Add sourcemap markings for each line of a string (#12086)

* Add sourcemap markings for each line of a string

Fixes https://github.com/babel/babel/issues/12083

* Fix for multiple newlines

* Optimize with indexOf

* Comment explaining newline search

packages/babel-generator/src/buffer.js

@@ -116,30 +116,54 @@ export default class Buffer {
     filename: ?string,
     force?: boolean,
   ): void {
-    // If there the line is ending, adding a new mapping marker is redundant
-    if (this._map && str[0] !== "\n") {
-      this._map.mark(
-        this._position.line,
-        this._position.column,
-        line,
-        column,
-        identifierName,
-        filename,
-        force,
-      );
-    }
-
     this._buf.push(str);
     this._last = str[str.length - 1];
 
-    for (let i = 0; i < str.length; i++) {
-      if (str[i] === "\n") {
-        this._position.line++;
-        this._position.column = 0;
-      } else {
-        this._position.column++;
-      }
+    // Search for newline chars. We search only for `\n`, since both `\r` and
+    // `\r\n` are normalized to `\n` during parse. We exclude `\u2028` and
+    // `\u2029` for performance reasons, they're so uncommon that it's probably
+    // ok. It's also unclear how other sourcemap utilities handle them...
+    let i = str.indexOf("\n");
+    let last = 0;
+
+    // If the string starts with a newline char, then adding a mark is redundant.
+    // This catches both "no newlines" and "newline after several chars".
+    if (i !== 0) {
+      this._mark(line, column, identifierName, filename, force);
     }
+
+    // Now, find each reamining newline char in the string.
+    while (i !== -1) {
+      this._position.line++;
+      this._position.column = 0;
+      last = i + 1;
+
+      // We mark the start of each line, which happens directly after this newline char
+      // unless this is the last char.
+      if (last < str.length) {
+        this._mark(++line, 0, identifierName, filename, force);
+      }
+      i = str.indexOf("\n", last);
+    }
+    this._position.column += str.length - last;
+  }
+
+  _mark(
+    line: number,
+    column: number,
+    identifierName: ?string,
+    filename: ?string,
+    force?: boolean,
+  ): void {
+    this._map?.mark(
+      this._position.line,
+      this._position.column,
+      line,
+      column,
+      identifierName,
+      filename,
+      force,
+    );
   }
 
   removeTrailingNewline(): void {

packages/babel-generator/test/fixtures/sourcemaps/string-literal-newline/input.js

@@ -0,0 +1,6 @@
+"before\
+after";
+
+"before\
+\
+after";

packages/babel-generator/test/fixtures/sourcemaps/string-literal-newline/output.js

@@ -0,0 +1,5 @@
+"before\
+after";
+"before\
+\
+after";

packages/babel-generator/test/fixtures/sourcemaps/string-literal-newline/source-map.json

@@ -0,0 +1,9 @@
+{
+  "mappings": "AAAA;AACA,MADA;AAGA;AACA;AACA,MAFA",
+  "names": [],
+  "sources": ["fixtures/sourcemaps/string-literal-newline/input.js"],
+  "sourcesContent": [
+    "\"before\\\nafter\";\n\n\"before\\\n\\\nafter\";"
+  ],
+  "version": 3
+}

packages/babel-generator/test/fixtures/sourcemaps/template-literal-newline/input.js

@@ -0,0 +1,27 @@
+// Newline
+`before
+after`;
+
+// Newline newline
+`before
+
+after`;
+
+// Newline LineContinuation
+`before
+\
+after`;
+
+// LineContinuation
+`before\
+after`;
+
+// LineContinuation newline
+`before\
+
+after`;
+
+// LineContinuation LineContinuation
+`before\
+\
+after`;

packages/babel-generator/test/fixtures/sourcemaps/template-literal-newline/output.js

@@ -0,0 +1,22 @@
+// Newline
+`before
+after`; // Newline newline
+
+`before
+
+after`; // Newline LineContinuation
+
+`before
+\
+after`; // LineContinuation
+
+`before\
+after`; // LineContinuation newline
+
+`before\
+
+after`; // LineContinuation LineContinuation
+
+`before\
+\
+after`;

packages/babel-generator/test/fixtures/sourcemaps/template-literal-newline/source-map.json

@@ -0,0 +1,9 @@
+{
+  "mappings": "AAAA;AACC;AACD,MADA,C,CAGA;;AACC;AACD;AACA,MAFA,C,CAIA;;AACC;AACD;AACA,MAFA,C,CAIA;;AACC;AACD,MADA,C,CAGA;;AACC;AACD;AACA,MAFA,C,CAIA;;AACC;AACD;AACA,MAFA",
+  "names": [],
+  "sources": ["fixtures/sourcemaps/template-literal-newline/input.js"],
+  "sourcesContent": [
+    "// Newline\n`before\nafter`;\n\n// Newline newline\n`before\n\nafter`;\n\n// Newline LineContinuation\n`before\n\\\nafter`;\n\n// LineContinuation\n`before\\\nafter`;\n\n// LineContinuation newline\n`before\\\n\nafter`;\n\n// LineContinuation LineContinuation\n`before\\\n\\\nafter`;"
+  ],
+  "version": 3
+}

packages/babel-generator/test/index.js

@@ -5,6 +5,7 @@ import * as t from "@babel/types";
 import fs from "fs";
 import path from "path";
 import fixtures from "@babel/helper-fixtures";
+import sourcemap from "source-map";
 
 describe("generation", function () {
   it("completeness", function () {
@@ -277,6 +278,48 @@ describe("generation", function () {
     expect(generated.code).toBe("function foo2() {\n  bar2;\n}");
   });
 
+  it("newline in template literal", () => {
+    const code = "`before\n\nafter`;";
+    const ast = parse(code, { filename: "inline" }).program;
+    const generated = generate(
+      ast,
+      {
+        filename: "inline",
+        sourceFileName: "inline",
+        sourceMaps: true,
+      },
+      code,
+    );
+
+    const consumer = new sourcemap.SourceMapConsumer(generated.map);
+    const loc = consumer.originalPositionFor({ line: 2, column: 1 });
+    expect(loc).toMatchObject({
+      column: 0,
+      line: 2,
+    });
+  });
+
+  it("newline in string literal", () => {
+    const code = "'before\\\n\\\nafter';";
+    const ast = parse(code, { filename: "inline" }).program;
+    const generated = generate(
+      ast,
+      {
+        filename: "inline",
+        sourceFileName: "inline",
+        sourceMaps: true,
+      },
+      code,
+    );
+
+    const consumer = new sourcemap.SourceMapConsumer(generated.map);
+    const loc = consumer.originalPositionFor({ line: 2, column: 1 });
+    expect(loc).toMatchObject({
+      column: 0,
+      line: 2,
+    });
+  });
+
   it("lazy source map generation", function () {
     const code = "function hi (msg) { console.log(msg); }\n";