Rewatch: honour explicit empty features list on dependencies

compute_active_features conflated "no consumer edge found" with
"consumer requested empty". A qualified dependency with
`"features": []` would land in the same state as an unreached dep
(requested set empty, no `any_all_request`) and trip the fallback that
forced all features active. That made it impossible to express an
untagged-only dependency build.

Track `saw_consumer_entry` explicitly. The all-features fallback now
fires only when no consumer edge was observed; an empty requested set
from an explicit `"features": []` is honoured as "no feature-gated
dirs". Regression test covers the explicit-empty case. Docs updated.

Signed-off-by: Jaap Frolich <jaap@tella.com>

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Signed-off-by: Jaap Frolich <jfrolich@gmail.com>

Author: jfrolich

CHANGELOG.md

@@ -23,6 +23,7 @@
 #### :rocket: New Feature
 
 - Rewatch: add `--prod` flag to `build`, `watch`, and `clean` to skip dev-dependencies and dev sources (`"type": "dev"`), enabling builds in environments where dev packages aren't installed (e.g. after `pnpm install --prod`). https://github.com/rescript-lang/rescript/pull/8347
+- Rewatch: feature-gated source directories. Tag a source entry with `"feature": "<name>"` and select with `--features a,b` (or per-dep in `dependencies` / `dev-dependencies`) to include optional slices of a package's source tree at build time. Top-level `features` map supports transitive implications. https://github.com/rescript-lang/rescript/pull/8379
 - Add `Dict.assignMany`, `Dict.concat`, `Dict.concatMany`, `Dict.concatAll`, `Array.concatAll` to the stdlib. https://github.com/rescript-lang/rescript/pull/8364
 - Implement `for...of` and `for await...of` loops. https://github.com/rescript-lang/rescript/pull/7887
 - Add support for dict spreads: `dict{...foo, "bar": 2, ...qux}`. https://github.com/rescript-lang/rescript/pull/8369

rewatch/Features.md

@@ -65,7 +65,7 @@ When consuming another ReScript package that uses features, switch the entry in
 Rules:
 
 - **Shorthand (`"@plain/dep"`)** — the consumer wants every feature of that dependency. This is the existing behavior; nothing changes for configs that don't opt into features.
-- **Object with `features`** — the consumer restricts the dependency to the listed features (and whatever they transitively imply through the dependency's own `features` map).
+- **Object with `features`** — the consumer restricts the dependency to the listed features (and whatever they transitively imply through the dependency's own `features` map). An explicit empty list (`"features": []`) means "only untagged source dirs, no feature-gated code".
 - **Object without `features`** — equivalent to the shorthand. All features active.
 
 When the same dependency is referenced by multiple consumers with different feature sets, the union of requests wins. If any consumer asks for all features, the dependency builds with all of its features. Features are always additive — enabling more features never removes modules, so the union is always safe.

rewatch/src/build/packages.rs

@@ -780,6 +780,7 @@ pub fn compute_active_features(
 
     for (package_name, package) in packages {
         let mut any_all_request = false;
+        let mut saw_consumer_entry = false;
         let mut requested: AHashSet<String> = AHashSet::new();
 
         if package.is_root {
@@ -795,6 +796,7 @@ pub fn compute_active_features(
                         if dep.name() != package_name {
                             continue;
                         }
+                        saw_consumer_entry = true;
                         match dep.features() {
                             None => any_all_request = true,
                             Some(list) => requested.extend(list.iter().cloned()),
@@ -811,6 +813,7 @@ pub fn compute_active_features(
                         if dep.name() != package_name {
                             continue;
                         }
+                        saw_consumer_entry = true;
                         match dep.features() {
                             None => any_all_request = true,
                             Some(list) => requested.extend(list.iter().cloned()),
@@ -819,9 +822,10 @@ pub fn compute_active_features(
                 }
             }
 
-            // Defensive: a package with no consumers (shouldn't happen in practice, but don't
-            // accidentally strip everything) is treated as all-features.
-            if !any_all_request && requested.is_empty() {
+            // Defensive: if no consumer edge was found at all, keep all features. An empty
+            // `requested` set by itself is a *valid* request (`"features": []` means
+            // "untagged only"), so only fall back when we truly observed no entries.
+            if !saw_consumer_entry {
                 any_all_request = true;
             }
         }
@@ -1622,6 +1626,7 @@ mod test {
                 modules: None,
                 path: PathBuf::from("."),
                 dirs: None,
+                gentype_dirs: None,
                 is_local_dep: true,
                 is_root: true,
             },
@@ -1658,4 +1663,63 @@ mod test {
             "prod must not inherit the dev-dependency shorthand's all-features request"
         );
     }
+
+    #[test]
+    fn compute_active_features_honours_explicit_empty_features_list() {
+        // Regression: `{"name": "dep", "features": []}` must be honoured as "no features, only
+        // untagged dirs". Previously the fallback for "no consumer edges" fired when `requested`
+        // was empty, forcing all features on even though the consumer explicitly asked for none.
+        let mut packages: AHashMap<String, super::Package> = AHashMap::new();
+
+        let mut root_config = config::tests::create_config(config::tests::CreateConfigArgs {
+            name: "root".to_string(),
+            bs_deps: vec![],
+            build_dev_deps: vec![],
+            allowed_dependents: None,
+            path: PathBuf::from("./rescript.json"),
+        });
+        root_config.sources = Some(config::OneOrMore::Single(config::Source::Shorthand(
+            "src".to_string(),
+        )));
+        root_config.dependencies = Some(vec![config::Dependency::Qualified(config::QualifiedDependency {
+            name: "dep".to_string(),
+            features: Some(vec![]),
+        })]);
+        packages.insert(
+            "root".to_string(),
+            super::Package {
+                name: "root".to_string(),
+                config: root_config,
+                source_folders: AHashSet::new(),
+                source_files: None,
+                namespace: super::Namespace::NoNamespace,
+                modules: None,
+                path: PathBuf::from("."),
+                dirs: None,
+                gentype_dirs: None,
+                is_local_dep: true,
+                is_root: true,
+            },
+        );
+
+        packages.insert(
+            "dep".to_string(),
+            root_package_with_features(
+                None,
+                vec![
+                    ("src", None),
+                    ("src-native", Some("native")),
+                    ("src-experimental", Some("experimental")),
+                ],
+            ),
+        );
+        packages.get_mut("dep").unwrap().is_root = false;
+
+        let active = super::compute_active_features(&packages, None, false).unwrap();
+        let dep_active = active.get("dep").unwrap();
+        assert!(
+            dep_active.is_empty(),
+            "an explicit empty features list should activate no feature-tagged dirs, got {dep_active:?}"
+        );
+    }
 }

rewatch/src/config.rs

@@ -1661,7 +1661,10 @@ pub mod tests {
         "#;
 
         let config = Config::new_from_json_string(json).expect("a valid json string");
-        assert_eq!(config.dependencies, Some(vec!["@testrepo/main".to_string()]));
+        assert_eq!(
+            config.dependencies,
+            Some(vec![Dependency::Shorthand("@testrepo/main".to_string())])
+        );
         assert_eq!(config.get_deprecations(), [DeprecationWarning::BsDependencies]);
     }
 
@@ -1714,7 +1717,10 @@ pub mod tests {
         "#;
 
         let config = Config::new_from_json_string(json).expect("a valid json string");
-        assert_eq!(config.dev_dependencies, Some(vec!["@testrepo/main".to_string()]));
+        assert_eq!(
+            config.dev_dependencies,
+            Some(vec![Dependency::Shorthand("@testrepo/main".to_string())])
+        );
         assert_eq!(config.get_deprecations(), [DeprecationWarning::BsDevDependencies]);
     }
 

tests/build_tests/cli_help/input.js

@@ -44,6 +44,7 @@ const buildHelp =
   "  -a, --after-build <AFTER_BUILD>  Run an additional command after build. E.g., play a sound or run a test suite when done compiling\n" +
   "  -q, --quiet...                   Decrease logging verbosity\n" +
   '      --warn-error <WARN_ERROR>    Override warning configuration from rescript.json. Example: --warn-error "+3+8+11+12+26+27+31+32+33+34+35+39+44+45+110"\n' +
+  "      --features <FEATURES>        Restrict the current package to a comma-separated set of features. Only source directories tagged with one of these features (plus untagged ones, and features they transitively imply through the top-level `features` map) are compiled. Omit the flag to build with all features active. Example: --features native,experimental\n" +
   "  -n, --no-timing [<NO_TIMING>]    Disable output timing [default: false] [possible values: true, false]\n" +
   '      --prod                       Skip dev-dependencies and dev sources (type: "dev")\n' +
   "  -h, --help                       Print help\n";