Utilize coercion information to guide never type fallback

This doesn't yet fully work -- running coercions seems to be prone to causing
errors in the environment, which creates a little noise in some cases. It should
be enough to let us re-run crater, though, I think.

Author: Mark-Simulacrum

compiler/rustc_typeck/src/check/cast.rs

@@ -54,8 +54,8 @@ use rustc_trait_selection::traits::error_reporting::report_object_safety_error;
 #[derive(Debug)]
 pub struct CastCheck<'tcx> {
     expr: &'tcx hir::Expr<'tcx>,
-    expr_ty: Ty<'tcx>,
-    cast_ty: Ty<'tcx>,
+    pub(crate) expr_ty: Ty<'tcx>,
+    pub(crate) cast_ty: Ty<'tcx>,
     cast_span: Span,
     span: Span,
 }

compiler/rustc_typeck/src/check/coercion.rs

@@ -62,7 +62,7 @@ use rustc_trait_selection::traits::{self, ObligationCause, ObligationCauseCode};
 use smallvec::{smallvec, SmallVec};
 use std::ops::Deref;
 
-struct Coerce<'a, 'tcx> {
+pub(crate) struct Coerce<'a, 'tcx> {
     fcx: &'a FnCtxt<'a, 'tcx>,
     cause: ObligationCause<'tcx>,
     use_lub: bool,
@@ -116,7 +116,7 @@ fn success<'tcx>(
 }
 
 impl<'f, 'tcx> Coerce<'f, 'tcx> {
-    fn new(
+    pub(crate) fn new(
         fcx: &'f FnCtxt<'f, 'tcx>,
         cause: ObligationCause<'tcx>,
         allow_two_phase: AllowTwoPhase,
@@ -146,7 +146,15 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
             .and_then(|InferOk { value: ty, obligations }| success(f(ty), ty, obligations))
     }
 
+    pub(crate) fn coerce_silent(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> CoerceResult<'tcx> {
+        self.coerce_(false, a, b)
+    }
+
     fn coerce(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> CoerceResult<'tcx> {
+        self.coerce_(true, a, b)
+    }
+
+    fn coerce_(&self, report_error: bool, a: Ty<'tcx>, b: Ty<'tcx>) -> CoerceResult<'tcx> {
         // First, remove any resolved type variables (at the top level, at least):
         let a = self.shallow_resolve(a);
         let b = self.shallow_resolve(b);
@@ -174,7 +182,7 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
         // NOTE: this is wrapped in a `commit_if_ok` because it creates
         // a "spurious" type variable, and we don't want to have that
         // type variable in memory if the coercion fails.
-        let unsize = self.commit_if_ok(|_| self.coerce_unsized(a, b));
+        let unsize = self.commit_if_ok(|_| self.coerce_unsized(report_error, a, b));
         match unsize {
             Ok(_) => {
                 debug!("coerce: unsize successful");
@@ -482,7 +490,12 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
     // &[T; n] or &mut [T; n] -> &[T]
     // or &mut [T; n] -> &mut [T]
     // or &Concrete -> &Trait, etc.
-    fn coerce_unsized(&self, mut source: Ty<'tcx>, mut target: Ty<'tcx>) -> CoerceResult<'tcx> {
+    fn coerce_unsized(
+        &self,
+        report_error: bool,
+        mut source: Ty<'tcx>,
+        mut target: Ty<'tcx>,
+    ) -> CoerceResult<'tcx> {
         debug!("coerce_unsized(source={:?}, target={:?})", source, target);
 
         source = self.shallow_resolve(source);
@@ -690,13 +703,15 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
 
                 // Object safety violations or miscellaneous.
                 Err(err) => {
-                    self.report_selection_error(
-                        obligation.clone(),
-                        &obligation,
-                        &err,
-                        false,
-                        false,
-                    );
+                    if report_error {
+                        self.report_selection_error(
+                            obligation.clone(),
+                            &obligation,
+                            &err,
+                            false,
+                            false,
+                        );
+                    }
                     // Treat this like an obligation and follow through
                     // with the unsizing - the lack of a coercion should
                     // be silent, as it causes a type mismatch later.
@@ -707,13 +722,15 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
         }
 
         if has_unsized_tuple_coercion && !self.tcx.features().unsized_tuple_coercion {
-            feature_err(
-                &self.tcx.sess.parse_sess,
-                sym::unsized_tuple_coercion,
-                self.cause.span,
-                "unsized tuple coercion is not stable enough for use and is subject to change",
-            )
-            .emit();
+            if report_error {
+                feature_err(
+                    &self.tcx.sess.parse_sess,
+                    sym::unsized_tuple_coercion,
+                    self.cause.span,
+                    "unsized tuple coercion is not stable enough for use and is subject to change",
+                )
+                .emit();
+            }
         }
 
         if has_trait_upcasting_coercion && !self.tcx().features().trait_upcasting {

compiler/rustc_typeck/src/check/fallback.rs

@@ -5,7 +5,8 @@ use rustc_data_structures::{
     graph::{iterate::DepthFirstSearch, vec_graph::VecGraph},
     stable_set::FxHashSet,
 };
-use rustc_middle::ty::{self, Ty};
+use rustc_infer::traits::ObligationCauseCode;
+use rustc_middle::ty::{self, adjustment::AllowTwoPhase, Ty, TypeFoldable};
 
 impl<'tcx> FnCtxt<'_, 'tcx> {
     /// Performs type inference fallback, returning true if any fallback
@@ -16,6 +17,48 @@ impl<'tcx> FnCtxt<'_, 'tcx> {
             self.fulfillment_cx.borrow_mut().pending_obligations()
         );
 
+        let mut cast_vars = Vec::new();
+
+        // FIXME: This seems to cause extra object safety errors. Not clear why; one would expect the probe and such to eat them.
+        for cast in self.deferred_cast_checks.borrow().iter() {
+            let source = cast.expr_ty;
+            let target = cast.cast_ty;
+            debug!("attempting coerce {:?} -> {:?}", source, target,);
+            let source = self.resolve_vars_with_obligations(source);
+
+            let cause = self.cause(rustc_span::DUMMY_SP, ObligationCauseCode::ExprAssignable);
+            // We don't ever need two-phase here since we throw out the result of the coercion
+            let coerce = crate::check::coercion::Coerce::new(self, cause, AllowTwoPhase::No);
+            if let Ok(infok) = self.probe(|_| coerce.coerce_silent(source, target)) {
+                for obligation in infok.obligations {
+                    if let ty::PredicateKind::Projection(predicate) =
+                        obligation.predicate.kind().skip_binder()
+                    {
+                        if !predicate.projection_ty.has_escaping_bound_vars() {
+                            // FIXME: We really *should* do this even with escaping bound
+                            // vars, but there's not much we can do here. In the worst case
+                            // (if this ends up being important) we just don't register a relationship and then end up falling back to !,
+                            // which is not terrible.
+
+                            if let Some(vid) = self
+                                .fulfillment_cx
+                                .borrow_mut()
+                                .normalize_projection_type(
+                                    &self.infcx,
+                                    obligation.param_env,
+                                    predicate.projection_ty,
+                                    obligation.cause.clone(),
+                                )
+                                .ty_vid()
+                            {
+                                cast_vars.push(vid);
+                            }
+                        }
+                    }
+                }
+            }
+        }
+
         // All type checking constraints were added, try to fallback unsolved variables.
         self.select_obligations_where_possible(false, |_| {});
 
@@ -36,7 +79,7 @@ impl<'tcx> FnCtxt<'_, 'tcx> {
         }
 
         let diverging_fallback =
-            self.calculate_diverging_fallback(&unsolved_variables, &relationships);
+            self.calculate_diverging_fallback(&unsolved_variables, &relationships, &cast_vars);
 
         // We do fallback in two passes, to try to generate
         // better error messages.
@@ -263,6 +306,7 @@ impl<'tcx> FnCtxt<'_, 'tcx> {
         &self,
         unsolved_variables: &[Ty<'tcx>],
         relationships: &FxHashMap<ty::TyVid, ty::FoundRelationships>,
+        cast_vars: &[ty::TyVid],
     ) -> FxHashMap<Ty<'tcx>, Ty<'tcx>> {
         debug!("calculate_diverging_fallback({:?})", unsolved_variables);
 
@@ -368,16 +412,26 @@ impl<'tcx> FnCtxt<'_, 'tcx> {
             let mut relationship = ty::FoundRelationships { self_in_trait: false, output: false };
 
             for (vid, rel) in relationships.iter() {
-                //if self.infcx.shallow_resolve(*ty).ty_vid().map(|t| self.infcx.root_var(t))
                 if self.infcx.root_var(*vid) == root_vid {
                     relationship.self_in_trait |= rel.self_in_trait;
                     relationship.output |= rel.output;
                 }
             }
 
+            let mut is_cast = false;
+            for vid in cast_vars.iter() {
+                if self.infcx.root_var(*vid) == root_vid {
+                    is_cast = true;
+                    break;
+                }
+            }
+
             if relationship.self_in_trait && relationship.output {
                 debug!("fallback to () - found trait and projection: {:?}", diverging_vid);
                 diverging_fallback.insert(diverging_ty, self.tcx.types.unit);
+            } else if is_cast {
+                debug!("fallback to () - interacted with cast: {:?}", diverging_vid);
+                diverging_fallback.insert(diverging_ty, self.tcx.types.unit);
             } else if can_reach_non_diverging {
                 debug!("fallback to () - reached non-diverging: {:?}", diverging_vid);
                 diverging_fallback.insert(diverging_ty, self.tcx.types.unit);

src/test/ui/issues/issue-22034.rs

@@ -1,3 +1,4 @@
+//~ ERROR expected a `FnOnce<()>` closure, found `()`
 #![feature(rustc_private)]
 
 extern crate libc;

src/test/ui/issues/issue-22034.stderr

@@ -1,5 +1,11 @@
+error[E0277]: expected a `FnOnce<()>` closure, found `()`
+   |
+   = help: the trait `FnOnce<()>` is not implemented for `()`
+   = note: wrap the `()` in a closure with no arguments: `|| { /* code */ }`
+   = note: required for the cast to the object type `dyn Fn()`
+
 error[E0277]: expected a `Fn<()>` closure, found `()`
-  --> $DIR/issue-22034.rs:8:16
+  --> $DIR/issue-22034.rs:9:16
    |
 LL |         &mut *(ptr as *mut dyn Fn())
    |                ^^^ expected an `Fn<()>` closure, found `()`
@@ -8,6 +14,6 @@ LL |         &mut *(ptr as *mut dyn Fn())
    = note: wrap the `()` in a closure with no arguments: `|| { /* code */ }`
    = note: required for the cast to the object type `dyn Fn()`
 
-error: aborting due to previous error
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0277`.

src/test/ui/never_type/fallback-closure-wrap.rs

@@ -1,8 +1,9 @@
+// check-pass
+
 use std::marker::PhantomData;
 
 fn main() {
     let error = Closure::wrap(Box::new(move || {
-    //~^ ERROR type mismatch
         panic!("Can't connect to server.");
     }) as Box<dyn FnMut()>);
 }