[WIP] reject more impossible trivial bounds (HRTBs and trivial after normalization)

compiler/rustc_typeck/src/check/wfcheck.rs

@@ -17,6 +17,7 @@ use rustc_infer::infer::outlives::obligations::TypeOutlives;
 use rustc_infer::infer::region_constraints::GenericKind;
 use rustc_infer::infer::{self, RegionckMode};
 use rustc_infer::infer::{InferCtxt, TyCtxtInferExt};
+use rustc_infer::traits::TraitEngine;
 use rustc_middle::hir::nested_filter;
 use rustc_middle::ty::subst::{GenericArgKind, InternalSubsts, Subst};
 use rustc_middle::ty::trait_def::TraitSpecializationKind;
@@ -1796,7 +1797,7 @@ fn report_bivariance(
 
 /// Feature gates RFC 2056 -- trivial bounds, checking for global bounds that
 /// aren't true.
-fn check_false_global_bounds(fcx: &FnCtxt<'_, '_>, mut span: Span, id: hir::HirId) {
+fn check_false_global_bounds(fcx: &FnCtxt<'_, '_>, span: Span, id: hir::HirId) {
     let empty_env = ty::ParamEnv::empty();
 
     let def_id = fcx.tcx.hir().local_def_id(id);
@@ -1807,33 +1808,64 @@ fn check_false_global_bounds(fcx: &FnCtxt<'_, '_>, mut span: Span, id: hir::HirI
 
     for obligation in implied_obligations {
         let pred = obligation.predicate;
+
+        // only use the span of the predicate clause (#90869)
+        let hir_node = fcx.tcx.hir().find(id);
+        let span = if let Some(hir::Generics { where_clause, .. }) =
+            hir_node.and_then(|node| node.generics())
+        {
+            let obligation_span = obligation.cause.span(fcx.tcx);
+            where_clause
+                .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)
+        } else {
+            span
+        };
+
         // Match the existing behavior.
         if pred.is_global() && !pred.has_late_bound_regions() {
             let pred = fcx.normalize_associated_types_in(span, pred);
-            let hir_node = fcx.tcx.hir().find(id);
-
-            // only use the span of the predicate clause (#90869)
-
-            if let Some(hir::Generics { where_clause, .. }) =
-                hir_node.and_then(|node| node.generics())
-            {
-                let obligation_span = obligation.cause.span(fcx.tcx);
-
-                span = where_clause
-                    .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);
-            }
 
             let obligation = traits::Obligation::new(
                 traits::ObligationCause::new(span, id, traits::TrivialBound),
                 empty_env,
                 pred,
             );
             fcx.register_predicate(obligation);
+        } else {
+            let infer::InferOk { value: norm_pred, obligations } =
+                fcx.normalize_associated_types_in_as_infer_ok(span, pred);
+            if norm_pred.is_global() {
+                fcx.probe(|_| {
+                    let mut fulfillment_cx = traits::FulfillmentContext::new_in_snapshot();
+                    for obligation in obligations {
+                        fulfillment_cx.register_predicate_obligation(&fcx, obligation);
+                    }
+                    fulfillment_cx.register_predicate_obligation(&fcx, traits::Obligation::new(
+                        traits::ObligationCause::new(span, id, traits::TrivialBound),
+                        empty_env,
+                        norm_pred,
+                    ));
+                    if !fulfillment_cx.select_all_or_error(&fcx).is_empty() {
+                        let mut diag = fcx
+                        .tcx
+                        .sess
+                        // NOTE: Error for the crater run
+                        .struct_span_warn(span, "where-clause bound is impossible to satisfy");
+                        diag.note(format!("the bound `{}` was previously accepted, but it may become a hard error in a future release", pred));
+                        if fcx.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

@@ -1,5 +1,5 @@
 error: fatal error triggered by #[rustc_error]
-  --> $DIR/bound-lifetime-in-binding-only.rs:71:1
+  --> $DIR/bound-lifetime-in-binding-only.rs:75:1
    |
 LL | fn main() { }
    | ^^^^^^^^^

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

@@ -55,6 +55,10 @@ fn elision<T: Fn() -> &i32>() {
 
 struct Parameterized<'a> { x: &'a str }
 
+impl<'a> Foo for Parameterized<'a> {
+    type Item = &'a i32;
+}
+
 #[cfg(ok)]
 fn ok1<T: for<'a> Fn(&Parameterized<'a>) -> &'a i32>() {
 }

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 {
+        //~^ WARN where-clause bound is impossible to satisfy
         let _x: <Self::Assoc as Foo>::Bar;
     }
 }

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

@@ -0,0 +1,11 @@
+warning: where-clause bound is impossible to satisfy
+  --> $DIR/issue-69398.rs:14:28
+   |
+LL |     fn broken(&self) where Self::Assoc: Foo {
+   |                            ^^^^^^^^^^^^^^^^
+   |
+   = note: the bound `<T as Broken>::Assoc: Foo` was previously accepted, but it may become a hard error in a future release
+   = 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

@@ -10,11 +10,18 @@ trait A {
 }
 
 impl A for [fn(&())] {
-    fn foo(&self) -> Self where Self: Copy { *(&[] as &[_]) }
+    fn foo(&self) -> Self where Self: Copy {
+        //~^ WARN where-clause bound is impossible to satisfy
+        //~| WARN where-clause bound is impossible to satisfy
+        //~| WARN where-clause bound is impossible to satisfy
+        *(&[] as &[_])
+    }
 }
 
 impl A for i32 {
-    fn foo(&self) -> Self { 3 }
+    fn foo(&self) -> Self {
+        3
+    }
 }
 
 fn main() {}

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

@@ -0,0 +1,29 @@
+warning: where-clause bound is impossible to satisfy
+  --> $DIR/issue-67696-const-prop-ice.rs:13:33
+   |
+LL |     fn foo(&self) -> Self where Self: Copy {
+   |                                 ^^^^^^^^^^
+   |
+   = note: the bound `[for<'r> fn(&'r ())]: Copy` was previously accepted, but it may become a hard error in a future release
+   = help: add `#![feature(trivial_bounds)]` to the crate attributes to allow it
+
+warning: where-clause bound is impossible to satisfy
+  --> $DIR/issue-67696-const-prop-ice.rs:13:33
+   |
+LL |     fn foo(&self) -> Self where Self: Copy {
+   |                                 ^^^^^^^^^^
+   |
+   = note: the bound `[for<'r> fn(&'r ())]: Clone` was previously accepted, but it may become a hard error in a future release
+   = help: add `#![feature(trivial_bounds)]` to the crate attributes to allow it
+
+warning: where-clause bound is impossible to satisfy
+  --> $DIR/issue-67696-const-prop-ice.rs:13:33
+   |
+LL |     fn foo(&self) -> Self where Self: Copy {
+   |                                 ^^^^^^^^^^
+   |
+   = note: the bound `[for<'r> fn(&'r ())]: Sized` was previously accepted, but it may become a hard error in a future release
+   = 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

@@ -62,7 +62,9 @@ fn return_str() -> str where str: Sized { //~ ERROR
 
 // This is currently accepted because the function pointer isn't
 // considered global.
+// FIXME(compiler-errors): Not for long..
 fn global_hr(x: fn(&())) where fn(&()): Foo { // OK
+    //~^ WARN where-clause bound is impossible to satisfy
     x.test();
 }
 

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

@@ -113,6 +113,15 @@ 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: where-clause bound is impossible to satisfy
+  --> $DIR/feature-gate-trivial_bounds.rs:66:32
+   |
+LL | fn global_hr(x: fn(&())) where fn(&()): Foo { // OK
+   |                                ^^^^^^^^^^^^
+   |
+   = note: the bound `for<'r> fn(&'r ()): Foo` was previously accepted, but it may become a hard error in a future release
+   = 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 ()>,
+    //~^ WARN where-clause bound is impossible to satisfy
 {
     type Dispatcher;
 }
@@ -26,6 +27,8 @@ where
 struct EthernetWorker<C>(C)
 where
     Ctx<()>: for<'a> BufferUdpStateContext<&'a ()>;
+    //~^ WARN where-clause bound is impossible to satisfy
+
 impl<C> EthernetWorker<C> {}
 //~^ ERROR: is not satisfied [E0277]
 

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

@@ -1,5 +1,14 @@
+warning: where-clause bound is impossible to satisfy
+  --> $DIR/issue-89118.rs:9:5
+   |
+LL |     Ctx<()>: for<'a> BufferUdpStateContext<&'a ()>,
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: the bound `for<'a> Ctx<()>: BufferUdpStateContext<&'a ()>` was previously accepted, but it may become a hard error in a future release
+   = 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 ()`
@@ -19,7 +28,7 @@ LL |     Ctx<()>: for<'a> BufferUdpStateContext<&'a ()>,
    |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `StackContext`
 
 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 ()`
@@ -38,8 +47,17 @@ LL | where
 LL |     Ctx<()>: for<'a> BufferUdpStateContext<&'a ()>,
    |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `StackContext`
 
+warning: where-clause bound is impossible to satisfy
+  --> $DIR/issue-89118.rs:29:5
+   |
+LL |     Ctx<()>: for<'a> BufferUdpStateContext<&'a ()>;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: the bound `for<'a> Ctx<()>: BufferUdpStateContext<&'a ()>` was previously accepted, but it may become a hard error in a future release
+   = 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:32:9
    |
 LL | impl<C> EthernetWorker<C> {}
    |         ^^^^^^^^^^^^^^^^^ the trait `for<'a> BufferMut` is not implemented for `&'a ()`
@@ -50,14 +68,14 @@ 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
 LL | where
 LL |     Ctx<()>: for<'a> BufferUdpStateContext<&'a ()>;
    |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `EthernetWorker`
 
-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 {
+        //~^ WARN where-clause bound is impossible to satisfy
         let _x: <Self::Assoc as Foo>::Bar;
     }
 }

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

@@ -0,0 +1,11 @@
+warning: where-clause bound is impossible to satisfy
+  --> $DIR/issue-36839.rs:14:28
+   |
+LL |     fn broken(&self) where Self::Assoc: Foo {
+   |                            ^^^^^^^^^^^^^^^^
+   |
+   = note: the bound `<T as Broken>::Assoc: Foo` was previously accepted, but it may become a hard error in a future release
+   = 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
+    //~^ WARN where-clause bound is impossible to satisfy
 {}
 
 fn main() {

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

@@ -1,5 +1,14 @@
+warning: where-clause bound is impossible to satisfy
+  --> $DIR/issue-39970.rs:15:5
+   |
+LL |     (): for<'a> Array<'a, Element=()>, // ICE
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: the bound `for<'a> <() as Array<'a>>::Element == ()` was previously accepted, but it may become a hard error in a future release
+   = help: add `#![feature(trivial_bounds)]` to the crate attributes to allow it
+
 error[E0271]: type mismatch resolving `for<'a> <() as Array<'a>>::Element == ()`
-  --> $DIR/issue-39970.rs:19:5
+  --> $DIR/issue-39970.rs:20:5
    |
 LL |     <() as Visit>::visit();
    |     ^^^^^^^^^^^^^^^^^^^^ type mismatch resolving `for<'a> <() as Array<'a>>::Element == ()`
@@ -15,6 +24,6 @@ note: required because of the requirements on the impl of `Visit` for `()`
 LL | impl Visit for () where
    |      ^^^^^     ^^
 
-error: aborting due to previous error
+error: aborting due to previous error; 1 warning emitted
 
 For more information about this error, try `rustc --explain E0271`.

src/test/ui/issues/issue-42796.rs

@@ -7,6 +7,7 @@ impl<T, Smoke> Mirror<Smoke> for T {
 }
 
 pub fn poison<S>(victim: String) where <String as Mirror<S>>::Image: Copy {
+    //~^ WARN where-clause bound is impossible to satisfy
     loop { drop(victim); }
 }
 

src/test/ui/issues/issue-42796.stderr

@@ -1,5 +1,14 @@
+warning: where-clause bound is impossible to satisfy
+  --> $DIR/issue-42796.rs:9:40
+   |
+LL | pub fn poison<S>(victim: String) where <String as Mirror<S>>::Image: Copy {
+   |                                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: the bound `<String as Mirror<S>>::Image: Copy` was previously accepted, but it may become a hard error in a future release
+   = help: add `#![feature(trivial_bounds)]` to the crate attributes to allow it
+
 error[E0382]: borrow of moved value: `s`
-  --> $DIR/issue-42796.rs:18:20
+  --> $DIR/issue-42796.rs:19:20
    |
 LL |     let s = "Hello!".to_owned();
    |         - move occurs because `s` has type `String`, which does not implement the `Copy` trait
@@ -11,6 +20,6 @@ LL |     println!("{}", s);
    |
    = note: this error originates in the macro `$crate::format_args_nl` (in Nightly builds, run with -Z macro-backtrace for more info)
 
-error: aborting due to previous error
+error: aborting due to previous error; 1 warning emitted
 
 For more information about this error, try `rustc --explain E0382`.

src/test/ui/mir/issue-91745.rs

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

src/test/ui/mir/issue-91745.stderr

@@ -0,0 +1,11 @@
+warning: where-clause bound is impossible to satisfy
+  --> $DIR/issue-91745.rs:14:28
+   |
+LL |     fn broken(&self) where Self::Assoc: Foo {
+   |                            ^^^^^^^^^^^^^^^^
+   |
+   = note: the bound `<T as Broken>::Assoc: Foo` was previously accepted, but it may become a hard error in a future release
+   = help: add `#![feature(trivial_bounds)]` to the crate attributes to allow it
+
+warning: 1 warning emitted
+