Remove default scope layer for desugaring (#106)

When desugaring introduces a `K"Identifer"` it should always decorate it
with an associates scope layer - either adopted from the users code, or
an internal layer created on the fly.

This ensures desugaring treats hygiene consistently with macro
expansion (thus ensuring that desugaring itself is hygienic).

Author: c42f

src/ast.jl

@@ -70,6 +70,21 @@ Id for scope layers in macro expansion
 """
 const LayerId = Int
 
+"""
+A `ScopeLayer` is a mechanism for automatic hygienic macros; every identifier
+is assigned to a particular layer and can only match against bindings which are
+themselves part of that layer.
+
+Normal code contains a single scope layer, whereas each macro expansion
+generates a new layer.
+"""
+struct ScopeLayer
+    id::LayerId
+    mod::Module
+    parent_layer::LayerId # Index of parent layer in a macro expansion. Equal to 0 for no parent
+    is_macro_expansion::Bool # FIXME
+end
+
 #-------------------------------------------------------------------------------
 # AST creation utilities
 _node_id(graph::SyntaxGraph, ex::SyntaxTree) = (check_compatible_graph(graph, ex); ex._id)
@@ -500,6 +515,10 @@ function adopt_scope(ex::SyntaxTree, scope_layer::LayerId)
     set_scope_layer(ex, ex, scope_layer, true)
 end
 
+function adopt_scope(ex::SyntaxTree, layer::ScopeLayer)
+    adopt_scope(ex, layer.id)
+end
+
 function adopt_scope(ex::SyntaxTree, ref::SyntaxTree)
     adopt_scope(ex, ref.scope_layer)
 end

src/macro_expansion.jl

@@ -1,20 +1,5 @@
 # Lowering pass 1: Macro expansion, simple normalizations and quote expansion
 
-"""
-A `ScopeLayer` is a mechanism for automatic hygienic macros; every identifier
-is assigned to a particular layer and can only match against bindings which are
-themselves part of that layer.
-
-Normal code contains a single scope layer, whereas each macro expansion
-generates a new layer.
-"""
-struct ScopeLayer
-    id::LayerId
-    mod::Module
-    parent_layer::LayerId # Index of parent layer in a macro expansion. Equal to 0 for no parent
-    is_macro_expansion::Bool # FIXME
-end
-
 struct MacroExpansionContext{GraphType} <: AbstractLoweringContext
     graph::GraphType
     bindings::Bindings

src/runtime.jl

@@ -343,9 +343,11 @@ function (g::GeneratedFunctionStub)(world::UInt, source::Method, @nospecialize a
     macro_world = typemax(UInt)
     ctx1 = MacroExpansionContext(graph, __module__, false, macro_world)
 
+    layer = only(ctx1.scope_layers)
+
     # Run code generator - this acts like a macro expander and like a macro
     # expander it gets a MacroContext.
-    mctx = MacroContext(syntax_graph(ctx1), g.srcref, ctx1.scope_layers[end])
+    mctx = MacroContext(syntax_graph(ctx1), g.srcref, layer)
     ex0 = g.gen(mctx, args...)
     if ex0 isa SyntaxTree
         if !is_compatible_graph(ctx1, ex0)
@@ -370,10 +372,10 @@ function (g::GeneratedFunctionStub)(world::UInt, source::Method, @nospecialize a
     # Wrap expansion in a non-toplevel lambda and run scope resolution
     ex2 = @ast ctx2 ex0 [K"lambda"(is_toplevel_thunk=false, toplevel_pure=true)
         [K"block"
-            (string(n)::K"Identifier" for n in g.argnames)...
+            (adopt_scope(string(n)::K"Identifier", layer) for n in g.argnames)...
         ]
         [K"block"
-            (string(n)::K"Identifier" for n in g.spnames)...
+            (adopt_scope(string(n)::K"Identifier", layer) for n in g.spnames)...
         ]
         ex2
     ]

src/scope_analysis.jl

@@ -12,16 +12,9 @@ function Base.isless(a::NameKey, b::NameKey)
     (a.name, a.layer) < (b.name, b.layer)
 end
 
-# Identifiers produced by lowering will have the following layer by default.
-#
-# To make new mutable variables without colliding names, lowering can
-# - generate new var_id's directly (like the gensyms used by the old system)
-# - create additional layers, though this may be unnecessary
-const _lowering_internal_layer = -1
-
 function NameKey(ex::SyntaxTree)
     @chk kind(ex) == K"Identifier"
-    NameKey(ex.name_val, get(ex, :scope_layer, _lowering_internal_layer))
+    NameKey(ex.name_val, ex.scope_layer)
 end
 
 #-------------------------------------------------------------------------------