add #![rustc_no_implicit_bounds]

compiler/rustc_feature/src/builtin_attrs.rs

@@ -1108,6 +1108,10 @@ pub static BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
         TEST, rustc_insignificant_dtor, Normal, template!(Word),
         WarnFollowing, EncodeCrossCrate::Yes
     ),
+    rustc_attr!(
+        TEST, rustc_no_implicit_bounds, CrateLevel, template!(Word),
+        WarnFollowing, EncodeCrossCrate::No
+    ),
     rustc_attr!(
         TEST, rustc_strict_coherence, Normal, template!(Word),
         WarnFollowing, EncodeCrossCrate::Yes

compiler/rustc_hir_analysis/src/hir_ty_lowering/bounds.rs

@@ -5,14 +5,14 @@ use rustc_errors::codes::*;
 use rustc_errors::struct_span_code_err;
 use rustc_hir as hir;
 use rustc_hir::def::{DefKind, Res};
-use rustc_hir::def_id::{DefId, LocalDefId};
+use rustc_hir::def_id::{CRATE_DEF_ID, DefId, LocalDefId};
 use rustc_hir::{AmbigArg, LangItem, PolyTraitRef};
 use rustc_middle::bug;
 use rustc_middle::ty::{
     self as ty, IsSuggestable, Ty, TyCtxt, TypeSuperVisitable, TypeVisitable, TypeVisitableExt,
     TypeVisitor, Upcast,
 };
-use rustc_span::{ErrorGuaranteed, Ident, Span, Symbol, kw};
+use rustc_span::{ErrorGuaranteed, Ident, Span, Symbol, kw, sym};
 use rustc_trait_selection::traits;
 use smallvec::SmallVec;
 use tracing::{debug, instrument};
@@ -188,6 +188,11 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
     ) {
         let tcx = self.tcx();
 
+        // Skip adding any default bounds if `#![rustc_no_implicit_bounds]`
+        if tcx.has_attr(CRATE_DEF_ID, sym::rustc_no_implicit_bounds) {
+            return;
+        }
+
         let meta_sized_did = tcx.require_lang_item(LangItem::MetaSized, span);
         let pointee_sized_did = tcx.require_lang_item(LangItem::PointeeSized, span);
 
@@ -408,24 +413,21 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         let tcx = self.tcx();
         let trait_id = tcx.lang_items().get(trait_);
         if let Some(trait_id) = trait_id
-            && self.do_not_provide_default_trait_bound(
-                trait_id,
-                hir_bounds,
-                self_ty_where_predicates,
-            )
+            && self.should_add_default_traits(trait_id, hir_bounds, self_ty_where_predicates)
         {
             add_trait_bound(tcx, bounds, self_ty, trait_id, span);
         }
     }
 
-    fn do_not_provide_default_trait_bound<'a>(
+    /// Returns `true` if default trait bound should be added.
+    fn should_add_default_traits<'a>(
         &self,
         trait_def_id: DefId,
         hir_bounds: &'a [hir::GenericBound<'tcx>],
         self_ty_where_predicates: Option<(LocalDefId, &'tcx [hir::WherePredicate<'tcx>])>,
     ) -> bool {
         let collected = collect_bounds(hir_bounds, self_ty_where_predicates, trait_def_id);
-        !collected.any()
+        !self.tcx().has_attr(CRATE_DEF_ID, sym::rustc_no_implicit_bounds) && !collected.any()
     }
 
     /// Lower HIR bounds into `bounds` given the self type `param_ty` and the overarching late-bound vars if any.

compiler/rustc_span/src/symbol.rs

@@ -1866,6 +1866,7 @@ symbols! {
         rustc_never_returns_null_ptr,
         rustc_never_type_options,
         rustc_no_implicit_autorefs,
+        rustc_no_implicit_bounds,
         rustc_no_mir_inline,
         rustc_nonnull_optimization_guaranteed,
         rustc_nounwind,

tests/ui/traits/next-solver/coherence/coherence-fulfill-overflow.rs

@@ -1,15 +1,17 @@
 //@ compile-flags: -Znext-solver=coherence
 
+#![feature(rustc_attrs)]
+#![rustc_no_implicit_bounds]
 #![recursion_limit = "10"]
 
 trait Trait {}
 
-struct W<T: ?Sized>(*const T);
+struct W<T>(*const T);
 trait TwoW {}
-impl<T: ?Sized + TwoW> TwoW for W<W<T>> {}
+impl<T: TwoW> TwoW for W<W<T>> {}
 
-impl<T: ?Sized + TwoW> Trait for W<T> {}
-impl<T: ?Sized + TwoW> Trait for T {}
+impl<T: TwoW> Trait for W<T> {}
+impl<T: TwoW> Trait for T {}
 //~^ ERROR conflicting implementations of trait `Trait` for type `W
 
 fn main() {}

tests/ui/traits/next-solver/coherence/coherence-fulfill-overflow.stderr

@@ -1,10 +1,10 @@
 error[E0119]: conflicting implementations of trait `Trait` for type `W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<_>>>>>>>>>>>>>>>>>>>>>>>`
-  --> $DIR/coherence-fulfill-overflow.rs:12:1
+  --> $DIR/coherence-fulfill-overflow.rs:14:1
    |
-LL | impl<T: ?Sized + TwoW> Trait for W<T> {}
-   | ------------------------------------- first implementation here
-LL | impl<T: ?Sized + TwoW> Trait for T {}
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<_>>>>>>>>>>>>>>>>>>>>>>>`
+LL | impl<T: TwoW> Trait for W<T> {}
+   | ---------------------------- first implementation here
+LL | impl<T: TwoW> Trait for T {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<_>>>>>>>>>>>>>>>>>>>>>>>`
 
 error: aborting due to 1 previous error
 

tests/ui/traits/next-solver/cycles/coinduction/incompleteness-unstable-result.rs

@@ -1,6 +1,7 @@
 //@ revisions: with without
 //@ compile-flags: -Znext-solver
 #![feature(rustc_attrs)]
+#![rustc_no_implicit_bounds]
 
 // This test is incredibly subtle. At its core the goal is to get a coinductive cycle,
 // which, depending on its root goal, either holds or errors. We achieve this by getting
@@ -17,20 +18,20 @@
 // test for that.
 
 #[rustc_coinductive]
-trait Trait<T: ?Sized, V: ?Sized, D: ?Sized> {}
-struct A<T: ?Sized>(*const T);
-struct B<T: ?Sized>(*const T);
+trait Trait<T, V, D> {}
+struct A<T>(*const T);
+struct B<T>(*const T);
 
-trait IncompleteGuidance<T: ?Sized, V: ?Sized> {}
-impl<T: ?Sized, U: ?Sized + 'static> IncompleteGuidance<U, u8> for T {}
-impl<T: ?Sized, U: ?Sized + 'static> IncompleteGuidance<U, i8> for T {}
-impl<T: ?Sized, U: ?Sized + 'static> IncompleteGuidance<U, i16> for T {}
+trait IncompleteGuidance<T, V> {}
+impl<T, U: 'static> IncompleteGuidance<U, u8> for T {}
+impl<T, U: 'static> IncompleteGuidance<U, i8> for T {}
+impl<T, U: 'static> IncompleteGuidance<U, i16> for T {}
 
-trait ImplGuidance<T: ?Sized, V: ?Sized> {}
-impl<T: ?Sized> ImplGuidance<u32, u8> for T {}
-impl<T: ?Sized> ImplGuidance<i32, i8> for T {}
+trait ImplGuidance<T, V> {}
+impl<T> ImplGuidance<u32, u8> for T {}
+impl<T> ImplGuidance<i32, i8> for T {}
 
-impl<T: ?Sized, U: ?Sized, V: ?Sized, D: ?Sized> Trait<U, V, D> for A<T>
+impl<T, U, V, D> Trait<U, V, D> for A<T>
 where
     T: IncompleteGuidance<U, V>,
     A<T>: Trait<U, D, V>,
@@ -39,17 +40,17 @@ where
 {
 }
 
-trait ToU8<T: ?Sized> {}
+trait ToU8<T> {}
 impl ToU8<u8> for () {}
 
-impl<T: ?Sized, U: ?Sized, V: ?Sized, D: ?Sized> Trait<U, V, D> for B<T>
+impl<T, U, V, D> Trait<U, V, D> for B<T>
 where
     T: ImplGuidance<U, V>,
     A<T>: Trait<U, V, D>,
 {
 }
 
-fn impls_trait<T: ?Sized + Trait<U, V, D>, U: ?Sized, V: ?Sized, D: ?Sized>() {}
+fn impls_trait<T: Trait<U, V, D>, U, V, D>() {}
 
 fn with_bound<X>()
 where

tests/ui/traits/next-solver/cycles/coinduction/incompleteness-unstable-result.with.stderr

@@ -1,25 +1,25 @@
 error[E0277]: the trait bound `A<X>: Trait<_, _, _>` is not satisfied
-  --> $DIR/incompleteness-unstable-result.rs:65:19
+  --> $DIR/incompleteness-unstable-result.rs:66:19
    |
 LL |     impls_trait::<A<X>, _, _, _>();
    |                   ^^^^ the trait `Trait<_, _, _>` is not implemented for `A<X>`
    |
    = help: the trait `Trait<U, V, D>` is implemented for `A<T>`
 note: required for `A<X>` to implement `Trait<_, _, _>`
-  --> $DIR/incompleteness-unstable-result.rs:33:50
+  --> $DIR/incompleteness-unstable-result.rs:34:18
    |
-LL | impl<T: ?Sized, U: ?Sized, V: ?Sized, D: ?Sized> Trait<U, V, D> for A<T>
-   |                                                  ^^^^^^^^^^^^^^     ^^^^
+LL | impl<T, U, V, D> Trait<U, V, D> for A<T>
+   |                  ^^^^^^^^^^^^^^     ^^^^
 ...
 LL |     A<T>: Trait<U, D, V>,
    |           -------------- unsatisfied trait bound introduced here
    = note: 8 redundant requirements hidden
    = note: required for `A<X>` to implement `Trait<_, _, _>`
 note: required by a bound in `impls_trait`
-  --> $DIR/incompleteness-unstable-result.rs:52:28
+  --> $DIR/incompleteness-unstable-result.rs:53:19
    |
-LL | fn impls_trait<T: ?Sized + Trait<U, V, D>, U: ?Sized, V: ?Sized, D: ?Sized>() {}
-   |                            ^^^^^^^^^^^^^^ required by this bound in `impls_trait`
+LL | fn impls_trait<T: Trait<U, V, D>, U, V, D>() {}
+   |                   ^^^^^^^^^^^^^^ required by this bound in `impls_trait`
 
 error: aborting due to 1 previous error
 

tests/ui/traits/next-solver/cycles/coinduction/incompleteness-unstable-result.without.stderr

@@ -1,25 +1,25 @@
 error[E0277]: the trait bound `A<X>: Trait<_, _, _>` is not satisfied
-  --> $DIR/incompleteness-unstable-result.rs:65:19
+  --> $DIR/incompleteness-unstable-result.rs:66:19
    |
 LL |     impls_trait::<A<X>, _, _, _>();
    |                   ^^^^ the trait `Trait<_, _, _>` is not implemented for `A<X>`
    |
    = help: the trait `Trait<U, V, D>` is implemented for `A<T>`
 note: required for `A<X>` to implement `Trait<_, _, _>`
-  --> $DIR/incompleteness-unstable-result.rs:33:50
+  --> $DIR/incompleteness-unstable-result.rs:34:18
    |
-LL | impl<T: ?Sized, U: ?Sized, V: ?Sized, D: ?Sized> Trait<U, V, D> for A<T>
-   |                                                  ^^^^^^^^^^^^^^     ^^^^
+LL | impl<T, U, V, D> Trait<U, V, D> for A<T>
+   |                  ^^^^^^^^^^^^^^     ^^^^
 ...
 LL |     A<T>: Trait<U, D, V>,
    |           -------------- unsatisfied trait bound introduced here
    = note: 8 redundant requirements hidden
    = note: required for `A<X>` to implement `Trait<_, _, _>`
 note: required by a bound in `impls_trait`
-  --> $DIR/incompleteness-unstable-result.rs:52:28
+  --> $DIR/incompleteness-unstable-result.rs:53:19
    |
-LL | fn impls_trait<T: ?Sized + Trait<U, V, D>, U: ?Sized, V: ?Sized, D: ?Sized>() {}
-   |                            ^^^^^^^^^^^^^^ required by this bound in `impls_trait`
+LL | fn impls_trait<T: Trait<U, V, D>, U, V, D>() {}
+   |                   ^^^^^^^^^^^^^^ required by this bound in `impls_trait`
 
 error: aborting due to 1 previous error
 

tests/ui/traits/next-solver/cycles/fixpoint-rerun-all-cycle-heads.rs

@@ -1,23 +1,24 @@
 //@ compile-flags: -Znext-solver
 #![feature(rustc_attrs)]
+#![rustc_no_implicit_bounds]
 
 // Check that we correctly rerun the trait solver for heads of cycles,
 // even if they are not the root.
 
-struct A<T: ?Sized>(*const T);
-struct B<T: ?Sized>(*const T);
-struct C<T: ?Sized>(*const T);
+struct A<T>(*const T);
+struct B<T>(*const T);
+struct C<T>(*const T);
 
 #[rustc_coinductive]
 trait Trait<'a, 'b> {}
 trait NotImplemented {}
 
-impl<'a, 'b, T: ?Sized> Trait<'a, 'b> for A<T> where B<T>: Trait<'a, 'b> {}
+impl<'a, 'b, T> Trait<'a, 'b> for A<T> where B<T>: Trait<'a, 'b> {}
 
 // With this the root of `B<T>` is `A<T>`, even if the other impl does
 // not have a cycle with `A<T>`. This candidate never applies because of
 // the `A<T>: NotImplemented` bound.
-impl<'a, 'b, T: ?Sized> Trait<'a, 'b> for B<T>
+impl<'a, 'b, T> Trait<'a, 'b> for B<T>
 where
     A<T>: Trait<'a, 'b>,
     A<T>: NotImplemented,
@@ -31,7 +32,7 @@ where
 // use the impl itself to prove that adds region constraints as we uniquified the
 // regions in the `A<T>: Trait<'a, 'b>` where-bound. As both the impl above
 // and the impl below now apply with some constraints, we failed with ambiguity.
-impl<'a, 'b, T: ?Sized> Trait<'a, 'b> for B<T>
+impl<'a, 'b, T> Trait<'a, 'b> for B<T>
 where
     A<T>: NotImplemented,
 {}
@@ -40,22 +41,22 @@ where
 //
 // Because of the coinductive cycle through `C<T>` it also requires
 // 'a to be 'static.
-impl<'a, T: ?Sized> Trait<'a, 'static> for B<T>
+impl<'a, T> Trait<'a, 'static> for B<T>
 where
     C<T>: Trait<'a, 'a>,
 {}
 
 // In the first iteration of `B<T>: Trait<'a, 'b>` we don't add any
 // constraints here, only after setting the provisional result to require
 // `'b == 'static` do we also add that constraint for `'a`.
-impl<'a, 'b, T: ?Sized> Trait<'a, 'b> for C<T>
+impl<'a, 'b, T> Trait<'a, 'b> for C<T>
 where
     B<T>: Trait<'a, 'b>,
 {}
 
-fn impls_trait<'a, 'b, T: Trait<'a, 'b> + ?Sized>() {}
+fn impls_trait<'a, 'b, T: Trait<'a, 'b>>() {}
 
-fn check<'a, T: ?Sized>() {
+fn check<'a, T>() {
     impls_trait::<'a, 'static, A<T>>();
     //~^ ERROR lifetime may not live long enough
 }

tests/ui/traits/next-solver/cycles/fixpoint-rerun-all-cycle-heads.stderr

@@ -1,7 +1,7 @@
 error: lifetime may not live long enough
-  --> $DIR/fixpoint-rerun-all-cycle-heads.rs:59:5
+  --> $DIR/fixpoint-rerun-all-cycle-heads.rs:60:5
    |
-LL | fn check<'a, T: ?Sized>() {
+LL | fn check<'a, T>() {
    |          -- lifetime `'a` defined here
 LL |     impls_trait::<'a, 'static, A<T>>();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ requires that `'a` must outlive `'static`

tests/ui/traits/next-solver/cycles/inductive-fixpoint-hang.rs

@@ -1,4 +1,6 @@
 //@ compile-flags: -Znext-solver
+#![feature(rustc_attrs)]
+#![rustc_no_implicit_bounds]
 
 // This currently hangs if we do not erase constraints from
 // overflow.
@@ -17,9 +19,9 @@
 // the solver to hang without hitting the recursion limit.
 trait Trait {}
 
-struct W<T: ?Sized>(*const T);
+struct W<T>(*const T);
 
-impl<T: ?Sized> Trait for W<W<T>>
+impl<T> Trait for W<W<T>>
 where
     W<T>: Trait,
     W<T>: Trait,

tests/ui/traits/next-solver/cycles/inductive-fixpoint-hang.stderr

@@ -1,11 +1,11 @@
 error[E0275]: overflow evaluating the requirement `W<_>: Trait`
-  --> $DIR/inductive-fixpoint-hang.rs:31:19
+  --> $DIR/inductive-fixpoint-hang.rs:33:19
    |
 LL |     impls_trait::<W<_>>();
    |                   ^^^^
    |
 note: required by a bound in `impls_trait`
-  --> $DIR/inductive-fixpoint-hang.rs:28:19
+  --> $DIR/inductive-fixpoint-hang.rs:30:19
    |
 LL | fn impls_trait<T: Trait>() {}
    |                   ^^^^^ required by this bound in `impls_trait`

tests/ui/traits/next-solver/cycles/provisional-cache-impacts-behavior.rs

@@ -1,6 +1,7 @@
 //@ compile-flags: -Znext-solver
 //@ check-pass
 #![feature(rustc_attrs)]
+#![rustc_no_implicit_bounds]
 
 // A test showcasing that using a provisional cache can differ
 // from only tracking stack entries.
@@ -59,9 +60,9 @@ trait B {}
 #[rustc_coinductive]
 trait C {}
 
-impl<T: ?Sized + B + C> A for T {}
-impl<T: ?Sized + A + C> B for T {}
-impl<T: ?Sized + B> C for T {}
+impl<T: B + C> A for T {}
+impl<T: A + C> B for T {}
+impl<T: B> C for T {}
 
 fn impls_a<T: A>() {}
 

tests/ui/traits/next-solver/dont-canonicalize-re-error.rs

@@ -1,4 +1,6 @@
 //@ compile-flags: -Znext-solver
+#![feature(rustc_attrs)]
+#![rustc_no_implicit_bounds]
 
 trait Tr<'a> {}
 
@@ -16,9 +18,9 @@ trait Tr<'a> {}
 // Then, when we recompute the goal `W<?0>: Constrain<'error>`, when
 // collecting ambiguities and overflows, we end up assembling a default
 // error candidate w/o ambiguity, which causes the goal to pass, and ICE.
-impl<'a, A: ?Sized> Tr<'a> for W<A> {}
-struct W<A: ?Sized>(A);
-impl<'a, A: ?Sized> Tr<'a> for A where A: Constrain<'a> {}
+impl<'a, A> Tr<'a> for W<A> {}
+struct W<A>(A);
+impl<'a, A> Tr<'a> for A where A: Constrain<'a> {}
 //~^ ERROR conflicting implementations of trait `Tr<'_>` for type `W<_>`
 
 trait Constrain<'a> {}