fix(codegen): map sourcemaps from visible output starts (#21662)

## Summary

move statement sourcemap anchors to the first emitted token after generated indentation

## Testing
- cargo test -p oxc_codegen indented_statement_mappings_start_after_generated_indent -- --nocapture
- cargo test -p oxc_codegen class_member_mappings_start_before_member_keys -- --nocapture
- cargo test -p oxc_transformer --test integrations -- --nocapture

## AI Usage
- Codex was used to help prepare this change.

crates/oxc_codegen/src/gen.rs

@@ -200,8 +200,8 @@ impl Gen for ExpressionStatement<'_> {
     fn r#gen(&self, p: &mut Codegen, _ctx: Context) {
         p.print_comments_at(self.span.start);
         if !p.options.minify && (p.indent > 0 || p.print_next_indent_as_space) {
-            p.add_source_mapping(self.span);
             p.print_indent();
+            p.add_source_mapping(self.span);
         }
         p.start_of_stmt = p.code_len();
         p.print_expression(&self.expression);
@@ -212,7 +212,6 @@ impl Gen for ExpressionStatement<'_> {
 impl Gen for IfStatement<'_> {
     fn r#gen(&self, p: &mut Codegen, ctx: Context) {
         p.print_comments_at(self.span.start);
-        p.add_source_mapping(self.span);
         p.print_indent();
         print_if(self, p, ctx);
     }
@@ -220,6 +219,7 @@ impl Gen for IfStatement<'_> {
 
 fn print_if(if_stmt: &IfStatement<'_>, p: &mut Codegen, ctx: Context) {
     p.print_space_before_identifier();
+    p.add_source_mapping(if_stmt.span);
     p.print_str("if");
     p.print_soft_space();
     p.print_ascii_byte(b'(');
@@ -308,9 +308,9 @@ impl Gen for BlockStatement<'_> {
 impl Gen for ForStatement<'_> {
     fn r#gen(&self, p: &mut Codegen, ctx: Context) {
         p.print_comments_at(self.span.start);
-        p.add_source_mapping(self.span);
         p.print_indent();
         p.print_space_before_identifier();
+        p.add_source_mapping(self.span);
         p.print_str("for");
         p.print_soft_space();
         p.print_ascii_byte(b'(');
@@ -341,9 +341,9 @@ impl Gen for ForStatement<'_> {
 impl Gen for ForInStatement<'_> {
     fn r#gen(&self, p: &mut Codegen, ctx: Context) {
         p.print_comments_at(self.span.start);
-        p.add_source_mapping(self.span);
         p.print_indent();
         p.print_space_before_identifier();
+        p.add_source_mapping(self.span);
         p.print_str("for");
         p.print_soft_space();
         p.print_ascii_byte(b'(');
@@ -361,9 +361,9 @@ impl Gen for ForInStatement<'_> {
 impl Gen for ForOfStatement<'_> {
     fn r#gen(&self, p: &mut Codegen, ctx: Context) {
         p.print_comments_at(self.span.start);
-        p.add_source_mapping(self.span);
         p.print_indent();
         p.print_space_before_identifier();
+        p.add_source_mapping(self.span);
         p.print_str("for");
         if self.r#await {
             p.print_str(" await");
@@ -408,9 +408,9 @@ impl Gen for ForStatementLeft<'_> {
 impl Gen for WhileStatement<'_> {
     fn r#gen(&self, p: &mut Codegen, ctx: Context) {
         p.print_comments_at(self.span.start);
-        p.add_source_mapping(self.span);
         p.print_indent();
         p.print_space_before_identifier();
+        p.add_source_mapping(self.span);
         p.print_str("while");
         p.print_soft_space();
         p.print_ascii_byte(b'(');
@@ -423,9 +423,9 @@ impl Gen for WhileStatement<'_> {
 impl Gen for DoWhileStatement<'_> {
     fn r#gen(&self, p: &mut Codegen, ctx: Context) {
         p.print_comments_at(self.span.start);
-        p.add_source_mapping(self.span);
         p.print_indent();
         p.print_space_before_identifier();
+        p.add_source_mapping(self.span);
         p.print_str("do");
         match &self.body {
             Statement::BlockStatement(block) => {
@@ -455,8 +455,8 @@ impl Gen for DoWhileStatement<'_> {
 impl Gen for EmptyStatement {
     fn r#gen(&self, p: &mut Codegen, _ctx: Context) {
         p.print_comments_at(self.span.start);
-        p.add_source_mapping(self.span);
         p.print_indent();
+        p.add_source_mapping(self.span);
         p.print_semicolon();
         p.print_soft_newline();
     }
@@ -465,9 +465,9 @@ impl Gen for EmptyStatement {
 impl Gen for ContinueStatement<'_> {
     fn r#gen(&self, p: &mut Codegen, ctx: Context) {
         p.print_comments_at(self.span.start);
-        p.add_source_mapping(self.span);
         p.print_indent();
         p.print_space_before_identifier();
+        p.add_source_mapping(self.span);
         p.print_str("continue");
         if let Some(label) = &self.label {
             p.print_soft_space();
@@ -480,9 +480,9 @@ impl Gen for ContinueStatement<'_> {
 impl Gen for BreakStatement<'_> {
     fn r#gen(&self, p: &mut Codegen, ctx: Context) {
         p.print_comments_at(self.span.start);
-        p.add_source_mapping(self.span);
         p.print_indent();
         p.print_space_before_identifier();
+        p.add_source_mapping(self.span);
         p.print_str("break");
         if let Some(label) = &self.label {
             p.print_soft_space();
@@ -495,9 +495,9 @@ impl Gen for BreakStatement<'_> {
 impl Gen for SwitchStatement<'_> {
     fn r#gen(&self, p: &mut Codegen, ctx: Context) {
         p.print_comments_at(self.span.start);
-        p.add_source_mapping(self.span);
         p.print_indent();
         p.print_space_before_identifier();
+        p.add_source_mapping(self.span);
         p.print_str("switch");
         p.print_soft_space();
         p.print_ascii_byte(b'(');
@@ -548,9 +548,9 @@ impl Gen for SwitchCase<'_> {
 impl Gen for ReturnStatement<'_> {
     fn r#gen(&self, p: &mut Codegen, _ctx: Context) {
         p.print_comments_at(self.span.start);
-        p.add_source_mapping(self.span);
         p.print_indent();
         p.print_space_before_identifier();
+        p.add_source_mapping(self.span);
         p.print_str("return");
         if let Some(arg) = &self.argument {
             p.print_soft_space();
@@ -564,10 +564,10 @@ impl Gen for LabeledStatement<'_> {
     fn r#gen(&self, p: &mut Codegen, ctx: Context) {
         p.print_comments_at(self.span.start);
         if !p.options.minify && (p.indent > 0 || p.print_next_indent_as_space) {
-            p.add_source_mapping(self.span);
             p.print_indent();
         }
         p.print_space_before_identifier();
+        p.add_source_mapping(self.span);
         self.label.print(p, ctx);
         p.print_colon();
         p.print_body(&self.body, false, ctx);
@@ -577,9 +577,9 @@ impl Gen for LabeledStatement<'_> {
 impl Gen for TryStatement<'_> {
     fn r#gen(&self, p: &mut Codegen, ctx: Context) {
         p.print_comments_at(self.span.start);
-        p.add_source_mapping(self.span);
         p.print_indent();
         p.print_space_before_identifier();
+        p.add_source_mapping(self.span);
         p.print_str("try");
         p.print_soft_space();
         p.print_block_statement(&self.block, ctx);
@@ -626,9 +626,9 @@ impl Gen for CatchClause<'_> {
 impl Gen for ThrowStatement<'_> {
     fn r#gen(&self, p: &mut Codegen, _ctx: Context) {
         p.print_comments_at(self.span.start);
-        p.add_source_mapping(self.span);
         p.print_indent();
         p.print_space_before_identifier();
+        p.add_source_mapping(self.span);
         p.print_str("throw");
         p.print_soft_space();
         p.print_expression(&self.argument);
@@ -639,9 +639,9 @@ impl Gen for ThrowStatement<'_> {
 impl Gen for WithStatement<'_> {
     fn r#gen(&self, p: &mut Codegen, ctx: Context) {
         p.print_comments_at(self.span.start);
-        p.add_source_mapping(self.span);
         p.print_indent();
         p.print_space_before_identifier();
+        p.add_source_mapping(self.span);
         p.print_str("with");
         p.print_ascii_byte(b'(');
         p.print_expression(&self.object);
@@ -653,9 +653,9 @@ impl Gen for WithStatement<'_> {
 impl Gen for DebuggerStatement {
     fn r#gen(&self, p: &mut Codegen, _ctx: Context) {
         p.print_comments_at(self.span.start);
-        p.add_source_mapping(self.span);
         p.print_indent();
         p.print_space_before_identifier();
+        p.add_source_mapping(self.span);
         p.print_str("debugger");
         p.print_semicolon_after_statement();
     }

crates/oxc_codegen/tests/integration/sourcemap.rs

@@ -9,6 +9,81 @@ use oxc_span::{SourceType, Span};
 
 use crate::tester::default_options;
 
+#[derive(Clone, Copy)]
+struct Position {
+    line: u32,
+    col: u32,
+}
+
+#[derive(Clone, Copy)]
+struct Mapping {
+    dst: Position,
+    src: Position,
+}
+
+fn pos(line: u32, col: u32) -> Position {
+    Position { line, col }
+}
+
+fn sourcemap_tokens(source_text: &str, source_type: SourceType) -> Vec<Mapping> {
+    let allocator = Allocator::default();
+    let ret = Parser::new(&allocator, source_text, source_type).parse();
+    assert!(ret.errors.is_empty(), "parse errors: {:?}", ret.errors);
+
+    Codegen::new()
+        .with_options(default_options())
+        .build(&ret.program)
+        .map
+        .expect("sourcemap should be generated")
+        .get_tokens()
+        .map(|token| Mapping {
+            dst: pos(token.get_dst_line(), token.get_dst_col()),
+            src: pos(token.get_src_line(), token.get_src_col()),
+        })
+        .collect()
+}
+
+fn has_mapping(tokens: &[Mapping], src: Position, dst: Position) -> bool {
+    tokens.iter().any(|token| {
+        token.src.line == src.line
+            && token.src.col == src.col
+            && token.dst.line == dst.line
+            && token.dst.col == dst.col
+    })
+}
+
+fn first_generated_position_for_source(tokens: &[Mapping], src: Position) -> Option<Position> {
+    tokens
+        .iter()
+        .filter(|token| token.src.line == src.line && token.src.col == src.col)
+        .map(|token| token.dst)
+        .min_by_key(|position| (position.line, position.col))
+}
+
+fn assert_source_maps_after_indent(
+    tokens: &[Mapping],
+    src: Position,
+    wrong_dst: Position,
+    correct_dst: Position,
+) {
+    assert!(!has_mapping(tokens, src, wrong_dst));
+    assert!(has_mapping(tokens, src, correct_dst));
+}
+
+fn assert_member_start_maps_before_key(
+    tokens: &[Mapping],
+    member_start: Position,
+    key_start: Position,
+) {
+    let member_start = first_generated_position_for_source(tokens, member_start)
+        .expect("member start should be mapped");
+    let key_start = first_generated_position_for_source(tokens, key_start)
+        .expect("member key should be mapped");
+
+    assert_eq!(member_start.line, key_start.line);
+    assert!(member_start.col < key_start.col);
+}
+
 /// Upstream may have modified the AST to include incorrect spans.
 /// e.g. <https://github.com/rolldown/rolldown/blob/v1.0.0-beta.19/crates/rolldown/src/utils/ecma_visitors/mod.rs>
 #[test]
@@ -89,6 +164,37 @@ fn no_invalid_tokens_beyond_source() {
     }
 }
 
+#[test]
+fn indented_statement_mappings_start_after_generated_indent() {
+    let tokens = sourcemap_tokens(
+        r"if (foo) {
+  bar();
+}",
+        SourceType::mjs(),
+    );
+
+    assert_source_maps_after_indent(&tokens, pos(1, 2), pos(1, 0), pos(1, 1));
+}
+
+#[test]
+fn class_member_mappings_start_before_member_keys() {
+    let tokens = sourcemap_tokens(
+        r"class Foo {
+  get value() {
+    return 1;
+  }
+
+  static load() {
+    return 1;
+  }
+}",
+        SourceType::ts(),
+    );
+
+    assert_member_start_maps_before_key(&tokens, pos(1, 2), pos(1, 6));
+    assert_member_start_maps_before_key(&tokens, pos(5, 2), pos(5, 9));
+}
+
 #[test]
 #[cfg(all(not(target_endian = "big"), target_pointer_width = "64"))] // we run big endian tests on docker that does not have node installed; skip 32-bit as well
 fn stacktrace_is_correct() {