--source-maps=false generates empty, double stringified sourcemaps.

[xnuk]

$ echo > file.ts # empty file

$ npx swc file.ts --source-maps=false
Successfully compiled 1 file with swc.


//#sourceMappingURL=data:application/json;charset=utf-8;base64,IntcInZlcnNpb25cIjozLFwic291cmNlc1wiOltdLFwibmFtZXNcIjpbXSxcIm1hcHBpbmdzXCI6XCJcIixcImZpbGVcIjpcInN0ZG91dFwifSI=

$ npx swc file.ts --source-maps=inline
Successfully compiled 1 file with swc.

//# sourceMappingURL=data:application/json;base64,eyJ2ZXJzaW9uIjozLCJzb3VyY2VzIjpbXSwibmFtZXMiOltdLCJtYXBwaW5ncyI6IiJ9

//#sourceMappingURL=data:application/json;charset=utf-8;base64,IntcInZlcnNpb25cIjozLFwic291cmNlc1wiOltdLFwibmFtZXNcIjpbXSxcIm1hcHBpbmdzXCI6XCJcIixcImZpbGVcIjpcInN0ZG91dFwifSI=

$ npx swc --version

@swc/cli: 0.1.55
@swc/core: 1.2.122

And unsurprisingly:

atob('IntcInZlcnNpb25cIjozLFwic291cmNlc1wiOltdLFwibmFtZXNcIjpbXSxcIm1hcHBpbmdzXCI6XCJcIixcImZpbGVcIjpcInN0ZG91dFwifSI')
=== '"{\\"version\\":3,\\"sources\\":[],\\"names\\":[],\\"mappings\\":\\"\\",\\"file\\":\\"stdout\\"}"';

atob('eyJ2ZXJzaW9uIjozLCJzb3VyY2VzIjpbXSwibmFtZXMiOltdLCJtYXBwaW5ncyI6IiJ9')
=== '{"version":3,"sources":[],"names":[],"mappings":""}';

This PR tries to fix these thing by touching async function concatResults in src/swc/file with:

• not JSON.stringifiying source maps
• not generating empty sourcemaps, by lazily creating new SourceMapGenerator.

Merry... Christmas?

[xnuk]

It's about a year, and still reproducible in @swc/cli v0.1.57 + @swc/core v1.3.21. Could someone review this one? @swc-project