recursive macro much shorter.

(A possible alternative would be to capture the backtrace first, then limit it to a few frames at the start and end of the chain and print ... inbetween. This would also work with multiple macros calling each other, which is not addressed by this PR - although the backtrace will still be halved)

Example:

macro_rules! m {
 ( 0 $($t:tt)* ) => ( m!($($t)*); );
 () => ( fn main() {0} );
}

m!(0 0 0 0 0 0 0 0 0 0 0 0 0 0 0);

On a semi-recent nightly, this yields:

test.rs:3:21: 3:22 error: mismatched types:
 expected `()`,
    found `_`
(expected (),
    found integral variable) [E0308]
test.rs:3  () => ( fn main() {0} );
                              ^
test.rs:1:1: 4:2 note: in expansion of m!
test.rs:2:23: 2:34 note: expansion site
test.rs:1:1: 4:2 note: in expansion of m!
test.rs:2:23: 2:34 note: expansion site
test.rs:1:1: 4:2 note: in expansion of m!
test.rs:2:23: 2:34 note: expansion site
test.rs:1:1: 4:2 note: in expansion of m!
test.rs:2:23: 2:34 note: expansion site
test.rs:1:1: 4:2 note: in expansion of m!
test.rs:2:23: 2:34 note: expansion site
test.rs:1:1: 4:2 note: in expansion of m!
test.rs:2:23: 2:34 note: expansion site
test.rs:1:1: 4:2 note: in expansion of m!
test.rs:2:23: 2:34 note: expansion site
test.rs:1:1: 4:2 note: in expansion of m!
test.rs:2:23: 2:34 note: expansion site
test.rs:1:1: 4:2 note: in expansion of m!
test.rs:2:23: 2:34 note: expansion site
test.rs:1:1: 4:2 note: in expansion of m!
test.rs:2:23: 2:34 note: expansion site
test.rs:1:1: 4:2 note: in expansion of m!
test.rs:2:23: 2:34 note: expansion site
test.rs:1:1: 4:2 note: in expansion of m!
test.rs:2:23: 2:34 note: expansion site
test.rs:1:1: 4:2 note: in expansion of m!
test.rs:2:23: 2:34 note: expansion site
test.rs:1:1: 4:2 note: in expansion of m!
test.rs:2:23: 2:34 note: expansion site
test.rs:1:1: 4:2 note: in expansion of m!
test.rs:2:23: 2:34 note: expansion site
test.rs:1:1: 4:2 note: in expansion of m!
test.rs:6:1: 6:35 note: expansion site
test.rs:3:21: 3:22 help: run `rustc --explain E0308` to see a detailed explanation
error: aborting due to previous error

After this patch:

test.rs:3:21: 3:22 error: mismatched types:
 expected `()`,
    found `_`
(expected (),
    found integral variable) [E0308]
test.rs:3  () => ( fn main() {0} );
                              ^
test.rs:2:23: 2:34 note: in this expansion of m!
test.rs:6:1: 6:35 note: in this expansion of m!
test.rs:3:21: 3:22 help: run `rustc --explain E0308` to see a detailed explanation
error: aborting due to previous error

[rust-highfive]

r? @nrc

(rust_highfive has picked a reviewer for you, use r? to override)

[jonas-schievink]

Another option that doesn't lose the macro definition site completely would be:

test.rs:6:1: 6:35 note: in this expansion of m! (defined in test.rs)
test.rs:6:1: 6:35 note: in this expansion of println! (defined in <std macros>)
test.rs:6:1: 6:35 note: in this expansion of for loop expansion

Codemap::span_to_filename() should work for this

[nrc]

@jonas-schievink the last option you suggest in the comment sounds like a good compromise to me. I am often frustrated by these macro expansion back traces, but OTOH I have occasionally found the definition site of the macro useful.

cc @rust-lang/compiler

[jonas-schievink]

Done!

Example:

<std macros>:1:33: 1:58 error: invalid reference to argument `0` (no arguments given)
<std macros>:1 ( $ fmt : expr ) => ( print ! ( concat ! ( $ fmt , "\n" ) ) ) ; (
                                               ^~~~~~~~~~~~~~~~~~~~~~~~~
<std macros>:1:33: 1:58 note: in this expansion of concat!
<std macros>:2:25: 2:56 note: in this expansion of format_args!
<std macros>:1:23: 1:60 note: in this expansion of print! (defined in <std macros>)
test.rs:3:25: 3:39 note: in this expansion of println! (defined in <std macros>)
test.rs:2:26: 2:37 note: in this expansion of m! (defined in test.rs)
test.rs:6:1: 6:35 note: in this expansion of m! (defined in test.rs)
error: aborting due to previous error

[eddyb]

👍 This looks nice.

[nikomatsakis]

Certainly progress.

[Aatch]

Looks good to me. I wonder if having the line/column in the "defined in file.rs" would be good. So note: in this expansion of m! (defined in test.rs:6:1).

[jonas-schievink]

@Aatch Since the spans are only correct for the local crate, in which I usually know where macros are defined (and grepping is trivial, especially when the file name is given), this would require checking if the macro is defined in the current crate, which - at a first glance - doesn't seem easy. Or just print <std macros>:1:1 like it's done now (ugh!).

[Aatch]

@jonas-schievink fair enough, just a thought.

[nrc]

@bors: r+

[bors]

📌 Commit 0be755c has been approved by nrc

[bors]

⌛ Testing commit 0be755c with merge f3e6d31...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-x-android-t, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-win-gnu-32-nopt-t, auto-win-gnu-32-opt, auto-win-gnu-64-nopt-t, auto-win-gnu-64-opt, auto-win-msvc-32-opt, auto-win-msvc-64-opt