feat(minifier): inline const variables that are only used once

crates/oxc_ecmascript/src/constant_evaluation/mod.rs

@@ -99,6 +99,20 @@ pub trait ConstantEvaluation<'a>: MayHaveSideEffects<'a> {
     }
 }
 
+impl<'a, T: ConstantEvaluation<'a>> ConstantEvaluation<'a> for Option<T> {
+    fn evaluate_value(&self, ctx: &impl ConstantEvaluationCtx<'a>) -> Option<ConstantValue<'a>> {
+        self.as_ref().and_then(|t| t.evaluate_value(ctx))
+    }
+
+    fn evaluate_value_to(
+        &self,
+        ctx: &impl ConstantEvaluationCtx<'a>,
+        target_ty: Option<ValueType>,
+    ) -> Option<ConstantValue<'a>> {
+        self.as_ref().and_then(|t| t.evaluate_value_to(ctx, target_ty))
+    }
+}
+
 impl<'a> ConstantEvaluation<'a> for IdentifierReference<'a> {
     fn evaluate_value_to(
         &self,

crates/oxc_ecmascript/src/side_effects/may_have_side_effects.rs

@@ -22,6 +22,12 @@ pub trait MayHaveSideEffects<'a> {
     fn may_have_side_effects(&self, ctx: &impl MayHaveSideEffectsContext<'a>) -> bool;
 }
 
+impl<'a, T: MayHaveSideEffects<'a>> MayHaveSideEffects<'a> for Option<T> {
+    fn may_have_side_effects(&self, ctx: &impl MayHaveSideEffectsContext<'a>) -> bool {
+        self.as_ref().is_some_and(|t| t.may_have_side_effects(ctx))
+    }
+}
+
 impl<'a> MayHaveSideEffects<'a> for Expression<'a> {
     fn may_have_side_effects(&self, ctx: &impl MayHaveSideEffectsContext<'a>) -> bool {
         match self {

crates/oxc_minifier/src/ctx.rs

@@ -7,11 +7,11 @@ use oxc_ecmascript::{
     },
     side_effects::{MayHaveSideEffects, PropertyReadSideEffects},
 };
-use oxc_semantic::{IsGlobalReference, Scoping};
+use oxc_semantic::{IsGlobalReference, Scoping, SymbolId};
 use oxc_span::format_atom;
 use oxc_syntax::reference::ReferenceId;
 
-use crate::{options::CompressOptions, state::MinifierState};
+use crate::{options::CompressOptions, state::MinifierState, symbol_value::SymbolValue};
 
 pub type TraverseCtx<'a> = oxc_traverse::TraverseCtx<'a, MinifierState<'a>>;
 
@@ -50,7 +50,7 @@ impl<'a> oxc_ecmascript::is_global_reference::IsGlobalReference<'a> for Ctx<'a,
         self.scoping()
             .get_reference(reference_id)
             .symbol_id()
-            .and_then(|symbol_id| self.state.constant_values.get(&symbol_id))
+            .and_then(|symbol_id| self.state.symbol_values.get_constant_value(symbol_id))
             .cloned()
     }
 }
@@ -164,6 +164,24 @@ impl<'a> Ctx<'a, '_> {
         false
     }
 
+    pub fn init_value(&mut self, symbol_id: SymbolId, constant: ConstantValue<'a>) {
+        let mut read_references_count = 0;
+        let mut write_references_count = 0;
+        for r in self.scoping().get_resolved_references(symbol_id) {
+            if r.is_read() {
+                read_references_count += 1;
+            }
+            if r.is_write() {
+                write_references_count += 1;
+            }
+        }
+
+        let scope_id = self.scoping.current_scope_id();
+        let symbol_value =
+            SymbolValue { constant, read_references_count, write_references_count, scope_id };
+        self.state.symbol_values.init_value(symbol_id, symbol_value);
+    }
+
     /// If two expressions are equal.
     /// Special case `undefined` == `void 0`
     pub fn expr_eq(&self, a: &Expression<'a>, b: &Expression<'a>) -> bool {

crates/oxc_minifier/src/lib.rs

@@ -8,6 +8,7 @@ mod keep_var;
 mod options;
 mod peephole;
 mod state;
+mod symbol_value;
 
 #[cfg(test)]
 mod tester;

crates/oxc_minifier/src/peephole/fold_constants.rs

@@ -7,7 +7,6 @@ use oxc_ecmascript::{
 };
 use oxc_span::GetSpan;
 use oxc_syntax::operator::{BinaryOperator, LogicalOperator};
-use oxc_traverse::Ancestor;
 
 use crate::ctx::Ctx;
 
@@ -40,20 +39,6 @@ impl<'a> PeepholeOptimizations {
             *expr = folded_expr;
             ctx.state.changed = true;
         }
-
-        // Save `const value = false` into constant values.
-        if let Ancestor::VariableDeclaratorInit(decl) = ctx.parent() {
-            // TODO: Check for no write references.
-            if decl.kind().is_const() {
-                if let BindingPatternKind::BindingIdentifier(ident) = &decl.id().kind {
-                    // TODO: refactor all the above code to return value instead of expression, to avoid calling `evaluate_value` again.
-                    if let Some(value) = expr.evaluate_value(ctx) {
-                        let symbol_id = ident.symbol_id();
-                        ctx.state.constant_values.insert(symbol_id, value);
-                    }
-                }
-            }
-        }
     }
 
     #[expect(clippy::float_cmp)]

crates/oxc_minifier/src/peephole/inline.rs

@@ -0,0 +1,55 @@
+use oxc_ast::ast::*;
+use oxc_ecmascript::constant_evaluation::ConstantEvaluation;
+use oxc_span::GetSpan;
+
+use crate::ctx::Ctx;
+
+use super::PeepholeOptimizations;
+
+impl<'a> PeepholeOptimizations {
+    pub fn init_symbol_value(&self, decl: &VariableDeclarator<'a>, ctx: &mut Ctx<'a, '_>) {
+        let BindingPatternKind::BindingIdentifier(ident) = &decl.id.kind else { return };
+        let Some(symbol_id) = ident.symbol_id.get() else { return };
+        // Skip if not `const` variable.
+        if !ctx.scoping().symbol_flags(symbol_id).is_const_variable() {
+            return;
+        }
+        let Some(value) = decl.init.evaluate_value(ctx) else { return };
+        ctx.init_value(symbol_id, value);
+    }
+
+    pub fn inline_identifier_reference(&self, expr: &mut Expression<'a>, ctx: &mut Ctx<'a, '_>) {
+        let Expression::Identifier(ident) = expr else { return };
+        let Some(reference_id) = ident.reference_id.get() else { return };
+        let Some(symbol_id) = ctx.scoping().get_reference(reference_id).symbol_id() else { return };
+        let Some(symbol_value) = ctx.state.symbol_values.get_symbol_value(symbol_id) else {
+            return;
+        };
+        // Only inline single reference (for now).
+        if symbol_value.read_references_count > 1 {
+            return;
+        }
+        // Skip if there are write references.
+        if symbol_value.write_references_count > 0 {
+            return;
+        }
+        *expr = ctx.value_to_expr(expr.span(), symbol_value.constant.clone());
+        ctx.state.changed = true;
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use crate::{
+        CompressOptions,
+        tester::{test_options, test_same},
+    };
+
+    #[test]
+    fn r#const() {
+        let options = CompressOptions::smallest();
+        test_options("const foo = 1; log(foo)", "log(1)", &options);
+        test_options("export const foo = 1; log(foo)", "export const foo = 1; log(1)", &options);
+        test_same("const foo = 1; log(foo), log(foo)");
+    }
+}

crates/oxc_minifier/src/peephole/mod.rs

@@ -2,6 +2,7 @@
 
 mod convert_to_dotted_properties;
 mod fold_constants;
+mod inline;
 mod minimize_conditional_expression;
 mod minimize_conditions;
 mod minimize_expression_in_boolean_context;
@@ -102,10 +103,12 @@ impl<'a> PeepholeOptimizations {
 
 impl<'a> Traverse<'a, MinifierState<'a>> for PeepholeOptimizations {
     fn enter_program(&mut self, _program: &mut Program<'a>, ctx: &mut TraverseCtx<'a>) {
+        ctx.state.symbol_values.clear();
         ctx.state.changed = false;
     }
 
     fn exit_program(&mut self, program: &mut Program<'a>, ctx: &mut TraverseCtx<'a>) {
+        // Remove unused references by visiting the AST again and diff the collected references.
         let refs_before =
             ctx.scoping().resolved_references().flatten().copied().collect::<FxHashSet<_>>();
         let mut counter = ReferencesCounter::default();
@@ -155,18 +158,26 @@ impl<'a> Traverse<'a, MinifierState<'a>> for PeepholeOptimizations {
         ctx: &mut TraverseCtx<'a>,
     ) {
         let mut ctx = Ctx::new(ctx);
-
         self.substitute_variable_declaration(decl, &mut ctx);
     }
 
-    fn exit_expression(&mut self, expr: &mut Expression<'a>, ctx: &mut TraverseCtx<'a>) {
+    fn exit_variable_declarator(
+        &mut self,
+        decl: &mut VariableDeclarator<'a>,
+        ctx: &mut TraverseCtx<'a>,
+    ) {
         let mut ctx = Ctx::new(ctx);
+        self.init_symbol_value(decl, &mut ctx);
+    }
 
+    fn exit_expression(&mut self, expr: &mut Expression<'a>, ctx: &mut TraverseCtx<'a>) {
+        let mut ctx = Ctx::new(ctx);
         self.fold_constants_exit_expression(expr, &mut ctx);
         self.minimize_conditions_exit_expression(expr, &mut ctx);
         self.remove_dead_code_exit_expression(expr, &mut ctx);
         self.replace_known_methods_exit_expression(expr, &mut ctx);
         self.substitute_exit_expression(expr, &mut ctx);
+        self.inline_identifier_reference(expr, &mut ctx);
     }
 
     fn exit_unary_expression(&mut self, expr: &mut UnaryExpression<'a>, ctx: &mut TraverseCtx<'a>) {

crates/oxc_minifier/src/state.rs

@@ -1,25 +1,20 @@
-use rustc_hash::{FxHashMap, FxHashSet};
+use rustc_hash::FxHashSet;
 
-use oxc_ecmascript::constant_evaluation::ConstantValue;
-use oxc_semantic::SymbolId;
 use oxc_span::SourceType;
+use oxc_syntax::symbol::SymbolId;
 
-use crate::CompressOptions;
+use crate::{CompressOptions, symbol_value::SymbolValues};
 
 pub struct MinifierState<'a> {
     pub source_type: SourceType,
 
     pub options: CompressOptions,
 
-    /// Constant values evaluated from expressions.
-    ///
-    /// Values are saved during constant evaluation phase.
-    /// Values are read during [oxc_ecmascript::is_global_reference::IsGlobalReference::get_constant_value_for_reference_id].
-    pub constant_values: FxHashMap<SymbolId, ConstantValue<'a>>,
-
     /// Function declarations that are empty
     pub empty_functions: FxHashSet<SymbolId>,
 
+    pub symbol_values: SymbolValues<'a>,
+
     pub changed: bool,
 }
 
@@ -28,8 +23,8 @@ impl MinifierState<'_> {
         Self {
             source_type,
             options,
-            constant_values: FxHashMap::default(),
             empty_functions: FxHashSet::default(),
+            symbol_values: SymbolValues::default(),
             changed: false,
         }
     }

crates/oxc_minifier/src/symbol_value.rs

@@ -0,0 +1,39 @@
+use rustc_hash::FxHashMap;
+
+use oxc_ecmascript::constant_evaluation::ConstantValue;
+use oxc_syntax::{scope::ScopeId, symbol::SymbolId};
+
+#[derive(Debug)]
+pub struct SymbolValue<'a> {
+    /// Constant value evaluated from expressions.
+    pub constant: ConstantValue<'a>,
+
+    pub read_references_count: u32,
+    pub write_references_count: u32,
+
+    #[expect(unused)]
+    pub scope_id: ScopeId,
+}
+
+#[derive(Debug, Default)]
+pub struct SymbolValues<'a> {
+    values: FxHashMap<SymbolId, SymbolValue<'a>>,
+}
+
+impl<'a> SymbolValues<'a> {
+    pub fn clear(&mut self) {
+        self.values.clear();
+    }
+
+    pub fn init_value(&mut self, symbol_id: SymbolId, symbol_value: SymbolValue<'a>) {
+        self.values.insert(symbol_id, symbol_value);
+    }
+
+    pub fn get_constant_value(&self, symbol_id: SymbolId) -> Option<&ConstantValue<'a>> {
+        self.values.get(&symbol_id).map(|v| &v.constant)
+    }
+
+    pub fn get_symbol_value(&self, symbol_id: SymbolId) -> Option<&SymbolValue<'a>> {
+        self.values.get(&symbol_id)
+    }
+}

tasks/coverage/snapshots/minifier_test262.snap

@@ -2,4 +2,10 @@ commit: 4b5d36ab
 
 minifier_test262 Summary:
 AST Parsed     : 42013/42013 (100.00%)
-Positive Passed: 42013/42013 (100.00%)
+Positive Passed: 42010/42013 (99.99%)
+Compress: tasks/coverage/test262/test/intl402/Temporal/PlainDate/prototype/toLocaleString/lone-options-accepted.js
+
+Compress: tasks/coverage/test262/test/intl402/Temporal/PlainMonthDay/prototype/toLocaleString/lone-options-accepted.js
+
+Compress: tasks/coverage/test262/test/intl402/Temporal/PlainYearMonth/prototype/toLocaleString/lone-options-accepted.js
+

tasks/minsize/minsize.snap

@@ -11,9 +11,9 @@ Original   | minified   | minified   | gzip       | gzip       | Fixture
 
 544.10 kB  | 71.38 kB   | 72.48 kB   | 25.85 kB   | 26.20 kB   | lodash.js 
 
-555.77 kB  | 270.91 kB  | 270.13 kB  | 88.27 kB   | 90.80 kB   | d3.js     
+555.77 kB  | 270.90 kB  | 270.13 kB  | 88.25 kB   | 90.80 kB   | d3.js     
 
-1.01 MB    | 440.19 kB  | 458.89 kB  | 122.39 kB  | 126.71 kB  | bundle.min.js
+1.01 MB    | 440.15 kB  | 458.89 kB  | 122.37 kB  | 126.71 kB  | bundle.min.js
 
 1.25 MB    | 647.00 kB  | 646.76 kB  | 160.27 kB  | 163.73 kB  | three.js