From e2047c94353a5d3880889cff441626db80e8f1f3 Mon Sep 17 00:00:00 2001
From: lauren <lauren@anysphere.co>
Date: Wed, 20 May 2026 17:35:30 -0700
Subject: [PATCH] [rust-compiler] Fix off-by-one in SWC source-location
 conversion

SWC's BytePos is 1-based (BytePos(0) is the DUMMY/synthetic sentinel)
while the Babel AST that the rest of the compiler consumes uses
0-based offsets in loc.{start,end}.{line,column,index}. `position()`
in the SWC convert_ast was binary-searching the 1-based offset
against the 0-based line_offsets table, producing line/column/index
values that were off by 1, and that flipped to "next line, column 0"
when the referenced byte happened to be the byte right before a `\n`.

Shift the offset down by 1 before the line lookup. BaseNode.start/end
keeps the SWC-native 1-based value because the SWC scope collector
keys its node_to_scope map on span.lo.0 and the HIR builder looks it
up via base.start.

Also drop the e2e harness's `oneBasedColumns = variant === 'swc'`
workaround that was masking part of this divergence.

Test plan:
- bash compiler/scripts/test-e2e.sh --variant swc:
    Before: Total 1682/1795 (113 failures)
    After:  Total 1742/1795 (53 failures, 60 fixed)
- bash compiler/scripts/test-e2e.sh --variant babel: 1788/1795 (unchanged)
- bash compiler/scripts/test-e2e.sh --variant oxc:   1702/1795 (unchanged)
- cargo test --workspace: 56 passed, 0 failed (unchanged)
---
 compiler/crates/react_compiler_swc/src/convert_ast.rs | 9 ++++++---
 compiler/scripts/test-e2e.ts                          | 4 +---
 2 files changed, 7 insertions(+), 6 deletions(-)

diff --git a/compiler/crates/react_compiler_swc/src/convert_ast.rs b/compiler/crates/react_compiler_swc/src/convert_ast.rs
index 8ced013e5442..4c77f2a15eb2 100644
--- a/compiler/crates/react_compiler_swc/src/convert_ast.rs
+++ b/compiler/crates/react_compiler_swc/src/convert_ast.rs
@@ -196,16 +196,19 @@ impl<'a> ConvertCtx<'a> {
         }
     }
 
+    /// `BytePos` is 1-based; emit 0-based `loc` to match Babel.
+    /// (`BaseNode.start`/`end` stays 1-based: `convert_scope` keys on it.)
     fn position(&self, offset: u32) -> Position {
-        let line_idx = match self.line_offsets.binary_search(&offset) {
+        let zero_based = offset.saturating_sub(1);
+        let line_idx = match self.line_offsets.binary_search(&zero_based) {
             Ok(idx) => idx,
             Err(idx) => idx.saturating_sub(1),
         };
         let line_start = self.line_offsets[line_idx];
         Position {
             line: (line_idx as u32) + 1,
-            column: offset - line_start,
-            index: Some(offset),
+            column: zero_based - line_start,
+            index: Some(zero_based),
         }
     }
 
diff --git a/compiler/scripts/test-e2e.ts b/compiler/scripts/test-e2e.ts
index 244bf4096bda..a010a86c4989 100644
--- a/compiler/scripts/test-e2e.ts
+++ b/compiler/scripts/test-e2e.ts
@@ -484,11 +484,9 @@ async function runVariant(
   for (let i = 0; i < fixtureInfos.length; i++) {
     const {fixturePath, relPath, source, firstLine, isFlow} = fixtureInfos[i];
     const tsCode = tsBaselines.get(fixturePath)!;
-    // TS baseline uses Babel (0-based columns), no adjustment needed
+    // All variants emit 0-based columns/indices.
     oneBasedColumns = false;
     const tsEvents = normalizeEvents(tsRawEvents.get(fixturePath)!);
-    // SWC uses 1-based columns/indices (BytePos); OXC is 0-based like Babel
-    oneBasedColumns = variant === 'swc';
 
     writeProgress(
       `  ${variant}: ${i + 1}/${fixtureInfos.length} (${s.passed} passed, ${s.failed} failed)`,