Watch folders based on BuildState #8219

nojaf · 2026-01-23T14:32:24Z

Instead of watching the entire current directory, I would watch specific folders based on the build state instead.

Could fix #8215

(@cknitt should not block 13 alpha 1, would like to the reporter to test this out first)

Copilot

Pull request overview

This PR changes the file watching strategy from watching the entire project directory recursively to watching only specific folders based on the build state. This addresses issue #8215 where users with large non-source directories (like .direnv with millions of symlinks) were hitting file watcher limits.

Changes:

Modified watcher.rs to compute and watch specific paths (source folders, config files, lib directory) instead of the entire project root
Added logic to re-register watches when rescript.json changes, ensuring the watch list stays synchronized with the project configuration
Added three new comprehensive tests to verify new file detection, config change handling, and non-source file filtering
Updated documentation to explain debug logging for watch mode

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
rewatch/src/watcher.rs	Core implementation: added `compute_watch_paths`, `register_watches`, and `unregister_watches` functions; modified watch initialization and rebuild logic to support targeted watching
rewatch/tests/watch/03-watch-new-file.sh	Tests that new files created in watched source directories are detected and compiled
rewatch/tests/watch/04-watch-config-change.sh	Tests that config changes trigger full rebuilds and watch re-registration
rewatch/tests/watch/05-watch-ignores-non-source.sh	Tests that files created outside source directories are correctly ignored
rewatch/tests/suite.sh	Added new watch tests and stale process detection
AGENTS.md	Added documentation for debug logging in watch mode

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rewatch/src/watcher.rs

pkg-pr-new · 2026-01-23T15:26:38Z

Open in StackBlitz

rescript

npm i https://pkg.pr.new/rescript-lang/rescript@8219

@rescript/darwin-arm64

npm i https://pkg.pr.new/rescript-lang/rescript/@rescript/darwin-arm64@8219

@rescript/darwin-x64

npm i https://pkg.pr.new/rescript-lang/rescript/@rescript/darwin-x64@8219

@rescript/linux-arm64

npm i https://pkg.pr.new/rescript-lang/rescript/@rescript/linux-arm64@8219

@rescript/linux-x64

npm i https://pkg.pr.new/rescript-lang/rescript/@rescript/linux-x64@8219

@rescript/runtime

npm i https://pkg.pr.new/rescript-lang/rescript/@rescript/runtime@8219

@rescript/win32-x64

npm i https://pkg.pr.new/rescript-lang/rescript/@rescript/win32-x64@8219

commit: 16293b9

LTibbetts · 2026-01-23T18:22:59Z

This fixes the issues I had in #8215. See this comment

cknitt · 2026-01-24T07:56:23Z

@codex review

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chatgpt-codex-connector · 2026-01-24T08:01:45Z

rewatch/src/watcher.rs

+        for source in &package.source_folders {
+            let dir = package.path.join(&source.dir);
+            if !dir.exists() {
+                continue;
+            }


Watch missing source dirs to avoid missed recompiles

The new watch list skips configured source folders that don’t exist at startup (if !dir.exists() { continue; }). Because the package root is only watched non-recursively and the event loop ignores non‑ReScript paths, creating that directory later (or adding files inside it) won’t emit any watched events, so new sources in that folder won’t trigger a rebuild until some unrelated full compile happens. This is a regression from the previous recursive watch and affects workflows where a listed source dir or qualified subdir is created after rewatch watch starts.

Useful? React with 👍 / 👎.

@nojaf Is this a valid concern? Configuring a source folder in rescript.json, then starting a watcher, and only then actually creating the source folder?

Do we already log a warning about the configured, but non-existing source folder elsewhere?
Build/watch probably shouldn't even start if a configured source folder is missing?

Yep, good catch. The current behaviour was to show a log message.

ERROR: Could not read folder: "blah". Specified in dependency: foo, located "/Users/nojaf/Projects/foo"...

Restored this.

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Pull request overview

Copilot reviewed 12 out of 12 changed files in this pull request and generated 3 comments.

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rewatch/src/watcher.rs

Copilot · 2026-01-24T15:16:19Z

rewatch/tests/watch/06-watch-missing-source-folder.sh

+node -e "
+  const fs = require('fs');
+  const config = JSON.parse(fs.readFileSync('$DEP01_CONFIG', 'utf8'));
+  config.sources = [{dir: 'nonexistent-folder'}, config.sources];


The array construction on line 24 is incorrect. The expression [{dir: 'nonexistent-folder'}, config.sources] creates a two-element array where the second element is the entire original sources array, not a concatenation. This should be [{dir: 'nonexistent-folder'}, ...config.sources] (with spread operator) or [{dir: 'nonexistent-folder'}].concat(config.sources).

Suggested change

config.sources = [{dir: 'nonexistent-folder'}, config.sources];

config.sources = [{dir: 'nonexistent-folder'}, ...config.sources];

In dep01/rescript.json, config.sources is a single sourceItem object ({dir: "src", subdirs: true}). So [{dir: 'nonexistent-folder'}, config.sources] produces a valid two-element array of source items — both are objects with dir. This is correct according to the schema.

nojaf added 2 commits January 23, 2026 15:20

Watch folders based on BuildState

fec645d

Add additional test cases

2d29583

nojaf requested a review from Copilot January 23, 2026 14:32

Copilot AI reviewed Jan 23, 2026

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nojaf added 3 commits January 23, 2026 15:43

Code review feedback

531d17c

Make test cross plat (I hope)

89fa645

Avoids sleeps in test

1760df9

nojaf marked this pull request as ready for review January 23, 2026 20:57

nojaf requested a review from cknitt January 23, 2026 20:58

chatgpt-codex-connector bot reviewed Jan 24, 2026

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nojaf added 3 commits January 24, 2026 16:06

Log error when source folder does not exist.

607ea3d

Merge branch 'master' into revisit-watch

3ddf6fd

Add changelog entry

87a0312

nojaf requested a review from Copilot January 24, 2026 15:09

Copilot started reviewing on behalf of nojaf January 24, 2026 15:10 View session

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nojaf added 4 commits January 24, 2026 16:31

Code review feedback

9dbf9f1

Avoid race condition (I hope)

81bee98

Trigger CI

c9416ef

Merge branch 'master' into revisit-watch

16293b9

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Watch folders based on BuildState #8219

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	config.sources = [{dir: 'nonexistent-folder'}, config.sources];
	config.sources = [{dir: 'nonexistent-folder'}, ...config.sources];

Watch folders based on BuildState #8219

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Watch folders based on BuildState #8219

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