hir_analysis: add missing sizedness bounds

Default sizedness bounds were not being added to
`explicit_super_predicates_of` and `explicit_implied_predicates_of`
which meant that a trait bound added to a associated type projection
would be missing the implied predicate of the default sizedness
supertrait of that trait.

An unexpected consequence of this change was that the check for multiple
principals was now finding an additional `MetaSized` principal when
eagerly expanding trait aliases - this required modifying lowering to no
longer add default bounds to trait aliases.

Author: davidtwco

compiler/rustc_hir_analysis/src/collect/predicates_of.rs

@@ -658,6 +658,14 @@ pub(super) fn implied_predicates_with_filter<'tcx>(
         | PredicateFilter::SelfOnly
         | PredicateFilter::SelfTraitThatDefines(_)
         | PredicateFilter::SelfAndAssociatedTypeBounds => {
+            icx.lowerer().add_sizedness_bounds(
+                &mut bounds,
+                self_param_ty,
+                superbounds,
+                None,
+                Some(trait_def_id),
+                item.span,
+            );
             icx.lowerer().add_default_super_traits(
                 trait_def_id,
                 &mut bounds,

compiler/rustc_hir_analysis/src/hir_ty_lowering/bounds.rs

@@ -215,14 +215,18 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
             && !collected.meta_sized.any()
             && !collected.pointee_sized.any()
         {
-            // `?Sized` is equivalent to `MetaSized` (but only add the bound if there aren't any
-            // other explicit ones) - this can happen for trait aliases as well as bounds.
-            add_trait_bound(tcx, bounds, self_ty, meta_sized_did, span);
-        } else if !collected.any() {
-            if trait_did.is_some() {
-                // If there are no explicit sizedness bounds on a trait then add a default
-                // `MetaSized` supertrait.
+            if trait_did.map(|did| !tcx.is_trait_alias(did.to_def_id())).unwrap_or(true) {
+                // `?Sized` is equivalent to `MetaSized` (but only add the bound if there aren't any
+                // other explicit ones and it isn't a trait alias).
                 add_trait_bound(tcx, bounds, self_ty, meta_sized_did, span);
+            }
+        } else if !collected.any() {
+            if let Some(trait_did) = trait_did {
+                if !tcx.is_trait_alias(trait_did.to_def_id()) {
+                    // If there are no explicit sizedness bounds on a trait and it is not a trait
+                    // alias then add a default `MetaSized` supertrait.
+                    add_trait_bound(tcx, bounds, self_ty, meta_sized_did, span);
+                }
             } else {
                 // If there are no explicit sizedness bounds on a parameter then add a default
                 // `Sized` bound.

tests/ui/sized-hierarchy/bound-on-assoc-type-projection.rs

@@ -0,0 +1,15 @@
+//@ check-pass
+#![crate_type = "lib"]
+#![feature(sized_hierarchy)]
+
+trait FalseDeref {
+    type Target: std::marker::PointeeSized;
+}
+
+trait Bar {}
+
+fn foo<T: FalseDeref>()
+where
+    T::Target: Bar,
+{
+}

tests/ui/sized-hierarchy/default-supertrait.rs

@@ -47,14 +47,11 @@ fn with_pointeesized_supertrait<T: PointeeSized + PointeeSized_>() {
     requires_pointeesized::<T>();
 }
 
-// `T` won't inherit the `const MetaSized` implicit supertrait of `Bare`, so there is an error on
-// the bound, which is expected.
+// `T` inherits the `const MetaSized` implicit supertrait of `Bare`.
 fn with_bare_trait<T: PointeeSized + Bare>() {
-//~^ ERROR the size for values of type `T` cannot be known
     requires_sized::<T>();
     //~^ ERROR the size for values of type `T` cannot be known
     requires_metasized::<T>();
-    //~^ ERROR the size for values of type `T` cannot be known
     requires_pointeesized::<T>();
 }
 

tests/ui/sized-hierarchy/default-supertrait.stderr

@@ -28,22 +28,6 @@ LL | trait NegPointeeSized: ?PointeeSized { }
    = help: add `#![feature(more_maybe_bounds)]` to the crate attributes to enable
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
 
-error[E0277]: the size for values of type `T` cannot be known
-  --> $DIR/default-supertrait.rs:52:38
-   |
-LL | fn with_bare_trait<T: PointeeSized + Bare>() {
-   |                                      ^^^^ doesn't have a known size
-   |
-note: required by a bound in `Bare`
-  --> $DIR/default-supertrait.rs:22:1
-   |
-LL | trait Bare {}
-   | ^^^^^^^^^^^^^ required by this bound in `Bare`
-help: consider further restricting type parameter `T` with unstable trait `MetaSized`
-   |
-LL | fn with_bare_trait<T: PointeeSized + Bare + std::marker::MetaSized>() {
-   |                                           ++++++++++++++++++++++++
-
 error[E0277]: the size for values of type `T` cannot be known at compilation time
   --> $DIR/default-supertrait.rs:35:22
    |
@@ -89,11 +73,10 @@ LL | fn with_pointeesized_supertrait<T: PointeeSized + PointeeSized_ + std::mark
    |                                                                 ++++++++++++++++++++++++
 
 error[E0277]: the size for values of type `T` cannot be known at compilation time
-  --> $DIR/default-supertrait.rs:54:22
+  --> $DIR/default-supertrait.rs:52:22
    |
 LL | fn with_bare_trait<T: PointeeSized + Bare>() {
    |                    - this type parameter needs to be `Sized`
-LL |
 LL |     requires_sized::<T>();
    |                      ^ doesn't have a size known at compile-time
    |
@@ -103,23 +86,7 @@ note: required by a bound in `requires_sized`
 LL | fn requires_sized<T: Sized>() {}
    |                      ^^^^^ required by this bound in `requires_sized`
 
-error[E0277]: the size for values of type `T` cannot be known
-  --> $DIR/default-supertrait.rs:56:26
-   |
-LL |     requires_metasized::<T>();
-   |                          ^ doesn't have a known size
-   |
-note: required by a bound in `requires_metasized`
-  --> $DIR/default-supertrait.rs:25:26
-   |
-LL | fn requires_metasized<T: MetaSized>() {}
-   |                          ^^^^^^^^^ required by this bound in `requires_metasized`
-help: consider further restricting type parameter `T` with unstable trait `MetaSized`
-   |
-LL | fn with_bare_trait<T: PointeeSized + Bare + std::marker::MetaSized>() {
-   |                                           ++++++++++++++++++++++++
-
-error: aborting due to 9 previous errors
+error: aborting due to 7 previous errors
 
 Some errors have detailed explanations: E0277, E0658.
 For more information about an error, try `rustc --explain E0277`.

tests/ui/traits/next-solver/normalize/normalize-param-env-2.stderr

@@ -19,23 +19,6 @@ error[E0275]: overflow evaluating the requirement `<() as A<T>>::Assoc: A<T>`
 LL |         Self::Assoc: A<T>,
    |                      ^^^^
 
-error[E0275]: overflow evaluating the requirement `<() as A<T>>::Assoc: MetaSized`
-  --> $DIR/normalize-param-env-2.rs:24:22
-   |
-LL |         Self::Assoc: A<T>,
-   |                      ^^^^
-   |
-note: required by a bound in `A`
-  --> $DIR/normalize-param-env-2.rs:9:1
-   |
-LL | / trait A<T> {
-LL | |     type Assoc;
-LL | |
-LL | |     fn f()
-...  |
-LL | | }
-   | |_^ required by this bound in `A`
-
 error[E0275]: overflow evaluating the requirement `<() as A<T>>::Assoc well-formed`
   --> $DIR/normalize-param-env-2.rs:24:22
    |
@@ -63,6 +46,6 @@ LL |     where
 LL |         Self::Assoc: A<T>,
    |                      ^^^^ required by this bound in `A::f`
 
-error: aborting due to 6 previous errors
+error: aborting due to 5 previous errors
 
 For more information about this error, try `rustc --explain E0275`.

tests/ui/traits/next-solver/normalize/normalize-param-env-4.next.stderr

@@ -4,20 +4,6 @@ error[E0275]: overflow evaluating the requirement `<T as Trait>::Assoc: Trait`
 LL |     <T as Trait>::Assoc: Trait,
    |                          ^^^^^
 
-error[E0275]: overflow evaluating the requirement `<T as Trait>::Assoc: MetaSized`
-  --> $DIR/normalize-param-env-4.rs:19:26
-   |
-LL |     <T as Trait>::Assoc: Trait,
-   |                          ^^^^^
-   |
-note: required by a bound in `Trait`
-  --> $DIR/normalize-param-env-4.rs:7:1
-   |
-LL | / trait Trait {
-LL | |     type Assoc;
-LL | | }
-   | |_^ required by this bound in `Trait`
-
-error: aborting due to 2 previous errors
+error: aborting due to 1 previous error
 
 For more information about this error, try `rustc --explain E0275`.

tests/ui/traits/non_lifetime_binders/drop-impl-pred.no.stderr

@@ -1,5 +1,5 @@
 warning: the feature `non_lifetime_binders` is incomplete and may not be safe to use and/or cause compiler crashes
-  --> $DIR/drop-impl-pred.rs:6:12
+  --> $DIR/drop-impl-pred.rs:5:12
    |
 LL | #![feature(non_lifetime_binders)]
    |            ^^^^^^^^^^^^^^^^^^^^
@@ -8,17 +8,24 @@ LL | #![feature(non_lifetime_binders)]
    = note: `#[warn(incomplete_features)]` on by default
 
 error[E0367]: `Drop` impl requires `H: Foo` but the struct it is implemented for does not
-  --> $DIR/drop-impl-pred.rs:19:15
+  --> $DIR/drop-impl-pred.rs:20:15
    |
 LL |     for<H> H: Foo,
    |               ^^^
    |
 note: the implementor must specify the same requirement
-  --> $DIR/drop-impl-pred.rs:12:1
+  --> $DIR/drop-impl-pred.rs:11:1
    |
 LL | struct Bar<T>(T) where T: Foo;
    | ^^^^^^^^^^^^^
 
-error: aborting due to 1 previous error; 1 warning emitted
+error[E0282]: type annotations needed
+  --> $DIR/drop-impl-pred.rs:16:18
+   |
+LL | impl<T> Drop for Bar<T>
+   |                  ^^^^^^ cannot infer type for struct `Bar<T>`
+
+error: aborting due to 2 previous errors; 1 warning emitted
 
-For more information about this error, try `rustc --explain E0367`.
+Some errors have detailed explanations: E0282, E0367.
+For more information about an error, try `rustc --explain E0282`.

tests/ui/traits/non_lifetime_binders/drop-impl-pred.rs

@@ -1,5 +1,4 @@
 //@ revisions: no yes
-//@[yes] check-pass
 
 // Issue 110557
 
@@ -15,6 +14,8 @@ struct Bar<T>(T) where T: Foo;
 struct Bar<T>(T) where for<H> H: Foo;
 
 impl<T> Drop for Bar<T>
+//[yes]~^ ERROR type annotations needed
+//[no]~^^ ERROR type annotations needed
 where
     for<H> H: Foo,
 //[no]~^ ERROR `Drop` impl requires `H: Foo` but the struct it is implemented for does not

tests/ui/traits/non_lifetime_binders/drop-impl-pred.yes.stderr

@@ -1,11 +1,18 @@
 warning: the feature `non_lifetime_binders` is incomplete and may not be safe to use and/or cause compiler crashes
-  --> $DIR/drop-impl-pred.rs:6:12
+  --> $DIR/drop-impl-pred.rs:5:12
    |
 LL | #![feature(non_lifetime_binders)]
    |            ^^^^^^^^^^^^^^^^^^^^
    |
    = note: see issue #108185 <https://github.com/rust-lang/rust/issues/108185> for more information
    = note: `#[warn(incomplete_features)]` on by default
 
-warning: 1 warning emitted
+error[E0282]: type annotations needed
+  --> $DIR/drop-impl-pred.rs:16:18
+   |
+LL | impl<T> Drop for Bar<T>
+   |                  ^^^^^^ cannot infer type for struct `Bar<T>`
+
+error: aborting due to 1 previous error; 1 warning emitted
 
+For more information about this error, try `rustc --explain E0282`.