Rollup merge of #81433 - lcnr:stop-looking-into-ty-alias, r=oli-obk

const_evaluatable: stop looking into type aliases

see https://rust-lang.zulipchat.com/#narrow/stream/260443-project-const-generics/topic/const_evaluatable.3A.20type.20alias

r? ````@oli-obk````

compiler/rustc_typeck/src/collect.rs

@@ -50,8 +50,6 @@ use rustc_span::{Span, DUMMY_SP};
 use rustc_target::spec::abi;
 use rustc_trait_selection::traits::error_reporting::suggestions::NextTypeParamName;
 
-use std::ops::ControlFlow;
-
 mod item_bounds;
 mod type_of;
 
@@ -2080,38 +2078,6 @@ fn const_evaluatable_predicates_of<'tcx>(
                 ));
             }
         }
-
-        // Look into `TyAlias`.
-        fn visit_ty(&mut self, ty: &'tcx hir::Ty<'tcx>) {
-            use ty::fold::{TypeFoldable, TypeVisitor};
-            struct TyAliasVisitor<'a, 'tcx> {
-                tcx: TyCtxt<'tcx>,
-                preds: &'a mut FxIndexSet<(ty::Predicate<'tcx>, Span)>,
-                span: Span,
-            }
-
-            impl<'a, 'tcx> TypeVisitor<'tcx> for TyAliasVisitor<'a, 'tcx> {
-                fn visit_const(&mut self, ct: &'tcx Const<'tcx>) -> ControlFlow<Self::BreakTy> {
-                    if let ty::ConstKind::Unevaluated(def, substs, None) = ct.val {
-                        self.preds.insert((
-                            ty::PredicateKind::ConstEvaluatable(def, substs).to_predicate(self.tcx),
-                            self.span,
-                        ));
-                    }
-                    ControlFlow::CONTINUE
-                }
-            }
-
-            if let hir::TyKind::Path(hir::QPath::Resolved(None, path)) = ty.kind {
-                if let Res::Def(DefKind::TyAlias, def_id) = path.res {
-                    let mut visitor =
-                        TyAliasVisitor { tcx: self.tcx, preds: &mut self.preds, span: path.span };
-                    self.tcx.type_of(def_id).visit_with(&mut visitor);
-                }
-            }
-
-            intravisit::walk_ty(self, ty)
-        }
     }
 
     let hir_id = tcx.hir().local_def_id_to_hir_id(def_id);

src/test/ui/const-generics/const_evaluatable_checked/simple_fail.full.stderr

@@ -1,9 +1,15 @@
+error[E0080]: evaluation of constant value failed
+  --> $DIR/simple_fail.rs:9:48
+   |
+LL | fn test<const N: usize>() -> Arr<N> where [u8; N - 1]: Sized {
+   |                                                ^^^^^ attempt to compute `0_usize - 1_usize`, which would overflow
+
 error[E0080]: evaluation of constant value failed
   --> $DIR/simple_fail.rs:6:33
    |
 LL | type Arr<const N: usize> = [u8; N - 1];
    |                                 ^^^^^ attempt to compute `0_usize - 1_usize`, which would overflow
 
-error: aborting due to previous error
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0080`.

src/test/ui/const-generics/const_evaluatable_checked/simple_fail.min.stderr

@@ -7,5 +7,14 @@ LL | type Arr<const N: usize> = [u8; N - 1];
    = help: const parameters may only be used as standalone arguments, i.e. `N`
    = help: use `#![feature(const_generics)]` and `#![feature(const_evaluatable_checked)]` to allow generic const expressions
 
-error: aborting due to previous error
+error: generic parameters may not be used in const operations
+  --> $DIR/simple_fail.rs:9:48
+   |
+LL | fn test<const N: usize>() -> Arr<N> where [u8; N - 1]: Sized {
+   |                                                ^ cannot perform const operation using `N`
+   |
+   = help: const parameters may only be used as standalone arguments, i.e. `N`
+   = help: use `#![feature(const_generics)]` and `#![feature(const_evaluatable_checked)]` to allow generic const expressions
+
+error: aborting due to 2 previous errors
 

src/test/ui/const-generics/const_evaluatable_checked/simple_fail.rs

@@ -1,12 +1,14 @@
 // revisions: full min
 #![cfg_attr(full, feature(const_generics))]
-#![feature(const_evaluatable_checked)]
+#![cfg_attr(full, feature(const_evaluatable_checked))]
 #![allow(incomplete_features)]
 
 type Arr<const N: usize> = [u8; N - 1]; //[full]~ ERROR evaluation of constant
 //[min]~^ ERROR generic parameters may not be used in const operations
 
-fn test<const N: usize>() -> Arr<N> where Arr<N>: Sized {
+fn test<const N: usize>() -> Arr<N> where [u8; N - 1]: Sized {
+//[min]~^ ERROR generic parameters may not be used in const operations
+//[full]~^^ ERROR evaluation of constant
     todo!()
 }
 

src/test/ui/const-generics/const_evaluatable_checked/ty-alias-substitution.rs

@@ -0,0 +1,14 @@
+// check-pass
+// Test that we correctly substitute generic arguments for type aliases.
+#![feature(const_generics, const_evaluatable_checked)]
+#![allow(incomplete_features)]
+
+type Alias<T, const N: usize> = [T; N + 1];
+
+fn foo<const M: usize>() -> Alias<u32, M>  where [u8; M + 1]: Sized {
+    [0; M + 1]
+}
+
+fn main() {
+    foo::<0>();
+}