Implement RTN in resolve_bound_vars and HIR ty lowering

compiler/rustc_ast_lowering/src/lib.rs

@@ -487,11 +487,12 @@ enum ParamMode {
 
 #[derive(Copy, Clone, Debug)]
 enum AllowReturnTypeNotation {
+    /// Only in types, since RTN is denied later during HIR lowering.
     Yes,
+    /// All other positions (path expr, method, use tree).
     No,
 }
 
-#[derive(Copy, Clone, Debug)]
 enum GenericArgsMode {
     ParenSugar,
     ReturnTypeNotation,
@@ -1239,7 +1240,6 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             qself,
             path,
             param_mode,
-            // We deny these after the fact in HIR->middle type lowering.
             AllowReturnTypeNotation::Yes,
             itctx,
             None,

compiler/rustc_ast_lowering/src/path.rs

@@ -16,10 +16,9 @@ use super::errors::{
     GenericTypeWithParentheses, UseAngleBrackets,
 };
 use super::{
-    AllowReturnTypeNotation, GenericArgsCtor, GenericArgsMode, ImplTraitContext, LifetimeRes,
-    LoweringContext, ParamMode, ResolverAstLoweringExt,
+    AllowReturnTypeNotation, GenericArgsCtor, GenericArgsMode, ImplTraitContext, ImplTraitPosition,
+    LifetimeRes, LoweringContext, ParamMode, ResolverAstLoweringExt,
 };
-use crate::ImplTraitPosition;
 
 impl<'a, 'hir> LoweringContext<'a, 'hir> {
     #[instrument(level = "trace", skip(self))]