Disallow non-lhs all-underscore variable names (#57626)

Addresses part of #57547. Currently, we only show the "all-underscore
variable name not allowed" error when the offending symbol is in value
position according to `compile`, but an earlier pass would keep them as
raw symbols regardless. This led to raw symbols making their way out of
lowering in an edge case.

This change: Reject all-underscore variables unless they're being
written to. Also, improve an error message slighly.

Author: mlechu

src/julia-syntax.scm

@@ -4659,9 +4659,9 @@ f(x) = yt(x)
           (let ((e1 (if (and arg-map (symbol? e))
                         (get arg-map e e)
                         e)))
-            (if (and value (or (underscore-symbol? e)
-                               (and (pair? e) (eq? (car e) 'globalref)
-                                    (underscore-symbol? (cadr e)))))
+            (if (or (underscore-symbol? e)
+                    (and (pair? e) (eq? (car e) 'globalref)
+                         (underscore-symbol? (cadr e))))
                 (error (string "all-underscore identifiers are write-only and their values cannot be used in expressions" (format-loc current-loc))))
             (cond (tail  (emit-return tail e1))
                   (value e1)

src/method.c

@@ -84,7 +84,8 @@ static jl_value_t *resolve_definition_effects(jl_value_t *expr, jl_module_t *mod
                                    int binding_effects, int eager_resolve)
 {
     if (jl_is_symbol(expr)) {
-        jl_error("Found raw symbol in code returned from lowering. Expected all symbols to have been resolved to GlobalRef or slots.");
+        jl_errorf("Found raw symbol %s in code returned from lowering. Expected all symbols to have been resolved to GlobalRef or slots.",
+                  jl_symbol_name((jl_sym_t*)expr));
     }
 
     if (!jl_is_expr(expr)) {

test/syntax.jl

@@ -2831,6 +2831,20 @@ end
 @m38386
 @test isempty(methods(f38386))
 
+@testset "non-lhs all-underscore vars should fail in lowering" begin
+    # OK
+    @test (_ = 1) === 1
+    @test ((_, _) = (1, 2)) == (1, 2)
+    @test Meta.isexpr(Meta.lower(Main, :(for _ in 1:2; 1; end)), :thunk)
+    @test (try; throw(1); catch _; 2; end) === 2
+    @test (let _ = 1; 2; end) === 2
+    # ERROR: syntax: all-underscore identifiers are write-only and their values cannot be used in expressions
+    @test Meta.isexpr(Meta.lower(Main, :(_ = 1; a = _)), :error)
+    @test Meta.isexpr(Meta.lower(Main, :(let; function f(); _; end; end)), :error)
+    @test Meta.isexpr(Meta.lower(Main, :(let; function f(); _; 1; end; end)), :error)
+    @test Meta.isexpr(Meta.lower(Main, :(begin; _; 1; end)), :error)
+end
+
 @testset "all-underscore varargs on the rhs" begin
     @test ncalls_in_lowered(quote _..., = a end, GlobalRef(Base, :rest)) == 0
     @test ncalls_in_lowered(quote ___..., = a end, GlobalRef(Base, :rest)) == 0