const_eval_checked: Support as casts in abstract consts

compiler/rustc_middle/src/mir/abstract_const.rs

@@ -1,6 +1,6 @@
 //! A subset of a mir body used for const evaluatability checking.
-use crate::mir;
-use crate::ty;
+use crate::mir::{self, CastKind};
+use crate::ty::{self, Ty};
 
 rustc_index::newtype_index! {
     /// An index into an `AbstractConst`.
@@ -17,6 +17,7 @@ pub enum Node<'tcx> {
     Binop(mir::BinOp, NodeId, NodeId),
     UnaryOp(mir::UnOp, NodeId),
     FunctionCall(NodeId, &'tcx [NodeId]),
+    Cast(CastKind, NodeId, Ty<'tcx>),
 }
 
 #[derive(Debug, Copy, Clone, PartialEq, Eq, HashStable, TyEncodable, TyDecodable)]

compiler/rustc_privacy/src/lib.rs

@@ -156,9 +156,10 @@ where
                 let leaf = leaf.subst(tcx, ct.substs);
                 self.visit_const(leaf)
             }
-            ACNode::Binop(..) | ACNode::UnaryOp(..) | ACNode::FunctionCall(_, _) => {
-                ControlFlow::CONTINUE
-            }
+            ACNode::Binop(..)
+            | ACNode::UnaryOp(..)
+            | ACNode::FunctionCall(_, _)
+            | ACNode::Cast(_, _, _) => ControlFlow::CONTINUE,
         })
     }
 

compiler/rustc_trait_selection/src/traits/const_evaluatable.rs

@@ -97,9 +97,10 @@ pub fn is_const_evaluatable<'cx, 'tcx>(
 
                         ControlFlow::CONTINUE
                     }
-                    Node::Binop(_, _, _) | Node::UnaryOp(_, _) | Node::FunctionCall(_, _) => {
-                        ControlFlow::CONTINUE
-                    }
+                    Node::Binop(_, _, _)
+                    | Node::UnaryOp(_, _)
+                    | Node::FunctionCall(_, _)
+                    | Node::Cast(_, _, _) => ControlFlow::CONTINUE,
                 });
 
                 match failure_kind {
@@ -304,6 +305,9 @@ impl<'a, 'tcx> AbstractConstBuilder<'a, 'tcx> {
                 self.nodes[func].used = true;
                 nodes.iter().for_each(|&n| self.nodes[n].used = true);
             }
+            Node::Cast(_, operand, _) => {
+                self.nodes[operand].used = true;
+            }
         }
 
         // Nodes start as unused.
@@ -408,6 +412,12 @@ impl<'a, 'tcx> AbstractConstBuilder<'a, 'tcx> {
                         self.locals[local] = self.add_node(Node::UnaryOp(op, operand), span);
                         Ok(())
                     }
+                    Rvalue::Cast(cast_kind, ref operand, ty) => {
+                        let operand = self.operand_to_node(span, operand)?;
+                        self.locals[local] =
+                            self.add_node(Node::Cast(cast_kind, operand, ty), span);
+                        Ok(())
+                    }
                     _ => self.error(Some(span), "unsupported rvalue")?,
                 }
             }
@@ -594,6 +604,7 @@ where
                 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),
         }
     }
 
@@ -676,6 +687,11 @@ 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_cast_kind, a_operand, a_ty), Node::Cast(b_cast_kind, b_operand, b_ty))
+            if (a_ty == b_ty) && (a_cast_kind == b_cast_kind) =>
+        {
+            try_unify(tcx, a.subtree(a_operand), b.subtree(b_operand))
+        }
         _ => false,
     }
 }

compiler/rustc_trait_selection/src/traits/object_safety.rs

@@ -838,9 +838,10 @@ fn contains_illegal_self_type_reference<'tcx, T: TypeFoldable<'tcx>>(
                         let leaf = leaf.subst(self.tcx, ct.substs);
                         self.visit_const(leaf)
                     }
-                    Node::Binop(..) | Node::UnaryOp(..) | Node::FunctionCall(_, _) => {
-                        ControlFlow::CONTINUE
-                    }
+                    Node::Binop(..)
+                    | Node::UnaryOp(..)
+                    | Node::FunctionCall(_, _)
+                    | Node::Cast(_, _, _) => ControlFlow::CONTINUE,
                 })
             } else {
                 ControlFlow::CONTINUE
@@ -859,9 +860,10 @@ fn contains_illegal_self_type_reference<'tcx, T: TypeFoldable<'tcx>>(
                             let leaf = leaf.subst(self.tcx, ct.substs);
                             self.visit_const(leaf)
                         }
-                        Node::Binop(..) | Node::UnaryOp(..) | Node::FunctionCall(_, _) => {
-                            ControlFlow::CONTINUE
-                        }
+                        Node::Binop(..)
+                        | Node::UnaryOp(..)
+                        | Node::FunctionCall(_, _)
+                        | Node::Cast(_, _, _) => ControlFlow::CONTINUE,
                     })
                 } else {
                     ControlFlow::CONTINUE

src/test/ui/const-generics/const_evaluatable_checked/abstract-const-as-cast-2.rs

@@ -0,0 +1,13 @@
+#![feature(const_evaluatable_checked, const_generics)]
+#![allow(incomplete_features)]
+
+trait Evaluatable<const N: u128> {}
+impl<const N: u128> Evaluatable<N> for () {}
+
+struct Foo<const N: u8>([u8; N as usize])
+//~^ Error: unconstrained generic constant
+//~| help: try adding a `where` bound using this expression: `where [(); N as usize]:`
+where
+    (): Evaluatable<{N as u128}>;
+
+fn main() {}

src/test/ui/const-generics/const_evaluatable_checked/abstract-const-as-cast-2.stderr

@@ -0,0 +1,10 @@
+error: unconstrained generic constant
+  --> $DIR/abstract-const-as-cast-2.rs:7:25
+   |
+LL | struct Foo<const N: u8>([u8; N as usize])
+   |                         ^^^^^^^^^^^^^^^^
+   |
+   = help: try adding a `where` bound using this expression: `where [(); N as usize]:`
+
+error: aborting due to previous error
+

src/test/ui/const-generics/const_evaluatable_checked/abstract-const-as-cast.rs

@@ -0,0 +1,9 @@
+// check-pass
+#![feature(const_evaluatable_checked, const_generics)]
+#![allow(incomplete_features)]
+
+struct Foo<const N: u8>([u8; N as usize])
+where
+    [(); N as usize]:;
+
+fn main() {}