Tweak "conflict markers" diagnostic by detecting git state #150233

estebank · 2025-12-21T21:22:08Z

When detecting "diff markers", inspect the current git repo's state to see if there is an active rebase. If that is the case, we try to identify whether the rebase is caused by git rebase or implicitly by git pull. In order to do that we query git to ask for the path of .git/rebase-merge and then inspect the files orig-head (the commit we are rebasing from), onto (the commit we are rebasing onto) and head-name (the branch being rebased). We modify the labels pointing at the markers to be clear on where each section of code is coming from.

git rebase case:

error: encountered diff marker
 --> src/main.rs:2:1
  |
2 | <<<<<<< HEAD
  | ^^^^^^^ between this marker and `=======` is the code that you're rebasing onto
3 |     println!("Hello, main!");
4 | =======
  | ------- between this marker and `>>>>>>>` is the code from branch 'branch-2' that you are rebasing
5 |     println!("Hello, branch!");
6 | >>>>>>> e644375 (branch)
  | ^^^^^^^ this marker concludes the conflict region
  |
  = note: conflict markers indicate that a merge was started but could not be completed due to merge conflicts
          to resolve a conflict, keep only the code you want and then delete the lines containing conflict markers
  = note: for an explanation on these markers from the `git` documentation:
          visit <https://git-scm.com/book/en/v2/Git-Tools-Advanced-Merging#_checking_out_conflicts>

git merge case:

error: encountered diff marker
 --> src/main.rs:2:1
  |
2 | <<<<<<< HEAD
  | ^^^^^^^ between this marker and `=======` is the code from branch 'branch-2'
3 |     println!("Hello, branch!");
4 | =======
  | ------- between this marker and `>>>>>>>` is the code from branch 'master'
5 |     println!("Hello, main!");
6 | >>>>>>> master
  | ^^^^^^^ this marker concludes the conflict region
  |
  = note: conflict markers indicate that a merge was started but could not be completed due to merge conflicts
          to resolve a conflict, keep only the code you want and then delete the lines containing conflict markers
  = note: for an explanation on these markers from the `git` documentation:
          visit <https://git-scm.com/book/en/v2/Git-Tools-Advanced-Merging#_checking_out_conflicts>

git pull --rebase case:

error: encountered diff marker
 --> src/main.rs:2:1
  |
2 | <<<<<<< HEAD
  | ^^^^^^^ between this marker and `=======` is the code from branch 'main'
3 |     println!("Hello, 2!");
4 | =======
  | ------- between this marker and `>>>>>>>` is the code from remote branch 'main' of github.com:estebank/pull-merge-conflict
5 |     println!("Hello, 1!");
6 | >>>>>>> 8191e7e4f9f82be45bdd4e71c37d2adcf4f88aa2
  | ^^^^^^^ this marker concludes the conflict region
  |
  = note: conflict markers indicate that a merge was started but could not be completed due to merge conflicts
          to resolve a conflict, keep only the code you want and then delete the lines containing conflict markers
  = note: for an explanation on these markers from the `git` documentation:
          visit <https://git-scm.com/book/en/v2/Git-Tools-Advanced-Merging#_checking_out_conflicts>

git pull --no-rebase case:

error: encountered diff marker
 --> src/main.rs:2:1
  |
2 | <<<<<<< HEAD
  | ^^^^^^^ between this marker and `=======` is the code from branch 'main' of github.com:estebank/pull-merge-conflict
3 |     println!("Hello, 1!");
4 | =======
  | ------- between this marker and `>>>>>>>` is the code from local branch 'main' before `git pull`
5 |     println!("Hello, 2!");
6 | >>>>>>> ebbeec7 (second)
  | ^^^^^^^ this marker concludes the conflict region
  |
  = note: conflict markers indicate that a merge was started but could not be completed due to merge conflicts
          to resolve a conflict, keep only the code you want and then delete the lines containing conflict markers
  = note: for an explanation on these markers from the `git` documentation:
          visit <https://git-scm.com/book/en/v2/Git-Tools-Advanced-Merging#_checking_out_conflicts>

Also, the generic help description is now removed.

rustbot · 2025-12-21T21:22:12Z

r? @petrochenkov

rustbot has assigned @petrochenkov.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

estebank · 2025-12-21T21:22:29Z

I'll push a run-make test for the different cases being handled shortly.

This is what it looks like in my two test repos

git rebase case:

git pull case:

and the current output:

Edit: done.

compiler/rustc_parse/src/parser/diagnostics.rs

estebank · 2025-12-22T14:39:57Z

compiler/rustc_parse/src/parser/diagnostics.rs

+                    format!("code you had in commit `{orig_head_sha}` that you are rebasing");
+            }
+            if let Ok(output) =
+                Command::new("git").args(["branch", "--points-at", onto_sha.trim()]).output()


Both calls to git branch --points-at (locals only and remotes) will incur a linear cost on the number of branches that exist in the repo. This can be a problem as there are some projects with lots of branches. We have the time cost of filtering them and the potential of lots branches pointing to the same tip, which is handled somewhat already.

The decision on whether the risk is acceptable, or we should not even try, is one I am flip-flopping and keep arguing with myself.

I reached out to the git mailing list to see if they could add the equivalent of branch-name so that this is unnecessary.

Changed to work only if the current directory cargo was invoked from is the root git repo and git has a default path configuration.

jyn514 · 2025-12-25T07:38:29Z

compiler/rustc_parse/src/parser/diagnostics.rs

+        // conflict might not have been caused by a git rebase, and we'll fall-back on the default
+        // diagnostic.
+        let git = |path: &str| {
+            std::fs::read_to_string(format!(".git/{path}"))


now this is definitely broken, certainly on worktrees and submodules and likely in other cases too. the fallback is to just not emit the diagnostic, which is ok i guess, but it will make UI tests very fragile and generally i don’t think this behaves how you expect.

i would rather you run git —rev-parse tbh.

i would rather you run git —rev-parse tbh

I'll look into potentially using the appropriate sub-crate from gitoxide for this (if not just taking the appropriate logic), given how unlikely it will be for people to accept us shelling out to git.

Edit: it looks like it'd be https://crates.io/crates/gix-discover.

Edit 2: the list of new deps this would add is not ideal:

byteorder@1.5.0
gix-actor@0.36.1
gix-hashtable@0.11.0
gix-date@0.11.1
same-file@1.0.6
dunce@1.0.5
hash32@0.3.1
gix-object@0.53.0
gix-lock@20.0.0
walkdir@2.5.0
gix-utils@0.3.1
winapi-util@0.1.11
jiff-tzdb-platform@0.1.3
gix-validate@0.10.1
gix-trace@0.1.16
winnow@0.7.14
gix-fs@0.18.0
sha1-checked@0.10.0
prodash@30.0.1
gix-discover@0.44.0
faster-hex@0.10.0
gix-ref@0.56.0
gix-features@0.45.0
heapless@0.8.0
jiff-tzdb@0.1.5
gix-hash@0.21.0
gix-path@0.10.22
gix-sec@0.12.2
gix-tempfile@20.0.0

When detecting "diff markers", inspect the current `git` repo's state to see if there is an active rebase. If that is the case, we try to identify whether the rebase is caused by `git rebase` or implicitly by `git pull`. In order to do that we query `git` to ask for the path of `.git/rebase-merge` and then inspect the files `orig-head` (the commit we are rebasing from), `onto` (the commit we are rebasing onto) and `head-name` (the branch being rebased). We modify the labels pointing at the markers to be clear on where each section of code is coming from. `git rebase` case: ``` error: encountered diff marker --> src/main.rs:2:1 | 2 | <<<<<<< HEAD | ^^^^^^^ between this marker and `=======` is the code from branch `master` we're rebasing onto 3 | println!("Hello, main!"); 4 | ======= | ------- between this marker and `>>>>>>>` is the code you had in branch `branch-2` that you are rebasing 5 | println!("Hello, branch!"); 6 | >>>>>>> e644375 (branch) | ^^^^^^^ this marker concludes the conflict region | = note: conflict markers indicate that a merge was started but could not be completed due to merge conflicts to resolve a conflict, keep only the code you want and then delete the lines containing conflict markers = note: for an explanation on these markers from the `git` documentation: visit <https://git-scm.com/book/en/v2/Git-Tools-Advanced-Merging#_checking_out_conflicts> ``` `git pull` case: ``` error: encountered diff marker --> src/main.rs:2:1 | 2 | <<<<<<< HEAD | ^^^^^^^ between this marker and `=======` is the code from the remote branch `origin/main` we're rebasing onto 3 | println!("Hello, 1!"); 4 | ======= | ------- between this marker and `>>>>>>>` is the code you had in branch `main` that you are rebasing 5 | println!("Hello, 2!"); 6 | >>>>>>> ebbeec7 (second) | ^^^^^^^ this marker concludes the conflict region | = note: conflict markers indicate that a merge was started but could not be completed due to merge conflicts to resolve a conflict, keep only the code you want and then delete the lines containing conflict markers = note: for an explanation on these markers from the `git` documentation: visit <https://git-scm.com/book/en/v2/Git-Tools-Advanced-Merging#_checking_out_conflicts> ``` Also, the generic `help` description had the `git pull` case flipped from what it should have been.

The logic now only works if `cargo` is called from the git root, otherwise it won't find where the git metadata is. We rely purely on inspecting the files with rebase metadata and don't attempt to resolve a rebase target SHA to a branch name. Reword slightly the labels.

…rectory

rustbot · 2025-12-26T19:33:05Z

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

bors · 2026-01-02T03:49:38Z

☔ The latest upstream changes (presumably #150575) made this pull request unmergeable. Please resolve the merge conflicts.

rust-bors · 2026-01-02T16:33:16Z

☔ The latest upstream changes made this pull request unmergeable. Please resolve the merge conflicts.

rustbot assigned petrochenkov Dec 21, 2025

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Dec 21, 2025

estebank commented Dec 21, 2025

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compiler/rustc_parse/src/parser/diagnostics.rs Outdated Show resolved Hide resolved

estebank force-pushed the git-conflict branch from c38cd37 to 8e52082 Compare December 22, 2025 12:34

estebank mentioned this pull request Dec 22, 2025

Query git state to get information on a currently ongoing rebase when encountering conflict markers rust-lang/compiler-team#955

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estebank commented Dec 22, 2025

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Noratrieb added S-waiting-on-MCP Status: PR has a compiler MCP and is waiting for the compiler MCP to complete. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Dec 23, 2025

rustbot added the A-run-make Area: port run-make Makefiles to rmake.rs label Dec 23, 2025

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jyn514 reviewed Dec 25, 2025

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estebank mentioned this pull request Dec 25, 2025

Tweak wording of diff marker diagnostic #150382

Merged

estebank added 10 commits December 25, 2025 21:09

Fix tidy check

7fb15e2

Do not mention target commit when we already mention source branch

9bb5f17

Account for conflict markers caused by git merge

98c1f84

Add the necessary .git contents to the tests

aaff570

update test descriptions

352ba21

Tweak output and wording

4c11415

More information for git merge and git pull --no-rebase

345ec30

Find parent .git directory, even when cargo build is run from a subdi…

7deca7a

…rectory

estebank force-pushed the git-conflict branch from d84c2d6 to 7deca7a Compare December 26, 2025 19:33

bors added the S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. label Jan 2, 2026

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