Invalid sourcemaps even without any AST changes

[mischnic]

Bug Report

• I would like to work on a fix or @DeMoorJasper might want to

Current behavior
This script parses a file and then stringifies the AST again without changing it:

const fs = require("fs");
const parse = require("@babel/parser").parse;
const generate = require("@babel/generator").default;

let inputCode = fs.readFileSync("index.js", "utf8");

let ast = parse(inputCode, { sourceFilename: "index.js" });

let { code: outputCode, map: outputMap } = generate(
	ast,
	{ sourceMaps: true },
	{ "index.js": inputCode }
);

fs.writeFileSync(
	"babel/index.js",
	outputCode + "\n//# sourceMappingURL=index.js.map"
);
fs.writeFileSync("babel/index.js.map", JSON.stringify(outputMap));

The bracket after the call arguments is mapped to the start of the line (just like console):

Input Code

console.log("Other!");

Expected behavior

For example Rollup's output:

Environment

  System:
    OS: macOS 10.15.5
  Binaries:
    Node: 14.14.0 - /usr/local/bin/node
    Yarn: 1.22.10 - /usr/local/bin/yarn
    npm: 6.14.8 - /usr/local/bin/npm
  npmPackages:
    @babel/core: ^7.12.3 => 7.12.3
    @babel/generator: ^7.12.5 => 7.12.5
    @babel/parser: ^7.12.5 => 7.12.5

Possible solution

It looks like characters that don't have their own AST node (e.g. brackets) just use the location of their parent node (CallExpression), without accounting for any child nodes that might be between the start of the parent and the character (the callee or the arguments).

Additional context
This might cause invalid sourcemaps with Parcel (after passing Babel's output to other tools).

Repo containing script & artifacts for Babel and Rollup: https://github.com/mischnic/parcel-invalid-sourcemaps

[babel-bot]

Hey @mischnic! We really appreciate you taking the time to report an issue. The collaborators on this project attempt to help as many people as possible, but we're a limited number of volunteers, so it's possible this won't be addressed swiftly.

If you need any help, or just have general Babel or JavaScript questions, we have a vibrant Slack community that typically always has someone willing to help. You can sign-up here for an invite."

[mischnic]

Looks like this is related to this function resetting the position after the argument is printed:

babel/packages/babel-generator/src/buffer.js

Lines 270 to 305 in 3227914

	withSource(prop: string, loc: Location, cb: () => void): void {
	if (!this._map) return cb();
	// Use the call stack to manage a stack of "source location" data because
	// the _sourcePosition object is mutated over the course of code generation,
	// and constantly copying it would be slower.
	const originalLine = this._sourcePosition.line;
	const originalColumn = this._sourcePosition.column;
	const originalFilename = this._sourcePosition.filename;
	const originalIdentifierName = this._sourcePosition.identifierName;
	this.source(prop, loc);
	cb();
	if (
	// If the current active position is forced, we only want to reactivate
	// the old position if it is different from the newest position.
	(!this._sourcePosition.force ||
	this._sourcePosition.line !== originalLine ||
	this._sourcePosition.column !== originalColumn ||
	this._sourcePosition.filename !== originalFilename) &&
	// Verify if reactivating this specific position has been disallowed.
	(!this._disallowedPop ||
	this._disallowedPop.line !== originalLine ||
	this._disallowedPop.column !== originalColumn ||
	this._disallowedPop.filename !== originalFilename)
	) {
	this._sourcePosition.line = originalLine;
	this._sourcePosition.column = originalColumn;
	this._sourcePosition.filename = originalFilename;
	this._sourcePosition.identifierName = originalIdentifierName;
	this._sourcePosition.force = false;
	this._disallowedPop = null;
	}
	}

[liuxingbaoyu]

fixed in #14967