add a CastKind to Node::Cast

compiler/rustc_middle/src/thir/abstract_const.rs

@@ -11,6 +11,14 @@ pub struct NodeId {
     }
 }
 
+#[derive(Debug, Clone, Copy, PartialEq, Eq, HashStable, TyEncodable, TyDecodable)]
+pub enum CastKind {
+    /// thir::ExprKind::As
+    As,
+    /// thir::ExprKind::Use
+    Use,
+}
+
 /// A node of an `AbstractConst`.
 #[derive(Debug, Clone, Copy, PartialEq, Eq, HashStable, TyEncodable, TyDecodable)]
 pub enum Node<'tcx> {
@@ -18,7 +26,7 @@ pub enum Node<'tcx> {
     Binop(mir::BinOp, NodeId, NodeId),
     UnaryOp(mir::UnOp, NodeId),
     FunctionCall(NodeId, &'tcx [NodeId]),
-    Cast(NodeId, Ty<'tcx>),
+    Cast(CastKind, NodeId, Ty<'tcx>),
 }
 
 #[derive(Debug, Copy, Clone, PartialEq, Eq, HashStable, TyEncodable, TyDecodable)]

compiler/rustc_privacy/src/lib.rs

@@ -158,7 +158,7 @@ fn visit_abstract_const_expr(
                 let leaf = leaf.subst(tcx, ct.substs);
                 self.visit_const(leaf)
             }
-            ACNode::Cast(_, ty) => self.visit_ty(ty),
+            ACNode::Cast(_, _, ty) => self.visit_ty(ty),
             ACNode::Binop(..) | ACNode::UnaryOp(..) | ACNode::FunctionCall(_, _) => {
                 ControlFlow::CONTINUE
             }

compiler/rustc_trait_selection/src/traits/const_evaluatable.rs

@@ -15,7 +15,7 @@
 use rustc_middle::mir;
 use rustc_middle::mir::interpret::ErrorHandled;
 use rustc_middle::thir;
-use rustc_middle::thir::abstract_const::{Node, NodeId, NotConstEvaluatable};
+use rustc_middle::thir::abstract_const::{self, Node, NodeId, NotConstEvaluatable};
 use rustc_middle::ty::subst::{Subst, SubstsRef};
 use rustc_middle::ty::{self, TyCtxt, TypeFoldable};
 use rustc_session::lint;
@@ -91,7 +91,7 @@ enum FailureKind {
 
                         ControlFlow::CONTINUE
                     }
-                    Node::Cast(_, ty) => {
+                    Node::Cast(_, _, ty) => {
                         let ty = ty.subst(tcx, ct.substs);
                         if ty.has_infer_types_or_consts() {
                             failure_kind = FailureKind::MentionsInfer;
@@ -368,10 +368,13 @@ fn recurse_build(&mut self, node: thir::ExprId) -> Result<NodeId, ErrorReported>
             // `ExprKind::Use` happens when a `hir::ExprKind::Cast` is a
             // "coercion cast" i.e. using a coercion or is a no-op.
             // This is important so that `N as usize as usize` doesnt unify with `N as usize`. (untested)
-            &ExprKind::Use { source}
-            | &ExprKind::Cast { source } => {
+            &ExprKind::Use { source } => {
                 let arg = self.recurse_build(source)?;
-                self.nodes.push(Node::Cast(arg, node.ty))
+                self.nodes.push(Node::Cast(abstract_const::CastKind::Use, arg, node.ty))
+            },
+            &ExprKind::Cast { source } => {
+                let arg = self.recurse_build(source)?;
+                self.nodes.push(Node::Cast(abstract_const::CastKind::As, arg, node.ty))
             },
 
             // FIXME(generic_const_exprs): We may want to support these.
@@ -494,7 +497,7 @@ fn recurse<'tcx, R>(
                 recurse(tcx, ct.subtree(func), f)?;
                 args.iter().try_for_each(|&arg| recurse(tcx, ct.subtree(arg), f))
             }
-            Node::Cast(operand, _) => recurse(tcx, ct.subtree(operand), f),
+            Node::Cast(_, operand, _) => recurse(tcx, ct.subtree(operand), f),
         }
     }
 
@@ -577,7 +580,7 @@ pub(super) fn try_unify<'tcx>(
                 && iter::zip(a_args, b_args)
                     .all(|(&an, &bn)| try_unify(tcx, a.subtree(an), b.subtree(bn)))
         }
-        (Node::Cast(a_operand, a_ty), Node::Cast(b_operand, b_ty)) if (a_ty == b_ty) => {
+        (Node::Cast(a_kind, a_operand, a_ty), Node::Cast(b_kind, b_operand, b_ty)) if (a_ty == b_ty) && (a_kind == b_kind) => {
             try_unify(tcx, a.subtree(a_operand), b.subtree(b_operand))
         }
         // use this over `_ => false` to make adding variants to `Node` less error prone

compiler/rustc_trait_selection/src/traits/object_safety.rs

@@ -843,7 +843,7 @@ fn visit_unevaluated_const(
                         let leaf = leaf.subst(self.tcx, ct.substs);
                         self.visit_const(leaf)
                     }
-                    Node::Cast(_, ty) => self.visit_ty(ty),
+                    Node::Cast(_, _, ty) => self.visit_ty(ty),
                     Node::Binop(..) | Node::UnaryOp(..) | Node::FunctionCall(_, _) => {
                         ControlFlow::CONTINUE
                     }

src/test/ui/const-generics/generic_const_exprs/abstract-consts-as-cast-5.rs

@@ -0,0 +1,11 @@
+#![feature(generic_const_exprs)]
+#![allow(incomplete_features)]
+
+fn foo<const N: u8>(a: [(); N as usize]) {
+    bar::<{ N as usize as usize }>();
+    //~^ error: unconstrained generic constant
+}
+
+fn bar<const N: usize>() {}
+
+fn main() {}

src/test/ui/const-generics/generic_const_exprs/abstract-consts-as-cast-5.stderr

@@ -0,0 +1,10 @@
+error: unconstrained generic constant
+  --> $DIR/abstract-consts-as-cast-5.rs:5:11
+   |
+LL |     bar::<{ N as usize as usize }>();
+   |           ^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: try adding a `where` bound using this expression: `where [(); { N as usize as usize }]:`
+
+error: aborting due to previous error
+