Define λ through the renaming mechanism.

[NoahStoryM]

Checklist

• Feature
• tests included

Description of change

To put it simply, this PR has the following changes:

Before changes:

(#%provide new-lambda new-λ)

(define-syntaxes (new-lambda new-λ)
   (let ([new-lambda ...])
     (values new-lambda new-lambda)))

After changes:

(#%provide new-lambda (rename new-lambda new-λ))

(define-syntax (new-lambda stx) ...)

I didn't know how to check the impact of this change on all packages, so I just ran raco setup and found no exception.

See also #4989 (comment).

[racket-discourse-github-bot]

This pull request has been mentioned on Racket Discourse. There might be relevant details there:

https://racket.discourse.group/t/renaming-mechanism-for-unicode-in-racket/2910/1

[mflatt]

Given the potential to change expansion behavior, I think it would be worth adding a note to the documentation for lambda and λ to record the change. I suggest @history[#:changed "8.13.0.5" @elem{Adjusted binding so that @racket[(free-identifier=? #'λ #'lambda)] produces @racket[#t].}] added before the closing brace of line 2106 of syntax.scrbl.

[samth]

It looks like this broke a bunch of packages. A representative failure is here: https://plt.cs.northwestern.edu/pkg-build/server/built/fail/phc-toolkit.txt

The failures mostly are in Typed Racket-related packages, but reverting the changes since 8.13 in Typed Racket does not fix the problems.

[mflatt]

It's not clear that those failures are related to this PR. In fact, they report version 8.13.0.4, but I bumped the version to 8.13.0.5 just after this merge.

Based on the error message, it seems more likely that the failure is related to a847564. I'm able to replicate the error for phc-toolkit, so I'll investigate more.

[mflatt]

Commit a79cd80 is intended to repair an expander problem that caused trouble for TR-related packages, including phc-toolkit.