feat(minifier): merge expressions in for-in statement head (#8811)

Compress `a; for (var b in c) d` into `for (var b in a, c) d`. This is possible when the left hand does not have a sideeffectful initializer. (Initializers on the left hand of for-in is Annex B thing.)

**References**
- [Spec of `ForIn/OfHeadEvaluation`](https://tc39.es/ecma262/multipage/ecmascript-language-statements-and-declarations.html#sec-runtime-semantics-forinofheadevaluation): `c` in the example above is passed as `expr` to this abstract operation. No side effect exists before Step 3.
- [Spec of the initializer in ForIn](https://tc39.es/ecma262/multipage/additional-ecmascript-features-for-web-browsers.html#sec-initializers-in-forin-statement-heads): See "The runtime semantics of ForInOfLoopEvaluation in 14.7.5.5 are augmented with the following:" part. Evaluation of Initializer is executed before `ForIn/OfHeadEvalution`. This is the reason why it cannot be compressed when a sideeffectful initializer exists.

crates/oxc_minifier/src/peephole/collapse_variable_declarations.rs

@@ -202,18 +202,17 @@ mod test {
         }
 
         #[test]
-        #[ignore]
         fn test_for_in() {
-            test("var a; for(a in b) foo()", "for (var a in b) foo()");
+            // test("var a; for(a in b) foo()", "for (var a in b) foo()");
             test("a = 0; for(a in b) foo()", "for (a in a = 0, b) foo();");
-            test_same("var a = 0; for(a in b) foo()");
+            // test_same("var a = 0; for(a in b) foo()");
 
             // We don't handle labels yet.
-            test_same("var a; a:for(a in b) foo()");
-            test_same("var a; a:b:for(a in b) foo()");
+            // test_same("var a; a:for(a in b) foo()");
+            // test_same("var a; a:b:for(a in b) foo()");
 
             // Verify FOR inside IFs.
-            test("if(x){var a; for(a in b) foo()}", "if(x) for(var a in b) foo()");
+            // test("if(x){var a; for(a in b) foo()}", "if(x) for(var a in b) foo()");
 
             // Any other expression.
             test("init(); for(a in b) foo()", "for (a in init(), b) foo();");
@@ -222,7 +221,7 @@ mod test {
             test_same("function f(){ for(a in b) foo() }");
 
             // We don't handle destructuring patterns yet.
-            test("var a; var b; for ([a, b] in c) foo();", "var a, b; for ([a, b] in c) foo();");
+            // test("var a; var b; for ([a, b] in c) foo();", "var a, b; for ([a, b] in c) foo();");
         }
 
         #[test]

crates/oxc_minifier/src/peephole/minimize_statements.rs

@@ -1,5 +1,6 @@
 use oxc_allocator::Vec;
 use oxc_ast::{ast::*, Visit};
+use oxc_ecmascript::side_effects::MayHaveSideEffects;
 use oxc_span::{cmp::ContentEq, GetSpan};
 use oxc_traverse::Ancestor;
 
@@ -340,6 +341,37 @@ impl<'a> PeepholeOptimizations {
                 }
                 result.push(Statement::ForStatement(for_stmt));
             }
+            Statement::ForInStatement(mut for_in_stmt) => {
+                // "a; for (var b in c) d" => "for (var b in a, c) d"
+                if let Some(Statement::ExpressionStatement(prev_expr_stmt)) = result.last_mut() {
+                    // Annex B.3.5 allows initializers in non-strict mode
+                    // <https://tc39.es/ecma262/multipage/additional-ecmascript-features-for-web-browsers.html#sec-initializers-in-forin-statement-heads>
+                    // If there's a side-effectful initializer, we should not move the previous statement inside.
+                    let has_side_effectful_initializer = {
+                        if let ForStatementLeft::VariableDeclaration(var_decl) = &for_in_stmt.left {
+                            if var_decl.declarations.len() == 1 {
+                                // only var can have a initializer
+                                var_decl.kind.is_var()
+                                    && var_decl.declarations[0].init.as_ref().is_some_and(|init| {
+                                        ctx.expression_may_have_side_effects(init)
+                                    })
+                            } else {
+                                // the spec does not allow multiple declarations though
+                                true
+                            }
+                        } else {
+                            false
+                        }
+                    };
+                    if !has_side_effectful_initializer {
+                        let a = &mut prev_expr_stmt.expression;
+                        for_in_stmt.right = Self::join_sequence(a, &mut for_in_stmt.right, ctx);
+                        result.pop();
+                        self.mark_current_function_as_changed();
+                    }
+                }
+                result.push(Statement::ForInStatement(for_in_stmt));
+            }
             stmt => result.push(stmt),
         }
     }

crates/oxc_minifier/src/peephole/statement_fusion.rs

@@ -45,14 +45,18 @@ mod test {
     }
 
     #[test]
-    #[ignore]
     fn fuse_into_for_in1() {
         test("a;b;c;for(x in y){}", "for(x in a,b,c,y);");
     }
 
     #[test]
     fn fuse_into_for_in2() {
+        // this should not be compressed into `for (var x = a() in b(), [0])`
+        // as the side effect order of `a()` and `b()` changes
         test_same("a();for(var x = b() in y);");
+        test("a = 1; for(var x = 2 in y);", "for(var x = 2 in a = 1, y);");
+        // this can be compressed because b() runs after a()
+        test("a(); for (var { x = b() } in y);", "for (var { x = b() } in a(), y);");
     }
 
     #[test]

tasks/minsize/minsize.snap

@@ -5,23 +5,23 @@ Original   | minified   | minified   | gzip       | gzip       | Fixture
 
 173.90 kB  | 59.55 kB   | 59.82 kB   | 19.19 kB   | 19.33 kB   | moment.js 
 
-287.63 kB  | 89.49 kB   | 90.07 kB   | 30.95 kB   | 31.95 kB   | jquery.js 
+287.63 kB  | 89.48 kB   | 90.07 kB   | 30.95 kB   | 31.95 kB   | jquery.js 
 
-342.15 kB  | 117.68 kB  | 118.14 kB  | 43.56 kB   | 44.37 kB   | vue.js    
+342.15 kB  | 117.68 kB  | 118.14 kB  | 43.57 kB   | 44.37 kB   | vue.js    
 
 544.10 kB  | 71.43 kB   | 72.48 kB   | 25.87 kB   | 26.20 kB   | lodash.js 
 
 555.77 kB  | 271.25 kB  | 270.13 kB  | 88.35 kB   | 90.80 kB   | d3.js     
 
 1.01 MB    | 440.96 kB  | 458.89 kB  | 122.50 kB  | 126.71 kB  | bundle.min.js
 
-1.25 MB    | 650.36 kB  | 646.76 kB  | 161.02 kB  | 163.73 kB  | three.js  
+1.25 MB    | 650.36 kB  | 646.76 kB  | 161.01 kB  | 163.73 kB  | three.js  
 
-2.14 MB    | 718.61 kB  | 724.14 kB  | 162.14 kB  | 181.07 kB  | victory.js
+2.14 MB    | 718.61 kB  | 724.14 kB  | 162.13 kB  | 181.07 kB  | victory.js
 
 3.20 MB    | 1.01 MB    | 1.01 MB    | 324.31 kB  | 331.56 kB  | echarts.js
 
 6.69 MB    | 2.30 MB    | 2.31 MB    | 468.88 kB  | 488.28 kB  | antd.js   
 
-10.95 MB   | 3.37 MB    | 3.49 MB    | 863.73 kB  | 915.50 kB  | typescript.js
+10.95 MB   | 3.37 MB    | 3.49 MB    | 863.74 kB  | 915.50 kB  | typescript.js