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The error message tells us not only that what we gave wasn’t a string, but also where the value originally came from. How would it know that?

Let’s run the open command again, but this time, we’ll look at the tags it gives back:

The span “start” and “end” here refer to where the underline will be in the line. If you count over 5, and then count up to 15, you’ll see it lines up with the “Cargo.toml” filename. This is how the error we saw earlier knew what to underline.