rustc_typeck: turn where Type:, into a WF(Type) predicate, instead of ignoring it.

src/librustc_typeck/collect.rs

@@ -1796,6 +1796,21 @@ fn explicit_predicates_of<'a, 'tcx>(
             &hir::WherePredicate::BoundPredicate(ref bound_pred) => {
                 let ty = icx.to_ty(&bound_pred.bounded_ty);
 
+                // Keep the type around in a WF predicate, in case of no bounds.
+                // That way, `where Ty:` is not a complete noop (see #53696).
+                if bound_pred.bounds.is_empty() {
+                    if let ty::Param(_) = ty.sty {
+                        // This is a `where T:`, which can be in the HIR from the
+                        // transformation that moves `?Sized` to `T`'s declaration.
+                        // We can skip the predicate because type parameters are
+                        // trivially WF, but also we *should*, to avoid exposing
+                        // users who never wrote `where Type:,` themselves, to
+                        // compiler/tooling bugs from not handling WF predicates.
+                    } else {
+                        predicates.push(ty::Predicate::WellFormed(ty));
+                    }
+                }
+
                 for bound in bound_pred.bounds.iter() {
                     match bound {
                         &hir::GenericBound::Trait(ref poly_trait_ref, _) => {

src/librustdoc/clean/mod.rs

@@ -1312,7 +1312,13 @@ impl<'a> Clean<WherePredicate> for ty::Predicate<'a> {
             Predicate::RegionOutlives(ref pred) => pred.clean(cx),
             Predicate::TypeOutlives(ref pred) => pred.clean(cx),
             Predicate::Projection(ref pred) => pred.clean(cx),
-            Predicate::WellFormed(_) => panic!("not user writable"),
+            Predicate::WellFormed(ty) => {
+                // This comes from `where Ty:` (i.e. no bounds) (see #53696).
+                WherePredicate::BoundPredicate {
+                    ty: ty.clean(cx),
+                    bounds: vec![],
+                }
+            }
             Predicate::ObjectSafe(_) => panic!("not user writable"),
             Predicate::ClosureKind(..) => panic!("not user writable"),
             Predicate::ConstEvaluatable(..) => panic!("not user writable"),

src/test/ui/chalkify/lower_trait_where_clause.rs

@@ -14,7 +14,15 @@ use std::fmt::{Debug, Display};
 use std::borrow::Borrow;
 
 #[rustc_dump_program_clauses] //~ ERROR program clause dump
-trait Foo<'a, 'b, S, T, U> where S: Debug, T: Borrow<U>, U: ?Sized, 'a: 'b, U: 'b {
+trait Foo<'a, 'b, S, T, U>
+where
+    S: Debug,
+    T: Borrow<U>,
+    U: ?Sized,
+    'a: 'b,
+    U: 'b,
+    Vec<T>:, // NOTE(#53696) this checks an empty list of bounds.
+{
     fn s(_: S) -> S;
     fn t(_: T) -> T;
     fn u(_: U) -> U;

src/test/ui/chalkify/lower_trait_where_clause.stderr

@@ -11,7 +11,8 @@ LL | #[rustc_dump_program_clauses] //~ ERROR program clause dump
    = note: Implemented(Self: Foo<'a, 'b, S, T, U>) :- FromEnv(Self: Foo<'a, 'b, S, T, U>).
    = note: RegionOutlives('a : 'b) :- FromEnv(Self: Foo<'a, 'b, S, T, U>).
    = note: TypeOutlives(U : 'b) :- FromEnv(Self: Foo<'a, 'b, S, T, U>).
-   = note: WellFormed(Self: Foo<'a, 'b, S, T, U>) :- Implemented(Self: Foo<'a, 'b, S, T, U>), WellFormed(S: std::marker::Sized), WellFormed(T: std::marker::Sized), WellFormed(S: std::fmt::Debug), WellFormed(T: std::borrow::Borrow<U>), RegionOutlives('a : 'b), TypeOutlives(U : 'b).
+   = note: WellFormed(Self: Foo<'a, 'b, S, T, U>) :- Implemented(Self: Foo<'a, 'b, S, T, U>), WellFormed(S: std::marker::Sized), WellFormed(T: std::marker::Sized), WellFormed(S: std::fmt::Debug), WellFormed(T: std::borrow::Borrow<U>), RegionOutlives('a : 'b), TypeOutlives(U : 'b), WellFormed(std::vec::Vec<T>).
+   = note: WellFormed(std::vec::Vec<T>) :- FromEnv(Self: Foo<'a, 'b, S, T, U>).
 
 error: aborting due to previous error
 

src/test/ui/wf/wf-fn-where-clause.rs

@@ -10,7 +10,6 @@
 
 // Test that we check where-clauses on fn items.
 
-#![feature(associated_type_defaults)]
 #![feature(rustc_attrs)]
 #![allow(dead_code)]
 
@@ -20,5 +19,9 @@ fn foo<T,U>() where T: ExtraCopy<U> //~ ERROR E0277
 {
 }
 
+fn bar() where Vec<dyn Copy>:, {}
+//~^ ERROR E0277
+//~| ERROR E0038
+
 #[rustc_error]
 fn main() { }

src/test/ui/wf/wf-fn-where-clause.stderr

@@ -1,5 +1,5 @@
 error[E0277]: the trait bound `U: std::marker::Copy` is not satisfied
-  --> $DIR/wf-fn-where-clause.rs:19:1
+  --> $DIR/wf-fn-where-clause.rs:18:1
    |
 LL | / fn foo<T,U>() where T: ExtraCopy<U> //~ ERROR E0277
 LL | | {
@@ -8,11 +8,30 @@ LL | | }
    |
    = help: consider adding a `where U: std::marker::Copy` bound
 note: required by `ExtraCopy`
-  --> $DIR/wf-fn-where-clause.rs:17:1
+  --> $DIR/wf-fn-where-clause.rs:16:1
    |
 LL | trait ExtraCopy<T:Copy> { }
    | ^^^^^^^^^^^^^^^^^^^^^^^
 
-error: aborting due to previous error
+error[E0277]: the size for values of type `(dyn std::marker::Copy + 'static)` cannot be known at compilation time
+  --> $DIR/wf-fn-where-clause.rs:22:1
+   |
+LL | fn bar() where Vec<dyn Copy>:, {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
+   |
+   = help: the trait `std::marker::Sized` is not implemented for `(dyn std::marker::Copy + 'static)`
+   = note: to learn more, visit <https://doc.rust-lang.org/book/second-edition/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
+   = note: required by `std::vec::Vec`
+
+error[E0038]: the trait `std::marker::Copy` cannot be made into an object
+  --> $DIR/wf-fn-where-clause.rs:22:1
+   |
+LL | fn bar() where Vec<dyn Copy>:, {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `std::marker::Copy` cannot be made into an object
+   |
+   = note: the trait cannot require that `Self : Sized`
+
+error: aborting due to 3 previous errors
 
-For more information about this error, try `rustc --explain E0277`.
+Some errors occurred: E0038, E0277.
+For more information about an error, try `rustc --explain E0038`.