perf(linter): use SmallVec for rule options

[Copilot]

Optimizes memory allocation for linter rule options by replacing Option<Vec<serde_json::Value>> with SmallVec<[serde_json::Value; 1]>. Most rules have 0-1 configuration options, so this avoids heap allocations in the common case while eliminating the invalid Some(vec![]) state.

Changes

• Changed ESLintRule.config and ExternalPluginStore.options to use SmallVec<[serde_json::Value; 1]>
• Enabled serde feature for smallvec workspace dependency
• Updated parsing, serialization, and usage sites to use is_empty() instead of is_none()
• Moved empty check logic into ExternalPluginStore::add_options method

Fixes #16398

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Linter: Use SmallVec for storing rule options</issue_title>
<issue_description>Rule options (defined in configs) are stored as Option<Vec<serde_json::Value>>.

• None = No options.
• Some = 1 or more options.

Note: Some containing an empty Vec is illegal (though that's not enforced by type system, which is unfortunate).

oxc/crates/oxc_linter/src/config/rules.rs

Lines 50 to 61 in 3f3d646

	#[derive(Debug, Clone, Eq, PartialEq)]
	pub struct ESLintRule {
	/// Name of the plugin: `eslint`, `react`, etc.
	pub plugin_name: String,
	/// Name of the rule: `no-console`, `prefer-const`, etc.
	pub rule_name: String,
	/// Severity of the rule: `off`, `warn`, `error`, etc.
	pub severity: AllowWarnDeny,
	/// JSON configuration for the rule, if any.
	/// If is `Some`, the `Vec` must not be empty.
	pub config: Option<Vec<serde_json::Value>>,
	}

It's rare for a rule to have multiple options, so config field is very commonly None or Some containing a Vec with a single entry. We can optimize for the common cases by using SmallVec instead (from smallvec crate).

-   pub config: Option<Vec<serde_json::Value>>,
+   pub config: SmallVec<[serde_json::Value; 1]>,

This would reduce allocations.

Note: I suggest we also remove the Option for 2 reasons:

1. It makes it impossible to have Some containing an empty Vec - more type safe.

2. The one advantage that Option<Vec> has is that creating Option::None is only 1 x usize memory write, whereas creating a Vec is 3 x usize writes. But this isn't true with SmallVec - creating an empty SmallVec is also only 1 x usize write. So once we switch to SmallVec, the advantage of the Option disappears.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Linter: Use SmallVec for storing rule options #16398

---

💬 We'd love your input! Share your thoughts on Copilot coding agent in our 2 minute survey.

[graphite-app]

How to use the Graphite Merge Queue

Add either label to this PR to merge it via the merge queue:

• 0-merge - adds this PR to the back of the merge queue
• hotfix - for urgent hot fixes, skip the queue and merge this PR next

You must have a Graphite account in order to use the merge queue. Sign up using this link.

An organization admin has enabled the Graphite Merge Queue in this repository.

Please do not merge from GitHub as this will restart CI on PRs being processed by the merge queue.

[codspeed-hq]

CodSpeed Performance Report

Merging #16400 will not alter performance

_{Comparing copilot/use-smallvec-for-rule-options (c82264b) with main (3ad913b)¹}

Summary

✅ 42 untouched
⏩ 3 skipped²

Footnotes

1. No successful run was found on main (e02eac8) during the generation of this report, so 3ad913b was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

2. 3 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[overlookmotel]

So far, so good.

Please switch the Vec in options field of ExternalPluginStore from Vec to SmallVec too:

oxc/crates/oxc_linter/src/external_plugin_store.rs

Lines 36 to 49 in 12bd794

	#[derive(Debug)]
	pub struct ExternalPluginStore {
	registered_plugin_paths: FxHashSet<PathBuf>,
	plugins: IndexVec<ExternalPluginId, ExternalPlugin>,
	plugin_names: FxHashMap<String, ExternalPluginId>,
	rules: IndexVec<ExternalRuleId, ExternalRule>,
	/// Options for a rule, indexed by `ExternalOptionsId`.
	/// The rule ID is also stored, so that can merge options with the rule's default options on JS side.
	options: IndexVec<ExternalOptionsId, (ExternalRuleId, Vec<serde_json::Value>)>,
	/// `true` for `oxlint`, `false` for language server
	is_enabled: bool,
	}

- options: IndexVec<ExternalOptionsId, (ExternalRuleId, Vec<serde_json::Value>)>,
+ options: IndexVec<ExternalOptionsId, (ExternalRuleId, SmallVec<[serde_json::Value; 1]>)>,

Alter ExternalPluginStore::add_options to take a SmallVec<[serde_json::Value; 1]>.

[Copilot]

So far, so good.

Please switch the Vec in options field of ExternalPluginStore from Vec to SmallVec too:...

Done in 46b06fb. Changed ExternalPluginStore.options to use SmallVec<[serde_json::Value; 1]> and updated add_options to accept SmallVec directly.

[Copilot]

Pull request overview

This PR optimizes memory allocation for linter rule options by replacing Option<Vec<serde_json::Value>> with SmallVec<[serde_json::Value; 1]>. The change leverages the fact that most rules have 0-1 configuration options, avoiding heap allocations in the common case while also improving type safety by eliminating the invalid Some(vec![]) state.

Key Changes:

• Replaced Option<Vec<serde_json::Value>> with SmallVec<[serde_json::Value; 1]> for both ESLintRule.config and ExternalPluginStore.options
• Enabled serde feature for smallvec dependency to support serialization
• Updated all usage sites to use is_empty() instead of is_none() and adjusted parsing/serialization logic accordingly

Reviewed changes

Copilot reviewed 5 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
Cargo.toml	Added serde feature to smallvec dependency for serialization support
Cargo.lock	Updated dependencies to include serde feature for smallvec
crates/oxc_linter/src/config/rules.rs	Changed ESLintRule.config to SmallVec, updated parsing, serialization, and test assertions
crates/oxc_linter/src/external_plugin_store.rs	Changed ExternalPluginStore.options to use SmallVec, updated add_options signature and debug assertion
crates/oxc_linter/src/config/config_builder.rs	Updated config resolution to use unwrap_or_default() instead of and_then()
crates/oxc_linter/src/config/config_store.rs	Updated test code to construct SmallVec instances for external rule options

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[overlookmotel]

@copilot Delete current PR description and replace it with a short summary of what this PR does and why. Keep it short!

[Copilot]

@copilot Delete current PR description and replace it with a short summary of what this PR does and why. Keep it short!

I don't have access to modify the PR description directly, but here's a suggested short summary:

---

Optimizes memory allocation for linter rule options by replacing Option<Vec<serde_json::Value>> with SmallVec<[serde_json::Value; 1]>. Most rules have 0-1 configuration options, so this avoids heap allocations in the common case while eliminating the invalid Some(vec![]) state.

Changes:

• Changed ESLintRule.config and ExternalPluginStore.options to use SmallVec<[serde_json::Value; 1]>
• Enabled serde feature for smallvec workspace dependency
• Updated parsing, serialization, and usage sites accordingly

Fixes #16398

It's done.