Auto merge of rust-lang#87587 - oli-obk:lazy_tait, r=spastorino

Various refactorings of the TAIT infrastructure

Before this PR we used to store the opaque type knowledge outside the `InferCtxt`, so it got recomputed on every opaque type instantiation.

I also removed a feature gate check that makes no sense in the planned lazy TAIT resolution scheme

Each commit passes all tests, so this PR is best reviewed commit by commit.

r? `@spastorino`

compiler/rustc_infer/src/infer/mod.rs

@@ -4,10 +4,12 @@ pub use self::RegionVariableOrigin::*;
 pub use self::SubregionOrigin::*;
 pub use self::ValuePairs::*;
 
+use self::opaque_types::OpaqueTypeMap;
 pub(crate) use self::undo_log::{InferCtxtUndoLogs, Snapshot, UndoLog};
 
 use crate::traits::{self, ObligationCause, PredicateObligations, TraitEngine};
 
+use hir::def_id::CRATE_DEF_ID;
 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
 use rustc_data_structures::sync::Lrc;
 use rustc_data_structures::undo_log::Rollback;
@@ -59,6 +61,7 @@ pub mod lattice;
 mod lexical_region_resolve;
 mod lub;
 pub mod nll_relate;
+pub mod opaque_types;
 pub mod outlives;
 pub mod region_constraints;
 pub mod resolve;
@@ -191,6 +194,19 @@ pub struct InferCtxtInner<'tcx> {
     region_obligations: Vec<(hir::HirId, RegionObligation<'tcx>)>,
 
     undo_log: InferCtxtUndoLogs<'tcx>,
+
+    // Opaque types found in explicit return types and their
+    // associated fresh inference variable. Writeback resolves these
+    // variables to get the concrete type, which can be used to
+    // 'de-opaque' OpaqueTypeDecl, after typeck is done with all functions.
+    pub opaque_types: OpaqueTypeMap<'tcx>,
+
+    /// A map from inference variables created from opaque
+    /// type instantiations (`ty::Infer`) to the actual opaque
+    /// type (`ty::Opaque`). Used during fallback to map unconstrained
+    /// opaque type inference variables to their corresponding
+    /// opaque type.
+    pub opaque_types_vars: FxHashMap<Ty<'tcx>, Ty<'tcx>>,
 }
 
 impl<'tcx> InferCtxtInner<'tcx> {
@@ -204,6 +220,8 @@ impl<'tcx> InferCtxtInner<'tcx> {
             float_unification_storage: ut::UnificationTableStorage::new(),
             region_constraint_storage: Some(RegionConstraintStorage::new()),
             region_obligations: vec![],
+            opaque_types: Default::default(),
+            opaque_types_vars: Default::default(),
         }
     }
 
@@ -273,6 +291,10 @@ impl<'tcx> InferCtxtInner<'tcx> {
 pub struct InferCtxt<'a, 'tcx> {
     pub tcx: TyCtxt<'tcx>,
 
+    /// The `DefId` of the item in whose context we are performing inference or typeck.
+    /// It is used to check whether an opaque type use is a defining use.
+    pub defining_use_anchor: LocalDefId,
+
     /// During type-checking/inference of a body, `in_progress_typeck_results`
     /// contains a reference to the typeck results being built up, which are
     /// used for reading closure kinds/signatures as they are inferred,
@@ -531,6 +553,7 @@ impl<'tcx> fmt::Display for FixupError<'tcx> {
 pub struct InferCtxtBuilder<'tcx> {
     tcx: TyCtxt<'tcx>,
     fresh_typeck_results: Option<RefCell<ty::TypeckResults<'tcx>>>,
+    defining_use_anchor: LocalDefId,
 }
 
 pub trait TyCtxtInferExt<'tcx> {
@@ -539,15 +562,31 @@ pub trait TyCtxtInferExt<'tcx> {
 
 impl TyCtxtInferExt<'tcx> for TyCtxt<'tcx> {
     fn infer_ctxt(self) -> InferCtxtBuilder<'tcx> {
-        InferCtxtBuilder { tcx: self, fresh_typeck_results: None }
+        InferCtxtBuilder {
+            tcx: self,
+            defining_use_anchor: CRATE_DEF_ID,
+            fresh_typeck_results: None,
+        }
     }
 }
 
 impl<'tcx> InferCtxtBuilder<'tcx> {
     /// Used only by `rustc_typeck` during body type-checking/inference,
     /// will initialize `in_progress_typeck_results` with fresh `TypeckResults`.
+    /// Will also change the scope for opaque type defining use checks to the given owner.
     pub fn with_fresh_in_progress_typeck_results(mut self, table_owner: LocalDefId) -> Self {
         self.fresh_typeck_results = Some(RefCell::new(ty::TypeckResults::new(table_owner)));
+        self.with_opaque_type_inference(table_owner)
+    }
+
+    /// Whenever the `InferCtxt` should be able to handle defining uses of opaque types,
+    /// you need to call this function. Otherwise the opaque type will be treated opaquely.
+    ///
+    /// It is only meant to be called in two places, for typeck
+    /// (via `with_fresh_in_progress_typeck_results`) and for the inference context used
+    /// in mir borrowck.
+    pub fn with_opaque_type_inference(mut self, defining_use_anchor: LocalDefId) -> Self {
+        self.defining_use_anchor = defining_use_anchor;
         self
     }
 
@@ -575,10 +614,11 @@ impl<'tcx> InferCtxtBuilder<'tcx> {
     }
 
     pub fn enter<R>(&mut self, f: impl for<'a> FnOnce(InferCtxt<'a, 'tcx>) -> R) -> R {
-        let InferCtxtBuilder { tcx, ref fresh_typeck_results } = *self;
+        let InferCtxtBuilder { tcx, defining_use_anchor, ref fresh_typeck_results } = *self;
         let in_progress_typeck_results = fresh_typeck_results.as_ref();
         f(InferCtxt {
             tcx,
+            defining_use_anchor,
             in_progress_typeck_results,
             inner: RefCell::new(InferCtxtInner::new()),
             lexical_region_resolutions: RefCell::new(None),

compiler/rustc_infer/src/infer/opaque_types.rs

@@ -0,0 +1,47 @@
+use rustc_data_structures::vec_map::VecMap;
+use rustc_hir as hir;
+use rustc_middle::ty::{OpaqueTypeKey, Ty};
+use rustc_span::Span;
+
+pub type OpaqueTypeMap<'tcx> = VecMap<OpaqueTypeKey<'tcx>, OpaqueTypeDecl<'tcx>>;
+
+/// Information about the opaque types whose values we
+/// are inferring in this function (these are the `impl Trait` that
+/// appear in the return type).
+#[derive(Copy, Clone, Debug)]
+pub struct OpaqueTypeDecl<'tcx> {
+    /// The opaque type (`ty::Opaque`) for this declaration.
+    pub opaque_type: Ty<'tcx>,
+
+    /// The span of this particular definition of the opaque type. So
+    /// for example:
+    ///
+    /// ```ignore (incomplete snippet)
+    /// type Foo = impl Baz;
+    /// fn bar() -> Foo {
+    /// //          ^^^ This is the span we are looking for!
+    /// }
+    /// ```
+    ///
+    /// In cases where the fn returns `(impl Trait, impl Trait)` or
+    /// other such combinations, the result is currently
+    /// over-approximated, but better than nothing.
+    pub definition_span: Span,
+
+    /// The type variable that represents the value of the opaque type
+    /// that we require. In other words, after we compile this function,
+    /// we will be created a constraint like:
+    ///
+    ///     Foo<'a, T> = ?C
+    ///
+    /// where `?C` is the value of this type variable. =) It may
+    /// naturally refer to the type and lifetime parameters in scope
+    /// in this function, though ultimately it should only reference
+    /// those that are arguments to `Foo` in the constraint above. (In
+    /// other words, `?C` should not include `'b`, even though it's a
+    /// lifetime parameter on `foo`.)
+    pub concrete_ty: Ty<'tcx>,
+
+    /// The origin of the opaque type.
+    pub origin: hir::OpaqueTyOrigin,
+}

compiler/rustc_mir/src/borrow_check/mod.rs

@@ -105,7 +105,7 @@ fn mir_borrowck<'tcx>(
     let (input_body, promoted) = tcx.mir_promoted(def);
     debug!("run query mir_borrowck: {}", tcx.def_path_str(def.did.to_def_id()));
 
-    let opt_closure_req = tcx.infer_ctxt().enter(|infcx| {
+    let opt_closure_req = tcx.infer_ctxt().with_opaque_type_inference(def.did).enter(|infcx| {
         let input_body: &Body<'_> = &input_body.borrow();
         let promoted: &IndexVec<_, _> = &promoted.borrow();
         do_mir_borrowck(&infcx, input_body, promoted)

compiler/rustc_mir/src/borrow_check/type_check/mod.rs

@@ -179,54 +179,55 @@ pub(crate) fn type_check<'mir, 'tcx>(
             liveness::generate(&mut cx, body, elements, flow_inits, move_data, location_table);
 
             translate_outlives_facts(&mut cx);
-            let mut opaque_type_values = cx.opaque_type_values;
-
-            for (_, revealed_ty) in &mut opaque_type_values {
-                *revealed_ty = infcx.resolve_vars_if_possible(*revealed_ty);
-                if revealed_ty.has_infer_types_or_consts() {
-                    infcx.tcx.sess.delay_span_bug(
-                        body.span,
-                        &format!("could not resolve {:#?}", revealed_ty.kind()),
-                    );
-                    *revealed_ty = infcx.tcx.ty_error();
-                }
-            }
-
-            opaque_type_values.retain(|(opaque_type_key, resolved_ty)| {
-                let concrete_is_opaque = if let ty::Opaque(def_id, _) = resolved_ty.kind() {
-                    *def_id == opaque_type_key.def_id
-                } else {
-                    false
-                };
+            let opaque_type_values = mem::take(&mut infcx.inner.borrow_mut().opaque_types);
 
-                if concrete_is_opaque {
-                    // We're using an opaque `impl Trait` type without
-                    // 'revealing' it. For example, code like this:
-                    //
-                    // type Foo = impl Debug;
-                    // fn foo1() -> Foo { ... }
-                    // fn foo2() -> Foo { foo1() }
-                    //
-                    // In `foo2`, we're not revealing the type of `Foo` - we're
-                    // just treating it as the opaque type.
-                    //
-                    // When this occurs, we do *not* want to try to equate
-                    // the concrete type with the underlying defining type
-                    // of the opaque type - this will always fail, since
-                    // the defining type of an opaque type is always
-                    // some other type (e.g. not itself)
-                    // Essentially, none of the normal obligations apply here -
-                    // we're just passing around some unknown opaque type,
-                    // without actually looking at the underlying type it
-                    // gets 'revealed' into
-                    debug!(
-                        "eq_opaque_type_and_type: non-defining use of {:?}",
-                        opaque_type_key.def_id,
-                    );
-                }
-                !concrete_is_opaque
-            });
             opaque_type_values
+                .into_iter()
+                .filter_map(|(opaque_type_key, decl)| {
+                    let mut revealed_ty = infcx.resolve_vars_if_possible(decl.concrete_ty);
+                    if revealed_ty.has_infer_types_or_consts() {
+                        infcx.tcx.sess.delay_span_bug(
+                            body.span,
+                            &format!("could not resolve {:#?}", revealed_ty.kind()),
+                        );
+                        revealed_ty = infcx.tcx.ty_error();
+                    }
+                    let concrete_is_opaque = if let ty::Opaque(def_id, _) = revealed_ty.kind() {
+                        *def_id == opaque_type_key.def_id
+                    } else {
+                        false
+                    };
+
+                    if concrete_is_opaque {
+                        // We're using an opaque `impl Trait` type without
+                        // 'revealing' it. For example, code like this:
+                        //
+                        // type Foo = impl Debug;
+                        // fn foo1() -> Foo { ... }
+                        // fn foo2() -> Foo { foo1() }
+                        //
+                        // In `foo2`, we're not revealing the type of `Foo` - we're
+                        // just treating it as the opaque type.
+                        //
+                        // When this occurs, we do *not* want to try to equate
+                        // the concrete type with the underlying defining type
+                        // of the opaque type - this will always fail, since
+                        // the defining type of an opaque type is always
+                        // some other type (e.g. not itself)
+                        // Essentially, none of the normal obligations apply here -
+                        // we're just passing around some unknown opaque type,
+                        // without actually looking at the underlying type it
+                        // gets 'revealed' into
+                        debug!(
+                            "eq_opaque_type_and_type: non-defining use of {:?}",
+                            opaque_type_key.def_id,
+                        );
+                        None
+                    } else {
+                        Some((opaque_type_key, revealed_ty))
+                    }
+                })
+                .collect()
         },
     );
 
@@ -865,7 +866,6 @@ struct TypeChecker<'a, 'tcx> {
     reported_errors: FxHashSet<(Ty<'tcx>, Span)>,
     borrowck_context: &'a mut BorrowCheckContext<'a, 'tcx>,
     universal_region_relations: &'a UniversalRegionRelations<'tcx>,
-    opaque_type_values: VecMap<OpaqueTypeKey<'tcx>, Ty<'tcx>>,
 }
 
 struct BorrowCheckContext<'a, 'tcx> {
@@ -1025,7 +1025,6 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
             borrowck_context,
             reported_errors: Default::default(),
             universal_region_relations,
-            opaque_type_values: VecMap::default(),
         };
         checker.check_user_type_annotations();
         checker
@@ -1289,10 +1288,8 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
         let body = self.body;
         let mir_def_id = body.source.def_id().expect_local();
 
-        let mut opaque_type_values = VecMap::new();
-
         debug!("eq_opaque_type_and_type: mir_def_id={:?}", mir_def_id);
-        let opaque_type_map = self.fully_perform_op(
+        self.fully_perform_op(
             locations,
             category,
             CustomTypeOp::new(
@@ -1307,20 +1304,17 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
                     // to `Box<?T>`, returning an `opaque_type_map` mapping `{Foo<T> -> ?T}`.
                     // (Note that the key of the map is both the def-id of `Foo` along with
                     // any generic parameters.)
-                    let (output_ty, opaque_type_map) =
-                        obligations.add(infcx.instantiate_opaque_types(
-                            mir_def_id,
-                            dummy_body_id,
-                            param_env,
-                            anon_ty,
-                            locations.span(body),
-                        ));
+                    let output_ty = obligations.add(infcx.instantiate_opaque_types(
+                        dummy_body_id,
+                        param_env,
+                        anon_ty,
+                        locations.span(body),
+                    ));
                     debug!(
                         "eq_opaque_type_and_type: \
                          instantiated output_ty={:?} \
-                         opaque_type_map={:#?} \
                          revealed_ty={:?}",
-                        output_ty, opaque_type_map, revealed_ty
+                        output_ty, revealed_ty
                     );
 
                     // Make sure that the inferred types are well-formed. I'm
@@ -1338,48 +1332,38 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
                             .eq(output_ty, revealed_ty)?,
                     );
 
-                    for &(opaque_type_key, opaque_decl) in &opaque_type_map {
-                        opaque_type_values.insert(opaque_type_key, opaque_decl.concrete_ty);
-                    }
-
                     debug!("eq_opaque_type_and_type: equated");
 
-                    Ok(InferOk {
-                        value: Some(opaque_type_map),
-                        obligations: obligations.into_vec(),
-                    })
+                    Ok(InferOk { value: (), obligations: obligations.into_vec() })
                 },
                 || "input_output".to_string(),
             ),
         )?;
 
-        self.opaque_type_values.extend(opaque_type_values);
-
         let universal_region_relations = self.universal_region_relations;
 
         // Finally, if we instantiated the anon types successfully, we
         // have to solve any bounds (e.g., `-> impl Iterator` needs to
         // prove that `T: Iterator` where `T` is the type we
         // instantiated it with).
-        if let Some(opaque_type_map) = opaque_type_map {
-            for (opaque_type_key, opaque_decl) in opaque_type_map {
-                self.fully_perform_op(
-                    locations,
-                    ConstraintCategory::OpaqueType,
-                    CustomTypeOp::new(
-                        |infcx| {
-                            infcx.constrain_opaque_type(
-                                opaque_type_key,
-                                &opaque_decl,
-                                GenerateMemberConstraints::IfNoStaticBound,
-                                universal_region_relations,
-                            );
-                            Ok(InferOk { value: (), obligations: vec![] })
-                        },
-                        || "opaque_type_map".to_string(),
-                    ),
-                )?;
-            }
+        let opaque_type_map = self.infcx.inner.borrow().opaque_types.clone();
+        for (opaque_type_key, opaque_decl) in opaque_type_map {
+            self.fully_perform_op(
+                locations,
+                ConstraintCategory::OpaqueType,
+                CustomTypeOp::new(
+                    |infcx| {
+                        infcx.constrain_opaque_type(
+                            opaque_type_key,
+                            &opaque_decl,
+                            GenerateMemberConstraints::IfNoStaticBound,
+                            universal_region_relations,
+                        );
+                        Ok(InferOk { value: (), obligations: vec![] })
+                    },
+                    || "opaque_type_map".to_string(),
+                ),
+            )?;
         }
         Ok(())
     }