exempt begin and underscore from macro hygiene (#40280)

Author: simeonschaub

src/macroexpand.scm

@@ -324,7 +324,7 @@
    m parent-scope inarg))
 
 (define (resolve-expansion-vars- e env m parent-scope inarg)
-  (cond ((or (eq? e 'end) (eq? e 'ccall) (eq? e 'cglobal))
+  (cond ((or (eq? e 'begin) (eq? e 'end) (eq? e 'ccall) (eq? e 'cglobal) (underscore-symbol? e))
          e)
         ((symbol? e)
          (let ((a (assq e env)))

test/syntax.jl

@@ -2751,3 +2751,15 @@ end
 @test eval(:(x = $(QuoteNode(Core.SlotNumber(1))))) == Core.SlotNumber(1)
 @test_throws ErrorException("syntax: SSAValue objects should not occur in an AST") eval(:(x = $(Core.SSAValue(1))))
 @test_throws ErrorException("syntax: Slot objects should not occur in an AST") eval(:(x = $(Core.SlotNumber(1))))
+
+macro m_underscore_hygiene()
+    return :(_ = 1)
+end
+
+@test @macroexpand(@m_underscore_hygiene()) == :(_ = 1)
+
+macro m_begin_hygiene(a)
+    return :($(esc(a))[begin])
+end
+
+@test @m_begin_hygiene([1, 2, 3]) == 1