hir_analysis: change {Meta,}Sized bounds to const

With const sizedness traits, the correct migration is from `Sized`
to `const Sized` and from `?Sized` to `const MetaSized`.

Author: davidtwco

compiler/rustc_hir_analysis/src/hir_ty_lowering/bounds.rs

@@ -122,8 +122,8 @@ fn collect_sizedness_bounds<'tcx>(
     (CollectedSizednessBounds { sized, metasized, pointeesized }, unbounds)
 }
 
-/// Add a trait bound for `did`.
-fn add_trait_bound<'tcx>(
+/// Add a trait predicate for `did`.
+fn add_trait_predicate<'tcx>(
     tcx: TyCtxt<'tcx>,
     bounds: &mut Vec<(ty::Clause<'tcx>, Span)>,
     self_ty: Ty<'tcx>,
@@ -136,6 +136,23 @@ fn add_trait_bound<'tcx>(
     bounds.insert(0, (trait_ref.upcast(tcx), span));
 }
 
+/// Add a host effect predicate for `did`.
+fn add_host_effect_predicate<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    bounds: &mut Vec<(ty::Clause<'tcx>, Span)>,
+    self_ty: Ty<'tcx>,
+    did: DefId,
+    span: Span,
+) {
+    let trait_ref = ty::TraitRef::new(tcx, did, [self_ty]);
+    let clause = ty::ClauseKind::HostEffect(ty::HostEffectPredicate {
+        trait_ref,
+        constness: ty::BoundConstness::Const,
+    })
+    .upcast(tcx);
+    bounds.insert(1, (clause, span));
+}
+
 /// Remove any bounds of `did`.
 fn remove_lang_item_bound<'tcx>(bounds: &mut Vec<(ty::Clause<'tcx>, Span)>, did: DefId) {
     let mut idx = None;
@@ -171,8 +188,8 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         remove_lang_item_bound(bounds, pointeesized_did);
     }
 
-    /// Adds a `MetaSized` bound to `bounds` (of a trait definition) if there are no other sizedness
-    /// bounds. Also removes `PointeeSized` params - see doc comment on
+    /// Adds a `const MetaSized` bound to `bounds` (of a trait definition) if there are no other
+    /// sizedness bounds. Also removes `PointeeSized` params - see doc comment on
     /// `adjust_sizedness_predicates`.
     pub(crate) fn adjust_sizedness_supertraits(
         &self,
@@ -195,16 +212,18 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
 
         let (collected, _unbounds) = collect_sizedness_bounds(tcx, hir_bounds, None, span);
         if !collected.any() && trait_did != pointeesized_did {
-            // If there are no explicit sizedness bounds then add a default `MetaSized` supertrait.
-            add_trait_bound(tcx, bounds, self_ty, metasized_did, span);
+            // If there are no explicit sizedness bounds then add a default `const MetaSized`
+            // supertrait.
+            add_trait_predicate(tcx, bounds, self_ty, metasized_did, span);
+            add_host_effect_predicate(tcx, bounds, self_ty, metasized_did, span);
         }
 
         // See doc comment on `adjust_sizedness_predicates`.
         remove_lang_item_bound(bounds, pointeesized_did);
     }
 
-    /// Add a default `Sized` bound if there are no other sizedness bounds and rewrite `?Sized`
-    /// to `MetaSized`. Also removes `PointeeSized` params - see doc comment on
+    /// Add a default `const Sized` bound if there are no other sizedness bounds and rewrite
+    /// `?Sized` to `MetaSized`. Also removes `PointeeSized` params - see doc comment on
     /// `adjust_sizedness_predicates`.
     pub(crate) fn adjust_sizedness_params_and_assoc_types(
         &self,
@@ -229,12 +248,14 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
             && !collected.metasized.any()
             && !collected.pointeesized.any()
         {
-            // `?Sized` is equivalent to `MetaSized` (but only add the bound if there aren't any
-            // other explicit ones)
-            add_trait_bound(tcx, bounds, self_ty, metasized_did, span);
+            // `?Sized` is equivalent to `const MetaSized` (but only add the bound if there aren't
+            // any other explicit ones)
+            add_trait_predicate(tcx, bounds, self_ty, metasized_did, span);
+            add_host_effect_predicate(tcx, bounds, self_ty, metasized_did, span);
         } else if !collected.any() {
-            // If there are no explicit sizedness bounds then add a default `Sized` bound.
-            add_trait_bound(tcx, bounds, self_ty, sized_did, span);
+            // If there are no explicit sizedness bounds then add a default `const Sized` bound.
+            add_trait_predicate(tcx, bounds, self_ty, sized_did, span);
+            add_host_effect_predicate(tcx, bounds, self_ty, sized_did, span);
         }
 
         // See doc comment on `adjust_sizedness_predicates`.

tests/ui/associated-consts/assoc-const-eq-bound-var-in-ty-not-wf.rs

@@ -12,8 +12,7 @@ fn take(
         K = { () }
     >,
 ) {}
-//~^^^^^^ ERROR implementation of `Project` is not general enough
-//~^^^^ ERROR higher-ranked subtype error
+//~^^^ ERROR higher-ranked subtype error
 //~| ERROR higher-ranked subtype error
 
 trait Project { type Out; }

tests/ui/associated-consts/assoc-const-eq-bound-var-in-ty-not-wf.stderr

@@ -12,14 +12,5 @@ LL |         K = { () }
    |
    = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
 
-error: implementation of `Project` is not general enough
-  --> $DIR/assoc-const-eq-bound-var-in-ty-not-wf.rs:9:4
-   |
-LL | fn take(
-   |    ^^^^ implementation of `Project` is not general enough
-   |
-   = note: `Project` would have to be implemented for the type `for<'a> fn(&'a str) -> &'a str`
-   = note: ...but `Project` is actually implemented for the type `fn(&'0 str) -> &'0 str`, for some specific lifetime `'0`
-
-error: aborting due to 3 previous errors
+error: aborting due to 2 previous errors
 

tests/ui/sized-hierarchy/const-bound.rs

@@ -0,0 +1,14 @@
+//@ check-pass
+//@ compile-flags: --crate-type=lib
+#![feature(const_trait_impl)]
+
+// This test can fail because of the implicit const bounds/supertraits.
+
+pub struct Bar;
+
+#[const_trait]
+pub trait Foo {}
+
+impl const Foo for Bar {}
+
+pub const fn bar<T: ~const Foo>() {}

tests/ui/where-clauses/ignore-err-clauses.rs

@@ -1,7 +1,6 @@
 use std::ops::Add;
 
 fn dbl<T>(x: T) -> <T as Add>::Output
-//~^ ERROR type annotations needed
 where
     T: Copy + Add,
     UUU: Copy,

tests/ui/where-clauses/ignore-err-clauses.stderr

@@ -1,16 +1,9 @@
 error[E0412]: cannot find type `UUU` in this scope
-  --> $DIR/ignore-err-clauses.rs:7:5
+  --> $DIR/ignore-err-clauses.rs:6:5
    |
 LL |     UUU: Copy,
    |     ^^^ not found in this scope
 
-error[E0282]: type annotations needed
-  --> $DIR/ignore-err-clauses.rs:3:14
-   |
-LL | fn dbl<T>(x: T) -> <T as Add>::Output
-   |              ^ cannot infer type for type parameter `T`
-
-error: aborting due to 2 previous errors
+error: aborting due to 1 previous error
 
-Some errors have detailed explanations: E0282, E0412.
-For more information about an error, try `rustc --explain E0282`.
+For more information about this error, try `rustc --explain E0412`.