Link for `rustc --explain E\d+`

[martinthomson]

The compiler helpfully provides explanations for most errors. Turning this text into a link to the relevant text would be helpful.

I don't know where this text is online; at worst, you can run rustc, but that seems wasteful.

[shepmaster]

This is already present earlier in the compilation message:

I'd accept a PR to apply the same logic to the --explain line. It would be good to test it with multiple error types.

[pickfire]

Or we could even use cargo-explain for colors.