Emit warning when bound is trivially false

compiler/rustc_typeck/src/check/wfcheck.rs

@@ -10,7 +10,7 @@ use rustc_hir::ItemKind;
 use rustc_infer::infer::outlives::env::{OutlivesEnvironment, RegionBoundPairs};
 use rustc_infer::infer::outlives::obligations::TypeOutlives;
 use rustc_infer::infer::{self, InferCtxt, TyCtxtInferExt};
-use rustc_infer::traits::Normalized;
+use rustc_infer::traits::{Normalized, TraitEngine, TraitEngineExt as _};
 use rustc_middle::ty::query::Providers;
 use rustc_middle::ty::subst::{GenericArgKind, InternalSubsts, Subst};
 use rustc_middle::ty::trait_def::TraitSpecializationKind;
@@ -27,7 +27,8 @@ use rustc_trait_selection::traits::query::evaluate_obligation::InferCtxtExt as _
 use rustc_trait_selection::traits::query::normalize::AtExt;
 use rustc_trait_selection::traits::query::NoSolution;
 use rustc_trait_selection::traits::{
-    self, ObligationCause, ObligationCauseCode, ObligationCtxt, WellFormedLoc,
+    self, FulfillmentContext, ObligationCause, ObligationCauseCode, ObligationCtxt,
+    SelectionContext, WellFormedLoc,
 };
 
 use std::cell::LazyCell;
@@ -1817,7 +1818,7 @@ impl<'tcx> WfCheckingCtxt<'_, 'tcx> {
     /// Feature gates RFC 2056 -- trivial bounds, checking for global bounds that
     /// aren't true.
     fn check_false_global_bounds(&mut self) {
-        let tcx = self.ocx.infcx.tcx;
+        let tcx = self.infcx.tcx;
         let mut span = self.span;
         let empty_env = ty::ParamEnv::empty();
 
@@ -1833,33 +1834,76 @@ impl<'tcx> WfCheckingCtxt<'_, 'tcx> {
             if let ty::PredicateKind::WellFormed(..) = obligation.predicate.kind().skip_binder() {
                 continue;
             }
-            let pred = obligation.predicate;
-            // Match the existing behavior.
-            if pred.is_global() && !pred.has_late_bound_regions() {
-                let pred = self.normalize(span, None, pred);
-                let hir_node = tcx.hir().find(self.body_id);
 
-                // only use the span of the predicate clause (#90869)
+            let pred = obligation.predicate;
 
-                if let Some(hir::Generics { predicates, .. }) =
-                    hir_node.and_then(|node| node.generics())
-                {
-                    let obligation_span = obligation.cause.span();
-
-                    span = predicates
-                        .iter()
-                        // There seems to be no better way to find out which predicate we are in
-                        .find(|pred| pred.span().contains(obligation_span))
-                        .map(|pred| pred.span())
-                        .unwrap_or(obligation_span);
-                }
+            // only use the span of the predicate clause (#90869)
+            let hir_node = tcx.hir().find(self.body_id);
+            if let Some(hir::Generics { predicates, .. }) =
+                hir_node.and_then(|node| node.generics())
+            {
+                let obligation_span = obligation.cause.span();
+                span = predicates
+                    .iter()
+                    // There seems to be no better way to find out which predicate we are in
+                    .find(|pred| pred.span().contains(obligation_span))
+                    .map(|pred| pred.span())
+                    .unwrap_or(obligation_span);
+            }
 
+            // Match the existing behavior.
+            if pred.is_global() && !pred.has_late_bound_regions() {
+                let normalized_pred = self.normalize(span, None, pred);
                 let obligation = traits::Obligation::new(
                     traits::ObligationCause::new(span, self.body_id, traits::TrivialBound),
                     empty_env,
-                    pred,
+                    normalized_pred,
                 );
-                self.ocx.register_obligation(obligation);
+                self.register_obligation(obligation);
+            } else {
+                self.infcx.probe(|_| {
+                    // Manually normalize because `wfcx.normalize` may report errors
+                    let Normalized { value: normalized_pred, obligations } = traits::normalize(
+                        &mut SelectionContext::new(self.infcx),
+                        self.param_env,
+                        ObligationCause::dummy(),
+                        pred,
+                    );
+                    if normalized_pred.is_global() {
+                        let mut fulfill_cx = FulfillmentContext::new_in_snapshot();
+                        fulfill_cx.register_predicate_obligations(self.infcx, obligations);
+                        fulfill_cx.register_predicate_obligation(
+                            self.infcx,
+                            traits::Obligation::new(
+                                traits::ObligationCause::new(
+                                    span,
+                                    self.body_id,
+                                    traits::TrivialBound,
+                                ),
+                                empty_env,
+                                normalized_pred,
+                            ),
+                        );
+                        if !fulfill_cx.select_all_or_error(self.infcx).is_empty() {
+                            let mut diag = self.tcx()
+                                .sess
+                                .struct_span_warn(
+                                    span,
+                                    format!("the where-clause bound `{pred}` is impossible to satisfy"),
+                                );
+                            diag.span_label(
+                                self.span,
+                                "this item cannot be referenced without causing an error",
+                            );
+                            if self.infcx.tcx.sess.opts.unstable_features.is_nightly_build() {
+                                diag.help(
+                                    "add `#![feature(trivial_bounds)]` to the crate attributes to allow it",
+                                );
+                            }
+                            diag.emit();
+                        }
+                    }
+                });
             }
         }
     }

src/test/ui/associated-types/bound-lifetime-in-binding-only.ok.stderr

@@ -1,8 +1,32 @@
+warning: the where-clause bound `for<'a> <Parameterized<'a> as Foo>::Item == &'a i32` is impossible to satisfy
+  --> $DIR/bound-lifetime-in-binding-only.rs:67:19
+   |
+LL |   fn ok3<T>() where for<'a> Parameterized<'a>: Foo<Item=&'a i32> {
+   |  _-                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+LL | |
+LL | |
+LL | | }
+   | |_- this item cannot be referenced without causing an error
+   |
+   = help: add `#![feature(trivial_bounds)]` to the crate attributes to allow it
+
+warning: the where-clause bound `for<'a> Parameterized<'a>: Foo` is impossible to satisfy
+  --> $DIR/bound-lifetime-in-binding-only.rs:67:19
+   |
+LL |   fn ok3<T>() where for<'a> Parameterized<'a>: Foo<Item=&'a i32> {
+   |  _-                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+LL | |
+LL | |
+LL | | }
+   | |_- this item cannot be referenced without causing an error
+   |
+   = help: add `#![feature(trivial_bounds)]` to the crate attributes to allow it
+
 error: fatal error triggered by #[rustc_error]
-  --> $DIR/bound-lifetime-in-binding-only.rs:71:1
+  --> $DIR/bound-lifetime-in-binding-only.rs:73:1
    |
 LL | fn main() { }
    | ^^^^^^^^^
 
-error: aborting due to previous error
+error: aborting due to previous error; 2 warnings emitted
 

src/test/ui/associated-types/bound-lifetime-in-binding-only.rs

@@ -65,6 +65,8 @@ fn ok2<T: for<'a,'b> Fn<(&'b Parameterized<'a>,), Output=&'a i32>>() {
 
 #[cfg(ok)]
 fn ok3<T>() where for<'a> Parameterized<'a>: Foo<Item=&'a i32> {
+    //[ok]~^ WARNING the where-clause bound
+    //[ok]~| WARNING the where-clause bound
 }
 
 #[rustc_error]

src/test/ui/associated-types/issue-59324.rs

@@ -12,7 +12,7 @@ pub trait ThriftService<Bug: NotFoo>:
 //~^ ERROR the trait bound `Bug: Foo` is not satisfied
 //~| ERROR the trait bound `Bug: Foo` is not satisfied
     Service<AssocType = <Bug as Foo>::OnlyFoo>
-{
+    {
     fn get_service(
     //~^ ERROR the trait bound `Bug: Foo` is not satisfied
         &self,

src/test/ui/associated-types/issue-69398.rs

@@ -12,6 +12,7 @@ pub trait Broken {
 impl<T> Broken for T {
     type Assoc = ();
     fn broken(&self) where Self::Assoc: Foo {
+        //~^ WARNING the where-clause bound
         let _x: <Self::Assoc as Foo>::Bar;
     }
 }

src/test/ui/associated-types/issue-69398.stderr

@@ -0,0 +1,14 @@
+warning: the where-clause bound `<T as Broken>::Assoc: Foo` is impossible to satisfy
+  --> $DIR/issue-69398.rs:14:28
+   |
+LL |       fn broken(&self) where Self::Assoc: Foo {
+   |  _____-                      ^^^^^^^^^^^^^^^^
+LL | |
+LL | |         let _x: <Self::Assoc as Foo>::Bar;
+LL | |     }
+   | |_____- this item cannot be referenced without causing an error
+   |
+   = help: add `#![feature(trivial_bounds)]` to the crate attributes to allow it
+
+warning: 1 warning emitted
+

src/test/ui/consts/issue-67696-const-prop-ice.rs

@@ -11,6 +11,9 @@ trait A {
 
 impl A for [fn(&())] {
     fn foo(&self) -> Self where Self: Copy { *(&[] as &[_]) }
+    //~^ WARNING the where-clause bound
+    //~| WARNING the where-clause bound
+    //~| WARNING the where-clause bound
 }
 
 impl A for i32 {

src/test/ui/consts/issue-67696-const-prop-ice.stderr

@@ -0,0 +1,26 @@
+warning: the where-clause bound `[for<'r> fn(&'r ())]: Copy` is impossible to satisfy
+  --> $DIR/issue-67696-const-prop-ice.rs:13:33
+   |
+LL |     fn foo(&self) -> Self where Self: Copy { *(&[] as &[_]) }
+   |     ----------------------------^^^^^^^^^^------------------- this item cannot be referenced without causing an error
+   |
+   = help: add `#![feature(trivial_bounds)]` to the crate attributes to allow it
+
+warning: the where-clause bound `[for<'r> fn(&'r ())]: Clone` is impossible to satisfy
+  --> $DIR/issue-67696-const-prop-ice.rs:13:33
+   |
+LL |     fn foo(&self) -> Self where Self: Copy { *(&[] as &[_]) }
+   |     ----------------------------^^^^^^^^^^------------------- this item cannot be referenced without causing an error
+   |
+   = help: add `#![feature(trivial_bounds)]` to the crate attributes to allow it
+
+warning: the where-clause bound `[for<'r> fn(&'r ())]: Sized` is impossible to satisfy
+  --> $DIR/issue-67696-const-prop-ice.rs:13:33
+   |
+LL |     fn foo(&self) -> Self where Self: Copy { *(&[] as &[_]) }
+   |     ----------------------------^^^^^^^^^^------------------- this item cannot be referenced without causing an error
+   |
+   = help: add `#![feature(trivial_bounds)]` to the crate attributes to allow it
+
+warning: 3 warnings emitted
+

src/test/ui/feature-gates/feature-gate-trivial_bounds.rs

@@ -63,6 +63,7 @@ fn return_str() -> str where str: Sized { //~ ERROR
 // This is currently accepted because the function pointer isn't
 // considered global.
 fn global_hr(x: fn(&())) where fn(&()): Foo { // OK
+    //~^ WARNING the where-clause bound
     x.test();
 }
 

src/test/ui/feature-gates/feature-gate-trivial_bounds.stderr

@@ -113,6 +113,18 @@ LL | fn return_str() -> str where str: Sized {
    = help: see issue #48214
    = help: add `#![feature(trivial_bounds)]` to the crate attributes to enable
 
-error: aborting due to 11 previous errors
+warning: the where-clause bound `for<'r> fn(&'r ()): Foo` is impossible to satisfy
+  --> $DIR/feature-gate-trivial_bounds.rs:65:32
+   |
+LL |   fn global_hr(x: fn(&())) where fn(&()): Foo { // OK
+   |  _-                              ^^^^^^^^^^^^
+LL | |
+LL | |     x.test();
+LL | | }
+   | |_- this item cannot be referenced without causing an error
+   |
+   = help: add `#![feature(trivial_bounds)]` to the crate attributes to allow it
+
+error: aborting due to 11 previous errors; 1 warning emitted
 
 For more information about this error, try `rustc --explain E0277`.

src/test/ui/higher-rank-trait-bounds/normalize-under-binder/issue-89118.rs

@@ -7,6 +7,7 @@ impl<B: BufferMut, C> BufferUdpStateContext<B> for C {}
 trait StackContext
 where
     Ctx<()>: for<'a> BufferUdpStateContext<&'a ()>,
+    //~^ WARNING the where-clause bound
 {
     type Dispatcher;
 }
@@ -26,6 +27,7 @@ where
 struct EthernetWorker<C>(C)
 where
     Ctx<()>: for<'a> BufferUdpStateContext<&'a ()>;
+    //~^ WARNING the where-clause bound
 impl<C> EthernetWorker<C> {}
 //~^ ERROR: is not satisfied [E0277]
 

src/test/ui/higher-rank-trait-bounds/normalize-under-binder/issue-89118.stderr

@@ -1,5 +1,20 @@
+warning: the where-clause bound `for<'a> Ctx<()>: BufferUdpStateContext<&'a ()>` is impossible to satisfy
+  --> $DIR/issue-89118.rs:9:5
+   |
+LL | / trait StackContext
+LL | | where
+LL | |     Ctx<()>: for<'a> BufferUdpStateContext<&'a ()>,
+   | |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+LL | |
+LL | | {
+LL | |     type Dispatcher;
+LL | | }
+   | |_- this item cannot be referenced without causing an error
+   |
+   = help: add `#![feature(trivial_bounds)]` to the crate attributes to allow it
+
 error[E0277]: the trait bound `for<'a> &'a (): BufferMut` is not satisfied
-  --> $DIR/issue-89118.rs:19:8
+  --> $DIR/issue-89118.rs:20:8
    |
 LL |     C: StackContext,
    |        ^^^^^^^^^^^^ the trait `for<'a> BufferMut` is not implemented for `&'a ()`
@@ -18,8 +33,18 @@ LL | where
 LL |     Ctx<()>: for<'a> BufferUdpStateContext<&'a ()>,
    |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `StackContext`
 
+warning: the where-clause bound `for<'a> Ctx<()>: BufferUdpStateContext<&'a ()>` is impossible to satisfy
+  --> $DIR/issue-89118.rs:29:5
+   |
+LL | / struct EthernetWorker<C>(C)
+LL | | where
+LL | |     Ctx<()>: for<'a> BufferUdpStateContext<&'a ()>;
+   | |_____^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^- this item cannot be referenced without causing an error
+   |
+   = help: add `#![feature(trivial_bounds)]` to the crate attributes to allow it
+
 error[E0277]: the trait bound `for<'a> &'a (): BufferMut` is not satisfied
-  --> $DIR/issue-89118.rs:29:9
+  --> $DIR/issue-89118.rs:31:9
    |
 LL | impl<C> EthernetWorker<C> {}
    |         ^^^^^^^^^^^^^^^^^ the trait `for<'a> BufferMut` is not implemented for `&'a ()`
@@ -30,7 +55,7 @@ note: required because of the requirements on the impl of `for<'a> BufferUdpStat
 LL | impl<B: BufferMut, C> BufferUdpStateContext<B> for C {}
    |                       ^^^^^^^^^^^^^^^^^^^^^^^^     ^
 note: required by a bound in `EthernetWorker`
-  --> $DIR/issue-89118.rs:28:14
+  --> $DIR/issue-89118.rs:29:14
    |
 LL | struct EthernetWorker<C>(C)
    |        -------------- required by a bound in this
@@ -39,7 +64,7 @@ LL |     Ctx<()>: for<'a> BufferUdpStateContext<&'a ()>;
    |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `EthernetWorker`
 
 error[E0277]: the trait bound `for<'a> &'a (): BufferMut` is not satisfied
-  --> $DIR/issue-89118.rs:22:20
+  --> $DIR/issue-89118.rs:23:20
    |
 LL |     type Handler = Ctx<C::Dispatcher>;
    |                    ^^^^^^^^^^^^^^^^^^ the trait `for<'a> BufferMut` is not implemented for `&'a ()`
@@ -58,6 +83,6 @@ LL | where
 LL |     Ctx<()>: for<'a> BufferUdpStateContext<&'a ()>,
    |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `StackContext`
 
-error: aborting due to 3 previous errors
+error: aborting due to 3 previous errors; 2 warnings emitted
 
 For more information about this error, try `rustc --explain E0277`.

src/test/ui/issues/issue-36839.rs

@@ -12,6 +12,7 @@ pub trait Broken {
 impl<T> Broken for T {
     type Assoc = ();
     fn broken(&self) where Self::Assoc: Foo {
+        //~^ WARNING the where-clause bound
         let _x: <Self::Assoc as Foo>::Bar;
     }
 }

src/test/ui/issues/issue-36839.stderr

@@ -0,0 +1,14 @@
+warning: the where-clause bound `<T as Broken>::Assoc: Foo` is impossible to satisfy
+  --> $DIR/issue-36839.rs:14:28
+   |
+LL |       fn broken(&self) where Self::Assoc: Foo {
+   |  _____-                      ^^^^^^^^^^^^^^^^
+LL | |
+LL | |         let _x: <Self::Assoc as Foo>::Bar;
+LL | |     }
+   | |_____- this item cannot be referenced without causing an error
+   |
+   = help: add `#![feature(trivial_bounds)]` to the crate attributes to allow it
+
+warning: 1 warning emitted
+

src/test/ui/issues/issue-39970.rs

@@ -13,6 +13,7 @@ impl<'a> Array<'a> for () {
 impl Visit for () where
     //(): for<'a> Array<'a, Element=&'a ()>, // No ICE
     (): for<'a> Array<'a, Element=()>, // ICE
+    //~^ WARNING the where-clause bound
 {}
 
 fn main() {