Make levenshtein distance case insensitive.

[MidasLamb]

When typing in a single character shortcut as a capital, it always
returns b as the suggestion as every one-letter abbreviation
is a lev distance 1 away from the capitalized one.
By making the levenshtein distance case insensitive, the case-mismatched
one-letter abbriviation (e.g. C to c) will be suggested, rather
than b

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @alexcrichton (or someone else) soon.

Please see the contribution instructions for more information.

[alexcrichton]

Thanks for the PR! Do you know if there's precedent for doing something like this? Do other levenshtein-using "did you mean" systems also do case-insensitive compares?

[MidasLamb]

I couldn't find any precedent for this, I just noticed that I accidentally had caps lock on and I found it weird that it would suggest b when I typed C.
Conceptually I find that C to c should have less distance than C to b, but for Levenshtein a change is a change, and there is no metric for "how far" the letters are apart that it needs to change into.
Potentially we could change the scoring such that changing capitalization has a cost (i.e. 0.5) as a special case, but that would be a bigger and more complex change (this basically does the same thing except with a cost of 0 for change of capitalization).

[alexcrichton]

Ok well at least seems reasonable to me. Can you add comments to the .to_lowercase() calls though as to why they're being made?

[MidasLamb]

Will do!

[joshtriplett]

Looks good to me!

@bors r+

[bors]

📌 Commit f0992e3 has been approved by joshtriplett

[bors]

⌛ Testing commit f0992e3 with merge 1f12b88...

[bors]

☀️ Test successful - checks-actions
Approved by: joshtriplett
Pushing 1f12b88 to master...