emit null source mappings for empty chunk content

Author: evanw

internal/linker/linker.go

@@ -5849,6 +5849,16 @@ func (c *linkerContext) generateChunkJS(chunkIndex int, chunkWaitGroup *sync.Wai
 			// Ignore empty source map chunks
 			if compileResult.SourceMapChunk.ShouldIgnore {
 				prevOffset.AdvanceBytes(compileResult.JS)
+
+				// Include a null entry in the source map
+				if c.options.SourceMap != config.SourceMapNone {
+					if n := len(compileResultsForSourceMap); n > 0 && !compileResultsForSourceMap[n-1].isNullEntry {
+						compileResultsForSourceMap = append(compileResultsForSourceMap, compileResultForSourceMap{
+							sourceIndex: compileResult.sourceIndex,
+							isNullEntry: true,
+						})
+					}
+				}
 			} else {
 				prevOffset = sourcemap.LineColumnOffset{}
 
@@ -6330,6 +6340,16 @@ func (c *linkerContext) generateChunkCSS(chunkIndex int, chunkWaitGroup *sync.Wa
 		// Ignore empty source map chunks
 		if compileResult.SourceMapChunk.ShouldIgnore {
 			prevOffset.AdvanceBytes(compileResult.CSS)
+
+			// Include a null entry in the source map
+			if c.options.SourceMap != config.SourceMapNone && compileResult.sourceIndex.IsValid() {
+				if n := len(compileResultsForSourceMap); n > 0 && !compileResultsForSourceMap[n-1].isNullEntry {
+					compileResultsForSourceMap = append(compileResultsForSourceMap, compileResultForSourceMap{
+						sourceIndex: compileResult.sourceIndex.GetIndex(),
+						isNullEntry: true,
+					})
+				}
+			}
 		} else {
 			prevOffset = sourcemap.LineColumnOffset{}
 
@@ -6902,6 +6922,7 @@ type compileResultForSourceMap struct {
 	sourceMapChunk  sourcemap.Chunk
 	generatedOffset sourcemap.LineColumnOffset
 	sourceIndex     uint32
+	isNullEntry     bool
 }
 
 func (c *linkerContext) generateSourceMapForChunk(
@@ -6929,6 +6950,9 @@ func (c *linkerContext) generateSourceMapForChunk(
 			continue
 		}
 		sourceIndexToSourcesIndex[result.sourceIndex] = nextSourcesIndex
+		if result.isNullEntry {
+			continue
+		}
 		file := &c.graph.Files[result.sourceIndex]
 
 		// Simple case: no nested source map
@@ -7044,28 +7068,38 @@ func (c *linkerContext) generateSourceMapForChunk(
 			startState.GeneratedColumn += prevColumnOffset
 		}
 
-		// Append the precomputed source map chunk
-		sourcemap.AppendSourceMapChunk(&j, prevEndState, startState, chunk.Buffer)
+		if result.isNullEntry {
+			// Emit a "null" mapping
+			chunk.Buffer.Data = []byte("A")
+			sourcemap.AppendSourceMapChunk(&j, prevEndState, startState, chunk.Buffer)
 
-		// Generate the relative offset to start from next time
-		prevOriginalName := prevEndState.OriginalName
-		prevEndState = chunk.EndState
-		prevEndState.SourceIndex += sourcesIndex
-		if chunk.Buffer.FirstNameOffset.IsValid() {
-			prevEndState.OriginalName += totalQuotedNameLen
+			// Only the generated position was advanced
+			prevEndState.GeneratedLine = startState.GeneratedLine
+			prevEndState.GeneratedColumn = startState.GeneratedColumn
 		} else {
-			// It's possible for a chunk to have mappings but for none of those
-			// mappings to have an associated name. The name is optional and is
-			// omitted when the mapping is for a non-name token or if the final
-			// and original names are the same. In that case we need to restore
-			// the previous original name end state since it wasn't modified after
-			// all. If we don't do this, then files after this will adjust their
-			// name offsets assuming that the previous generated mapping has this
-			// file's offset, which is wrong.
-			prevEndState.OriginalName = prevOriginalName
-		}
-		prevColumnOffset = chunk.FinalGeneratedColumn
-		totalQuotedNameLen += len(chunk.QuotedNames)
+			// Append the precomputed source map chunk
+			sourcemap.AppendSourceMapChunk(&j, prevEndState, startState, chunk.Buffer)
+
+			// Generate the relative offset to start from next time
+			prevOriginalName := prevEndState.OriginalName
+			prevEndState = chunk.EndState
+			prevEndState.SourceIndex += sourcesIndex
+			if chunk.Buffer.FirstNameOffset.IsValid() {
+				prevEndState.OriginalName += totalQuotedNameLen
+			} else {
+				// It's possible for a chunk to have mappings but for none of those
+				// mappings to have an associated name. The name is optional and is
+				// omitted when the mapping is for a non-name token or if the final
+				// and original names are the same. In that case we need to restore
+				// the previous original name end state since it wasn't modified after
+				// all. If we don't do this, then files after this will adjust their
+				// name offsets assuming that the previous generated mapping has this
+				// file's offset, which is wrong.
+				prevEndState.OriginalName = prevOriginalName
+			}
+			prevColumnOffset = chunk.FinalGeneratedColumn
+			totalQuotedNameLen += len(chunk.QuotedNames)
+		}
 
 		// If this was all one line, include the column offset from the start
 		if prevEndState.GeneratedLine == 0 {

internal/sourcemap/sourcemap.go

@@ -2,6 +2,7 @@ package sourcemap
 
 import (
 	"bytes"
+	"strings"
 	"unicode/utf8"
 
 	"github.com/evanw/esbuild/internal/ast"
@@ -426,20 +427,28 @@ func AppendSourceMapChunk(j *helpers.Joiner, prevEndState SourceMapState, startS
 	// case below instead. Original names are optional and are often omitted, so
 	// we handle it uniformly by saving an index to the first original name,
 	// which may or may not be a part of the first mapping.
+	var sourceIndex int
+	var originalLine int
+	var originalColumn int
+	omitSource := false
 	generatedColumn, i := DecodeVLQ(buffer.Data, semicolons)
-	sourceIndex, i := DecodeVLQ(buffer.Data, i)
-	originalLine, i := DecodeVLQ(buffer.Data, i)
-	originalColumn, i := DecodeVLQ(buffer.Data, i)
+	if i == len(buffer.Data) || strings.IndexByte(",;", buffer.Data[i]) != -1 {
+		omitSource = true
+	} else {
+		sourceIndex, i = DecodeVLQ(buffer.Data, i)
+		originalLine, i = DecodeVLQ(buffer.Data, i)
+		originalColumn, i = DecodeVLQ(buffer.Data, i)
+	}
 
 	// Rewrite the first mapping to be relative to the end state of the previous
 	// chunk. We now know what the end state is because we're in the second pass
 	// where all chunks have already been generated.
-	startState.SourceIndex += sourceIndex
 	startState.GeneratedColumn += generatedColumn
+	startState.SourceIndex += sourceIndex
 	startState.OriginalLine += originalLine
 	startState.OriginalColumn += originalColumn
 	prevEndState.HasOriginalName = false // This is handled separately below
-	rewritten, _ := appendMappingToBuffer(nil, j.LastByte(), prevEndState, startState)
+	rewritten, _ := appendMappingToBuffer(nil, j.LastByte(), prevEndState, startState, omitSource)
 	j.AddBytes(rewritten)
 
 	// Next, if there's an original name, we need to rewrite that as well to be
@@ -458,17 +467,21 @@ func AppendSourceMapChunk(j *helpers.Joiner, prevEndState SourceMapState, startS
 	j.AddBytes(buffer.Data[i:])
 }
 
-func appendMappingToBuffer(buffer []byte, lastByte byte, prevState SourceMapState, currentState SourceMapState) ([]byte, ast.Index32) {
+func appendMappingToBuffer(
+	buffer []byte, lastByte byte, prevState SourceMapState, currentState SourceMapState, omitSource bool,
+) ([]byte, ast.Index32) {
 	// Put commas in between mappings
 	if lastByte != 0 && lastByte != ';' && lastByte != '"' {
 		buffer = append(buffer, ',')
 	}
 
 	// Record the mapping (note that the generated line is recorded using ';' elsewhere)
 	buffer = encodeVLQ(buffer, currentState.GeneratedColumn-prevState.GeneratedColumn)
-	buffer = encodeVLQ(buffer, currentState.SourceIndex-prevState.SourceIndex)
-	buffer = encodeVLQ(buffer, currentState.OriginalLine-prevState.OriginalLine)
-	buffer = encodeVLQ(buffer, currentState.OriginalColumn-prevState.OriginalColumn)
+	if !omitSource {
+		buffer = encodeVLQ(buffer, currentState.SourceIndex-prevState.SourceIndex)
+		buffer = encodeVLQ(buffer, currentState.OriginalLine-prevState.OriginalLine)
+		buffer = encodeVLQ(buffer, currentState.OriginalColumn-prevState.OriginalColumn)
+	}
 
 	// Record the optional original name
 	var nameOffset ast.Index32
@@ -820,7 +833,7 @@ func (b *ChunkBuilder) appendMappingWithoutRemapping(currentState SourceMapState
 	}
 
 	var nameOffset ast.Index32
-	b.sourceMap, nameOffset = appendMappingToBuffer(b.sourceMap, lastByte, b.prevState, currentState)
+	b.sourceMap, nameOffset = appendMappingToBuffer(b.sourceMap, lastByte, b.prevState, currentState, false)
 	prevOriginalName := b.prevState.OriginalName
 	b.prevState = currentState
 	if !currentState.HasOriginalName {