Detect Merge Conflict Text

[Havvy]

I saw a tweet about git merge conflict text in source code, and wondered what the Rust compiler does. As such, I created a play.rust-lang.org sample.

It currently detects more than two equals in a row, doesn't know what it means, and bails. Specifically, it looks like this:

rustc 1.12.0 (3191fbae9 2016-09-23)
error: expected one of `!`, `+`, `->`, `::`, `>`, or `as`, found `==`
 --> <anon>:4:1
  |
4 | =======
  | ^^

error: aborting due to previous error

I propose that when this happens, the compiler should look for the git merge conflict issues, and if it finds it, give a specific error message related to what is actually happening.

The error message could show the start and end of the merge sections, and actually give searchable text for what this strange textual artifact is for those who've never seen it before.

For an example of how it'd look:

error: Merge conflict detected.
 --> <anon>:2:1
  |
2 |<<<<<<< HEAD
  |^^^^^^^^^^^^ Note: Beginning of commit HEAD here.
  |
4 | =======
  | ^^^^^^^ Note: Divider between commits HEAD and ab890fb here.
  |
5 |>>>>>>> ab890fb
  |^^^^^^^^^^^^^^^ Note: End of commit ab890fb here.

error: aborting due to previous error

The only con I can think of is that we're specifically caring about the semantics of one tool that isn't really related to Rust. But at the same time, git is widely used.

[8573]

The only con I can think of is that we're specifically caring about the semantics of one tool that isn't really related to Rust.

Perhaps it could use this diagnostic iff a Git repository is detected?

[solson]

Or do other merge tools use similar marks?

[8573]

Hm, I do seem to recall that <<<...===....>>> notation predating Git.

[8573]

Looks like it dates back to RCS, or even earlier.

[Havvy]

Alright, removed the "Git" part from this.

[8573]

Still, I think trying to detect a VCS (lazily and memoizedly) would be useful for giving the user more context.

[hanna-kruppe]

Still, I think trying to detect a VCS (lazily and memoizedly) would be useful for giving the user more context.

Why? Since the format is apparently not specific to one VCS, the compiler can just say "merge conflict" and perhaps drop the term "commit". The user (if they are in a position to resolve the merge conflict) will presumably know which VCS they are using. Not to mention that a merge conflict marker remains a merge conflict marker when it lives outside a VCS directory — for example, because the source code was downloaded from crates.io. Add to that the work needed to implement this feature and the danger of the detection being overly brittle, and it's a clear 👎 for me.

[TimNN]

In my personal opinion a functionality like this does not belong in the rust compiler. The current error message already identifies one of the three locations, which I think should be enough.

[arielb1]

GCC returns the following error:

test.c:1:1: error: version control conflict marker in file
 <<<<<<< HEAD
 ^~~~~~~

[8573]

Still, I think trying to detect a VCS (lazily and memoizedly) would be useful for giving the user more context.
Why?

While I still think it could be useful, I've failed to think of any feature for which it would be necessary, save one: giving the user VCS-specific tips or URLs of documentation re resolving merge conflicts — and of course rustc could instead (in e.g. its --explain text) merely give the user a list of such and have them pick.

I am and have been quite okay with a generic "merge conflict detected" message, although looking at GCC's above, I think I would prefer "merge conflict marker detected", as it doesn't necessarily mean there's an actual merge conflict.

[jfirebaugh]

Duplicate of #32059.