expander: avoid equal?-based hash table for module cache

Since a thread can be terminated while accessing the cache, it
can't be an `equal?`-based table where the lock can get lost.

racket/src/expander/eval/module-cache.rkt

@@ -9,7 +9,7 @@
          module-cache-set!
          module-cache-ref)
 
-(define module-cache (make-weak-hash))
+(define module-cache (make-weak-hasheq))
 
 (define (make-module-cache-key hash-code)
   ;; The result is preserved to retain the cache entry, and
@@ -20,7 +20,10 @@
   ;; relative to the enclosing module, and that part of
   ;; the syntax object is unmarshaled once and used for
   ;; all instances of the module.
-  (and hash-code (list hash-code (current-load-relative-directory))))
+  (and hash-code
+       ;; Encode as a symbol so we can use an eq?-based hash table
+       ;; (i.e., explot the low-level lock on the symbol table)
+       (string->symbol (format "~s" (list hash-code (current-load-relative-directory))))))
 
 (define (module-cache-set! key proc)
   (hash-set! module-cache key (make-ephemeron key proc)))

racket/src/racket/src/startup.inc

@@ -33804,10 +33804,11 @@ static const char *startup_source =
 "(let-values(((or-part_265)(if insp_11(inspector-superior? insp_11 guard-insp_4) #f)))"
 "(if or-part_265 or-part_265(inspector-superior?(current-code-inspector) guard-insp_4)))))"
 "(let-values() #t))))))))))"
-"(define-values(module-cache)(make-weak-hash))"
+"(define-values(module-cache)(make-weak-hasheq))"
 "(define-values"
 "(make-module-cache-key)"
-"(lambda(hash-code_4)(begin(if hash-code_4(list hash-code_4(current-load-relative-directory)) #f))))"
+"(lambda(hash-code_4)"
+"    (begin (if hash-code_4 (string->symbol (format \"~s\" (list hash-code_4 (current-load-relative-directory)))) #f))))"
 "(define-values"
 "(module-cache-set!)"
 "(lambda(key_11 proc_9)(begin(hash-set! module-cache key_11(make-ephemeron key_11 proc_9)))))"