Auto merge of #88149 - Mark-Simulacrum:prep-never-type, r=jackh726

Refactor fallback code to prepare for never type

This PR contains cherry-picks of some of `@nikomatsakis's` work from #79366, and shouldn't (AFAICT) represent any change in behavior. However, the refactoring is good regardless of the never type work being landed, and will reduce the size of those eventual PR(s) (and rebase pain).

I am not personally an expert on this code, and the commits are essentially 100% `@nikomatsakis's,` but they do seem reasonable to me by my understanding. Happy to edit with review, of course. Commits are best reviewed in sequence rather than all together.

r? `@jackh726` perhaps?

compiler/rustc_infer/src/infer/combine.rs

@@ -22,14 +22,14 @@
 // is also useful to track which value is the "expected" value in
 // terms of error reporting.
 
-use super::equate::Equate;
 use super::glb::Glb;
 use super::lub::Lub;
 use super::sub::Sub;
 use super::type_variable::TypeVariableValue;
 use super::unify_key::replace_if_possible;
 use super::unify_key::{ConstVarValue, ConstVariableValue};
 use super::unify_key::{ConstVariableOrigin, ConstVariableOriginKind};
+use super::{equate::Equate, type_variable::Diverging};
 use super::{InferCtxt, MiscVariable, TypeTrace};
 
 use crate::traits::{Obligation, PredicateObligations};
@@ -643,8 +643,13 @@ impl TypeRelation<'tcx> for Generalizer<'_, 'tcx> {
                                 .inner
                                 .borrow_mut()
                                 .type_variables()
-                                .new_var(self.for_universe, false, origin);
+                                .new_var(self.for_universe, Diverging::NotDiverging, origin);
                             let u = self.tcx().mk_ty_var(new_var_id);
+
+                            // Record that we replaced `vid` with `new_var_id` as part of a generalization
+                            // operation. This is needed to detect cyclic types. To see why, see the
+                            // docs in the `type_variables` module.
+                            self.infcx.inner.borrow_mut().type_variables().sub(vid, new_var_id);
                             debug!("generalize: replacing original vid={:?} with new={:?}", vid, u);
                             Ok(u)
                         }
@@ -881,7 +886,7 @@ impl TypeRelation<'tcx> for ConstInferUnifier<'_, 'tcx> {
                             *self.infcx.inner.borrow_mut().type_variables().var_origin(vid);
                         let new_var_id = self.infcx.inner.borrow_mut().type_variables().new_var(
                             self.for_universe,
-                            false,
+                            Diverging::NotDiverging,
                             origin,
                         );
                         let u = self.tcx().mk_ty_var(new_var_id);

compiler/rustc_infer/src/infer/mod.rs

@@ -23,7 +23,7 @@ use rustc_middle::infer::unify_key::{ConstVarValue, ConstVariableValue};
 use rustc_middle::infer::unify_key::{ConstVariableOrigin, ConstVariableOriginKind, ToType};
 use rustc_middle::mir::interpret::EvalToConstValueResult;
 use rustc_middle::traits::select;
-use rustc_middle::ty::error::{ExpectedFound, TypeError, UnconstrainedNumeric};
+use rustc_middle::ty::error::{ExpectedFound, TypeError};
 use rustc_middle::ty::fold::{TypeFoldable, TypeFolder};
 use rustc_middle::ty::relate::RelateResult;
 use rustc_middle::ty::subst::{GenericArg, GenericArgKind, InternalSubsts, SubstsRef};
@@ -46,7 +46,7 @@ use self::region_constraints::{GenericKind, RegionConstraintData, VarInfos, Veri
 use self::region_constraints::{
     RegionConstraintCollector, RegionConstraintStorage, RegionSnapshot,
 };
-use self::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
+use self::type_variable::{Diverging, TypeVariableOrigin, TypeVariableOriginKind};
 
 pub mod at;
 pub mod canonical;
@@ -679,10 +679,27 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         t.fold_with(&mut self.freshener())
     }
 
-    pub fn type_var_diverges(&'a self, ty: Ty<'_>) -> bool {
+    /// Returns whether `ty` is a diverging type variable or not.
+    /// (If `ty` is not a type variable at all, returns not diverging.)
+    ///
+    /// No attempt is made to resolve `ty`.
+    pub fn type_var_diverges(&'a self, ty: Ty<'_>) -> Diverging {
         match *ty.kind() {
             ty::Infer(ty::TyVar(vid)) => self.inner.borrow_mut().type_variables().var_diverges(vid),
-            _ => false,
+            _ => Diverging::NotDiverging,
+        }
+    }
+
+    /// Returns the origin of the type variable identified by `vid`, or `None`
+    /// if this is not a type variable.
+    ///
+    /// No attempt is made to resolve `ty`.
+    pub fn type_var_origin(&'a self, ty: Ty<'tcx>) -> Option<TypeVariableOrigin> {
+        match *ty.kind() {
+            ty::Infer(ty::TyVar(vid)) => {
+                Some(*self.inner.borrow_mut().type_variables().var_origin(vid))
+            }
+            _ => None,
         }
     }
 
@@ -695,28 +712,6 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         freshen::TypeFreshener::new(self, true)
     }
 
-    pub fn type_is_unconstrained_numeric(&'a self, ty: Ty<'_>) -> UnconstrainedNumeric {
-        use rustc_middle::ty::error::UnconstrainedNumeric::Neither;
-        use rustc_middle::ty::error::UnconstrainedNumeric::{UnconstrainedFloat, UnconstrainedInt};
-        match *ty.kind() {
-            ty::Infer(ty::IntVar(vid)) => {
-                if self.inner.borrow_mut().int_unification_table().probe_value(vid).is_some() {
-                    Neither
-                } else {
-                    UnconstrainedInt
-                }
-            }
-            ty::Infer(ty::FloatVar(vid)) => {
-                if self.inner.borrow_mut().float_unification_table().probe_value(vid).is_some() {
-                    Neither
-                } else {
-                    UnconstrainedFloat
-                }
-            }
-            _ => Neither,
-        }
-    }
-
     pub fn unsolved_variables(&self) -> Vec<Ty<'tcx>> {
         let mut inner = self.inner.borrow_mut();
         let mut vars: Vec<Ty<'_>> = inner
@@ -969,29 +964,62 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         );
     }
 
+    /// Processes a `Coerce` predicate from the fulfillment context.
+    /// This is NOT the preferred way to handle coercion, which is to
+    /// invoke `FnCtxt::coerce` or a similar method (see `coercion.rs`).
+    ///
+    /// This method here is actually a fallback that winds up being
+    /// invoked when `FnCtxt::coerce` encounters unresolved type variables
+    /// and records a coercion predicate. Presently, this method is equivalent
+    /// to `subtype_predicate` -- that is, "coercing" `a` to `b` winds up
+    /// actually requiring `a <: b`. This is of course a valid coercion,
+    /// but it's not as flexible as `FnCtxt::coerce` would be.
+    ///
+    /// (We may refactor this in the future, but there are a number of
+    /// practical obstacles. Among other things, `FnCtxt::coerce` presently
+    /// records adjustments that are required on the HIR in order to perform
+    /// the coercion, and we don't currently have a way to manage that.)
+    pub fn coerce_predicate(
+        &self,
+        cause: &ObligationCause<'tcx>,
+        param_env: ty::ParamEnv<'tcx>,
+        predicate: ty::PolyCoercePredicate<'tcx>,
+    ) -> Option<InferResult<'tcx, ()>> {
+        let subtype_predicate = predicate.map_bound(|p| ty::SubtypePredicate {
+            a_is_expected: false, // when coercing from `a` to `b`, `b` is expected
+            a: p.a,
+            b: p.b,
+        });
+        self.subtype_predicate(cause, param_env, subtype_predicate)
+    }
+
     pub fn subtype_predicate(
         &self,
         cause: &ObligationCause<'tcx>,
         param_env: ty::ParamEnv<'tcx>,
         predicate: ty::PolySubtypePredicate<'tcx>,
     ) -> Option<InferResult<'tcx, ()>> {
-        // Subtle: it's ok to skip the binder here and resolve because
-        // `shallow_resolve` just ignores anything that is not a type
-        // variable, and because type variable's can't (at present, at
+        // Check for two unresolved inference variables, in which case we can
+        // make no progress. This is partly a micro-optimization, but it's
+        // also an opportunity to "sub-unify" the variables. This isn't
+        // *necessary* to prevent cycles, because they would eventually be sub-unified
+        // anyhow during generalization, but it helps with diagnostics (we can detect
+        // earlier that they are sub-unified).
+        //
+        // Note that we can just skip the binders here because
+        // type variables can't (at present, at
         // least) capture any of the things bound by this binder.
         //
-        // NOTE(nmatsakis): really, there is no *particular* reason to do this
-        // `shallow_resolve` here except as a micro-optimization.
-        // Naturally I could not resist.
-        let two_unbound_type_vars = {
-            let a = self.shallow_resolve(predicate.skip_binder().a);
-            let b = self.shallow_resolve(predicate.skip_binder().b);
-            a.is_ty_var() && b.is_ty_var()
-        };
-
-        if two_unbound_type_vars {
-            // Two unbound type variables? Can't make progress.
-            return None;
+        // Note that this sub here is not just for diagnostics - it has semantic
+        // effects as well.
+        let r_a = self.shallow_resolve(predicate.skip_binder().a);
+        let r_b = self.shallow_resolve(predicate.skip_binder().b);
+        match (r_a.kind(), r_b.kind()) {
+            (&ty::Infer(ty::TyVar(a_vid)), &ty::Infer(ty::TyVar(b_vid))) => {
+                self.inner.borrow_mut().type_variables().sub(a_vid, b_vid);
+                return None;
+            }
+            _ => {}
         }
 
         Some(self.commit_if_ok(|_snapshot| {
@@ -1020,25 +1048,29 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         })
     }
 
-    pub fn next_ty_var_id(&self, diverging: bool, origin: TypeVariableOrigin) -> TyVid {
+    pub fn next_ty_var_id(&self, diverging: Diverging, origin: TypeVariableOrigin) -> TyVid {
         self.inner.borrow_mut().type_variables().new_var(self.universe(), diverging, origin)
     }
 
     pub fn next_ty_var(&self, origin: TypeVariableOrigin) -> Ty<'tcx> {
-        self.tcx.mk_ty_var(self.next_ty_var_id(false, origin))
+        self.tcx.mk_ty_var(self.next_ty_var_id(Diverging::NotDiverging, origin))
     }
 
     pub fn next_ty_var_in_universe(
         &self,
         origin: TypeVariableOrigin,
         universe: ty::UniverseIndex,
     ) -> Ty<'tcx> {
-        let vid = self.inner.borrow_mut().type_variables().new_var(universe, false, origin);
+        let vid = self.inner.borrow_mut().type_variables().new_var(
+            universe,
+            Diverging::NotDiverging,
+            origin,
+        );
         self.tcx.mk_ty_var(vid)
     }
 
     pub fn next_diverging_ty_var(&self, origin: TypeVariableOrigin) -> Ty<'tcx> {
-        self.tcx.mk_ty_var(self.next_ty_var_id(true, origin))
+        self.tcx.mk_ty_var(self.next_ty_var_id(Diverging::Diverges, origin))
     }
 
     pub fn next_const_var(
@@ -1152,7 +1184,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                 // as the substitutions for the default, `(T, U)`.
                 let ty_var_id = self.inner.borrow_mut().type_variables().new_var(
                     self.universe(),
-                    false,
+                    Diverging::NotDiverging,
                     TypeVariableOrigin {
                         kind: TypeVariableOriginKind::TypeParameterDefinition(
                             param.name,

compiler/rustc_infer/src/infer/nll_relate/mod.rs

@@ -22,6 +22,7 @@
 //!   constituents)
 
 use crate::infer::combine::ConstEquateRelation;
+use crate::infer::type_variable::Diverging;
 use crate::infer::InferCtxt;
 use crate::infer::{ConstVarValue, ConstVariableValue};
 use rustc_data_structures::fx::FxHashMap;
@@ -920,7 +921,8 @@ where
                             // Replacing with a new variable in the universe `self.universe`,
                             // it will be unified later with the original type variable in
                             // the universe `_universe`.
-                            let new_var_id = variables.new_var(self.universe, false, origin);
+                            let new_var_id =
+                                variables.new_var(self.universe, Diverging::NotDiverging, origin);
 
                             let u = self.tcx().mk_ty_var(new_var_id);
                             debug!("generalize: replacing original vid={:?} with new={:?}", vid, u);

compiler/rustc_infer/src/infer/outlives/mod.rs

@@ -19,6 +19,7 @@ pub fn explicit_outlives_bounds<'tcx>(
         .filter_map(move |kind| match kind {
             ty::PredicateKind::Projection(..)
             | ty::PredicateKind::Trait(..)
+            | ty::PredicateKind::Coerce(..)
             | ty::PredicateKind::Subtype(..)
             | ty::PredicateKind::WellFormed(..)
             | ty::PredicateKind::ObjectSafe(..)

compiler/rustc_infer/src/infer/sub.rs

@@ -85,19 +85,15 @@ impl TypeRelation<'tcx> for Sub<'combine, 'infcx, 'tcx> {
         let a = infcx.inner.borrow_mut().type_variables().replace_if_possible(a);
         let b = infcx.inner.borrow_mut().type_variables().replace_if_possible(b);
         match (a.kind(), b.kind()) {
-            (&ty::Infer(TyVar(a_vid)), &ty::Infer(TyVar(b_vid))) => {
+            (&ty::Infer(TyVar(_)), &ty::Infer(TyVar(_))) => {
                 // Shouldn't have any LBR here, so we can safely put
                 // this under a binder below without fear of accidental
                 // capture.
                 assert!(!a.has_escaping_bound_vars());
                 assert!(!b.has_escaping_bound_vars());
 
                 // can't make progress on `A <: B` if both A and B are
-                // type variables, so record an obligation. We also
-                // have to record in the `type_variables` tracker that
-                // the two variables are equal modulo subtyping, which
-                // is important to the occurs check later on.
-                infcx.inner.borrow_mut().type_variables().sub(a_vid, b_vid);
+                // type variables, so record an obligation.
                 self.fields.obligations.push(Obligation::new(
                     self.fields.trace.cause.clone(),
                     self.fields.param_env,

compiler/rustc_infer/src/infer/type_variable.rs

@@ -75,14 +75,30 @@ pub struct TypeVariableStorage<'tcx> {
     ///     ?1 <: ?3
     ///     Box<?3> <: ?1
     ///
-    /// This works because `?1` and `?3` are unified in the
-    /// `sub_relations` relation (not in `eq_relations`). Then when we
-    /// process the `Box<?3> <: ?1` constraint, we do an occurs check
-    /// on `Box<?3>` and find a potential cycle.
+    /// Without this second table, what would happen in a case like
+    /// this is that we would instantiate `?1` with a generalized
+    /// type like `Box<?6>`. We would then relate `Box<?3> <: Box<?6>`
+    /// and infer that `?3 <: ?6`. Next, since `?1` was instantiated,
+    /// we would process `?1 <: ?3`, generalize `?1 = Box<?6>` to `Box<?9>`,
+    /// and instantiate `?3` with `Box<?9>`. Finally, we would relate
+    /// `?6 <: ?9`. But now that we instantiated `?3`, we can process
+    /// `?3 <: ?6`, which gives us `Box<?9> <: ?6`... and the cycle
+    /// continues. (This is `occurs-check-2.rs`.)
+    ///
+    /// What prevents this cycle is that when we generalize
+    /// `Box<?3>` to `Box<?6>`, we also sub-unify `?3` and `?6`
+    /// (in the generalizer). When we then process `Box<?6> <: ?3`,
+    /// the occurs check then fails because `?6` and `?3` are sub-unified,
+    /// and hence generalization fails.
     ///
     /// This is reasonable because, in Rust, subtypes have the same
     /// "skeleton" and hence there is no possible type such that
     /// (e.g.)  `Box<?3> <: ?3` for any `?3`.
+    ///
+    /// In practice, we sometimes sub-unify variables in other spots, such
+    /// as when processing subtype predicates. This is not necessary but is
+    /// done to aid diagnostics, as it allows us to be more effective when
+    /// we guide the user towards where they should insert type hints.
     sub_relations: ut::UnificationTableStorage<ty::TyVid>,
 }
 
@@ -119,7 +135,13 @@ pub enum TypeVariableOriginKind {
 
 pub(crate) struct TypeVariableData {
     origin: TypeVariableOrigin,
-    diverging: bool,
+    diverging: Diverging,
+}
+
+#[derive(Copy, Clone, Debug)]
+pub enum Diverging {
+    NotDiverging,
+    Diverges,
 }
 
 #[derive(Copy, Clone, Debug)]
@@ -173,7 +195,7 @@ impl<'tcx> TypeVariableTable<'_, 'tcx> {
     ///
     /// Note that this function does not return care whether
     /// `vid` has been unified with something else or not.
-    pub fn var_diverges(&self, vid: ty::TyVid) -> bool {
+    pub fn var_diverges(&self, vid: ty::TyVid) -> Diverging {
         self.storage.values.get(vid.index as usize).diverging
     }
 
@@ -238,7 +260,7 @@ impl<'tcx> TypeVariableTable<'_, 'tcx> {
     pub fn new_var(
         &mut self,
         universe: ty::UniverseIndex,
-        diverging: bool,
+        diverging: Diverging,
         origin: TypeVariableOrigin,
     ) -> ty::TyVid {
         let eq_key = self.eq_relations().new_key(TypeVariableValue::Unknown { universe });

compiler/rustc_infer/src/traits/util.rs

@@ -158,6 +158,10 @@ impl Elaborator<'tcx> {
                 // Currently, we do not "elaborate" predicates like `X <: Y`,
                 // though conceivably we might.
             }
+            ty::PredicateKind::Coerce(..) => {
+                // Currently, we do not "elaborate" predicates like `X -> Y`,
+                // though conceivably we might.
+            }
             ty::PredicateKind::Projection(..) => {
                 // Nothing to elaborate in a projection predicate.
             }

compiler/rustc_lint/src/builtin.rs

@@ -1652,6 +1652,7 @@ impl<'tcx> LateLintPass<'tcx> for TrivialConstraints {
                     ObjectSafe(..) |
                     ClosureKind(..) |
                     Subtype(..) |
+                    Coerce(..) |
                     ConstEvaluatable(..) |
                     ConstEquate(..) |
                     TypeWellFormedFromEnv(..) => continue,

compiler/rustc_middle/src/ty/error.rs

@@ -71,12 +71,6 @@ pub enum TypeError<'tcx> {
     TargetFeatureCast(DefId),
 }
 
-pub enum UnconstrainedNumeric {
-    UnconstrainedFloat,
-    UnconstrainedInt,
-    Neither,
-}
-
 /// Explains the source of a type err in a short, human readable way. This is meant to be placed
 /// in parentheses after some larger message. You should also invoke `note_and_explain_type_err()`
 /// afterwards to present additional details, particularly when it comes to lifetime-related

compiler/rustc_middle/src/ty/flags.rs

@@ -231,6 +231,10 @@ impl FlagComputation {
                 self.add_ty(a);
                 self.add_ty(b);
             }
+            ty::PredicateKind::Coerce(ty::CoercePredicate { a, b }) => {
+                self.add_ty(a);
+                self.add_ty(b);
+            }
             ty::PredicateKind::Projection(ty::ProjectionPredicate { projection_ty, ty }) => {
                 self.add_projection_ty(projection_ty);
                 self.add_ty(ty);