Empty template spans break sourcemaps

[Septh]

I have set up a repro here: https://github.com/Septh/sucrase-broken-sourcemaps.

After cloning and installing, run node sucrase.js to compile the TypeScript code; you can then upload the resulting index.js file to Evan Wallace's excellent source-map-visualization tool.

Here's what I get:

(permalink)

As you can see, the original column number stays 'stuck' at the end of the first template string.

[Septh]

I did some digging and I think i figured out what's going on.

The issue arises only when the tokenizer generates zero-length tt.template tokens:

`foo ${var} bar`  // <- sourcemaps are correct
`foo ${var}`      // <- sourcemaps are wrong (last tt.template is 2:10-2:10)
`${var} bar`      // <- sourcemaps are wrong (first tt.template is 3:2-3:2)

(the line:column values come from the playground).

When a token is empty, the computeSourceColumns function in /src/computeSourceMap.ts fails to map the following tokens to their original source code positions because the condition at line 79 will never be true again.