Take 2 on use of global x in value position

Disallows "use" of the value of `global x`, except in tail position in
top level thunks - in those cases, return `nothing` so that `global x`
can be used in the value position of a top level thunk. This is normally
harmless as one cannot observe this value, except in special
circumstances - namely the return value of `eval()` (and things which
call eval, such as `include()`).

While we're thinking about this, also disallow a bare `local x` in a top
level thunk because this cannot have useful side effects and is just
confusing when it occurs outside a block construct. (This is not
currently disallowed for `local` arising from macro expansions because
it's not an obvious user error in that case. It could possibly arise as
a valid macro expansion of a trivial case, for some macros?)

Author: c42f

src/desugaring.jl

@@ -2173,9 +2173,6 @@ function expand_decls(ctx, ex)
             throw(LoweringError(ex, "invalid syntax in variable declaration"))
         end
     end
-    if kind(stmts[end]) in KSet"local global"
-        push!(stmts, @ast ctx ex "nothing"::K"core")
-    end
     makenode(ctx, ex, K"block", stmts)
 end
 

src/kinds.jl

@@ -117,6 +117,8 @@ function _register_kinds()
             # (That is, it's removable in the same sense as
             #  `@assume_effects :removable`.)
             "removable"
+            # Variable type declaration; `x::T = rhs` will be temporarily
+            # desugared to include `(decl x T)`
             "decl"
             # [K"captured_local" index]
             # A local variable captured into a global method. Contains the

src/linear_ir.jl

@@ -792,12 +792,20 @@ function compile(ctx::LinearIRContext, ex, needs_value, in_tail_pos)
         emit(ctx, ex)
         nothing
     elseif k == K"global"
-        if needs_value
-            throw(LoweringError(ex, "misplaced global declaration in value position"))
-        end
         emit(ctx, ex)
         ctx.is_toplevel_thunk && emit_latestworld(ctx, ex)
-        nothing
+        if needs_value
+            if in_tail_pos && ctx.is_toplevel_thunk
+                # Permit "statement-like" globals at top level but potentially
+                # inside blocks.
+                compile(ctx, nothing_(ctx, ex), needs_value, in_tail_pos)
+            else
+                throw(LoweringError(ex,
+                    "global declaration doesn't read the variable and can't return a value"))
+            end
+        else
+            nothing
+        end
     elseif k == K"meta"
         emit(ctx, ex)
         if needs_value

src/macro_expansion.jl

@@ -461,6 +461,11 @@ function expand_forms_1(ctx::MacroExpansionContext, ex::SyntaxTree)
 end
 
 @fzone "JL: macroexpand" function expand_forms_1(mod::Module, ex::SyntaxTree, expr_compat_mode::Bool, macro_world::UInt)
+    if kind(ex) == K"local"
+        # This error assumes we're expanding the body of a top level thunk but
+        # we might want to make that more explicit in the pass system.
+        throw(LoweringError(ex, "local declarations have no effect outside a scope"))
+    end
     graph = ensure_attributes(syntax_graph(ex),
                               var_id=IdTag,
                               scope_layer=LayerId,

src/scope_analysis.jl

@@ -427,7 +427,10 @@ function _resolve_scopes(ctx, ex::SyntaxTree)
     # elseif k == K"global"
     #     ex
     elseif k == K"local"
-        makeleaf(ctx, ex, K"TOMBSTONE")
+        # Local declarations have a value of `nothing` according to flisp
+        # lowering.
+        # TODO: Should local decls be disallowed in value position?
+        @ast ctx ex "nothing"::K"core"
     elseif k == K"decl"
         ex_out = mapchildren(e->_resolve_scopes(ctx, e), ctx, ex)
         name = ex_out[1]

test/decls.jl

@@ -11,12 +11,13 @@ end
 
 # In value position, yield the right hand side, not `x`
 @test JuliaLowering.include_string(test_mod, """
-local x::Int = 1.0
+begin
+    local x::Int = 1.0
+end
 """) === 1.0
 
-# Decsl in value position without assignment return nothing
+# Global decl in value position without assignment returns nothing
 @test JuliaLowering.include_string(test_mod, "global x_no_assign") === nothing
-@test JuliaLowering.include_string(test_mod, "local x_no_assign") === nothing
 
 # Unadorned declarations
 @test JuliaLowering.include_string(test_mod, """

test/decls_ir.jl

@@ -1,6 +1,8 @@
 ########################################
 # Local declaration with type
-local x::T = 1
+begin
+    local x::T = 1
+end
 #---------------------
 1   (newvar slot₁/x)
 2   1
@@ -18,20 +20,73 @@ local x::T = 1
 14  (return %₂)
 
 ########################################
-# Local declaration in value position without assignment
+# Error: Local declarations outside a scope are disallowed
+# See https://github.com/JuliaLang/julia/issues/57483
 local x
 #---------------------
+LoweringError:
+local x
+└─────┘ ── local declarations have no effect outside a scope
+
+########################################
+# Local declaration allowed in tail position
+begin
+    local x
+end
+#---------------------
 1   (newvar slot₁/x)
 2   (return core.nothing)
 
 ########################################
-# Global declaration in value position without assignment
+# Local declaration allowed in value position
+# TODO: This may be a bug in flisp lowering - should we reconsider this?
+let
+    y = local x
+end
+#---------------------
+1   (newvar slot₁/x)
+2   core.nothing
+3   (= slot₂/y %₂)
+4   (return %₂)
+
+########################################
+# Global declaration allowed in tail position
 global x
 #---------------------
 1   (global TestMod.x)
 2   latestworld
 3   (return core.nothing)
 
+########################################
+# Global declaration allowed in tail position, nested
+begin
+    global x
+end
+#---------------------
+1   (global TestMod.x)
+2   latestworld
+3   (return core.nothing)
+
+########################################
+# Error: Global declaration not allowed in tail position in functions
+function f()
+    global x
+end
+#---------------------
+LoweringError:
+function f()
+    global x
+#          ╙ ── global declaration doesn't read the variable and can't return a value
+end
+
+########################################
+# Error: Global declaration not allowed in value position
+y = global x
+#---------------------
+LoweringError:
+y = global x
+#          ╙ ── global declaration doesn't read the variable and can't return a value
+
 ########################################
 # const
 const xx = 10