Rollup merge of #89915 - jackh726:outlives_cleanup, r=nikomatsakis

Some outlives cleanup

No semantic changes here, only moving code around + using `LocalDefId` instead of `HirId`

r? ````@nikomatsakis````

3 files changed, 219 insertions, 3 deletions

diff --git a/compiler/rustc_infer/src/infer/outlives/components.rs b/compiler/rustc_infer/src/infer/outlives/components.rs
new file mode 100644
index 00000000000..98f926e9d76
--- /dev/null
+++ b/compiler/rustc_infer/src/infer/outlives/components.rs
@@ -0,0 +1,215 @@
+// The outlines relation `T: 'a` or `'a: 'b`. This code frequently
+// refers to rules defined in RFC 1214 (`OutlivesFooBar`), so see that
+// RFC for reference.
+
+use rustc_data_structures::sso::SsoHashSet;
+use rustc_middle::ty::subst::{GenericArg, GenericArgKind};
+use rustc_middle::ty::{self, Ty, TyCtxt, TypeFoldable};
+use smallvec::{smallvec, SmallVec};
+
+#[derive(Debug)]
+pub enum Component<'tcx> {
+    Region(ty::Region<'tcx>),
+    Param(ty::ParamTy),
+    UnresolvedInferenceVariable(ty::InferTy),
+
+    // Projections like `T::Foo` are tricky because a constraint like
+    // `T::Foo: 'a` can be satisfied in so many ways. There may be a
+    // where-clause that says `T::Foo: 'a`, or the defining trait may
+    // include a bound like `type Foo: 'static`, or -- in the most
+    // conservative way -- we can prove that `T: 'a` (more generally,
+    // that all components in the projection outlive `'a`). This code
+    // is not in a position to judge which is the best technique, so
+    // we just product the projection as a component and leave it to
+    // the consumer to decide (but see `EscapingProjection` below).
+    Projection(ty::ProjectionTy<'tcx>),
+
+    // In the case where a projection has escaping regions -- meaning
+    // regions bound within the type itself -- we always use
+    // the most conservative rule, which requires that all components
+    // outlive the bound. So for example if we had a type like this:
+    //
+    //     for<'a> Trait1<  <T as Trait2<'a,'b>>::Foo  >
+    //                      ~~~~~~~~~~~~~~~~~~~~~~~~~
+    //
+    // then the inner projection (underlined) has an escaping region
+    // `'a`. We consider that outer trait `'c` to meet a bound if `'b`
+    // outlives `'b: 'c`, and we don't consider whether the trait
+    // declares that `Foo: 'static` etc. Therefore, we just return the
+    // free components of such a projection (in this case, `'b`).
+    //
+    // However, in the future, we may want to get smarter, and
+    // actually return a "higher-ranked projection" here. Therefore,
+    // we mark that these components are part of an escaping
+    // projection, so that implied bounds code can avoid relying on
+    // them. This gives us room to improve the regionck reasoning in
+    // the future without breaking backwards compat.
+    EscapingProjection(Vec<Component<'tcx>>),
+}
+
+/// Push onto `out` all the things that must outlive `'a` for the condition
+/// `ty0: 'a` to hold. Note that `ty0` must be a **fully resolved type**.
+pub fn push_outlives_components(
+    tcx: TyCtxt<'tcx>,
+    ty0: Ty<'tcx>,
+    out: &mut SmallVec<[Component<'tcx>; 4]>,
+) {
+    let mut visited = SsoHashSet::new();
+    compute_components(tcx, ty0, out, &mut visited);
+    debug!("components({:?}) = {:?}", ty0, out);
+}
+
+fn compute_components(
+    tcx: TyCtxt<'tcx>,
+    ty: Ty<'tcx>,
+    out: &mut SmallVec<[Component<'tcx>; 4]>,
+    visited: &mut SsoHashSet<GenericArg<'tcx>>,
+) {
+    // Descend through the types, looking for the various "base"
+    // components and collecting them into `out`. This is not written
+    // with `collect()` because of the need to sometimes skip subtrees
+    // in the `subtys` iterator (e.g., when encountering a
+    // projection).
+    match *ty.kind() {
+            ty::FnDef(_, substs) => {
+                // HACK(eddyb) ignore lifetimes found shallowly in `substs`.
+                // This is inconsistent with `ty::Adt` (including all substs)
+                // and with `ty::Closure` (ignoring all substs other than
+                // upvars, of which a `ty::FnDef` doesn't have any), but
+                // consistent with previous (accidental) behavior.
+                // See https://github.com/rust-lang/rust/issues/70917
+                // for further background and discussion.
+                for child in substs {
+                    match child.unpack() {
+                        GenericArgKind::Type(ty) => {
+                            compute_components(tcx, ty, out, visited);
+                        }
+                        GenericArgKind::Lifetime(_) => {}
+                        GenericArgKind::Const(_) => {
+                            compute_components_recursive(tcx, child, out, visited);
+                        }
+                    }
+                }
+            }
+
+            ty::Array(element, _) => {
+                // Don't look into the len const as it doesn't affect regions
+                compute_components(tcx, element, out, visited);
+            }
+
+            ty::Closure(_, ref substs) => {
+                let tupled_ty = substs.as_closure().tupled_upvars_ty();
+                compute_components(tcx, tupled_ty, out, visited);
+            }
+
+            ty::Generator(_, ref substs, _) => {
+                // Same as the closure case
+                let tupled_ty = substs.as_generator().tupled_upvars_ty();
+                compute_components(tcx, tupled_ty, out, visited);
+
+                // We ignore regions in the generator interior as we don't
+                // want these to affect region inference
+            }
+
+            // All regions are bound inside a witness
+            ty::GeneratorWitness(..) => (),
+
+            // OutlivesTypeParameterEnv -- the actual checking that `X:'a`
+            // is implied by the environment is done in regionck.
+            ty::Param(p) => {
+                out.push(Component::Param(p));
+            }
+
+            // For projections, we prefer to generate an obligation like
+            // `<P0 as Trait<P1...Pn>>::Foo: 'a`, because this gives the
+            // regionck more ways to prove that it holds. However,
+            // regionck is not (at least currently) prepared to deal with
+            // higher-ranked regions that may appear in the
+            // trait-ref. Therefore, if we see any higher-ranke regions,
+            // we simply fallback to the most restrictive rule, which
+            // requires that `Pi: 'a` for all `i`.
+            ty::Projection(ref data) => {
+                if !data.has_escaping_bound_vars() {
+                    // best case: no escaping regions, so push the
+                    // projection and skip the subtree (thus generating no
+                    // constraints for Pi). This defers the choice between
+                    // the rules OutlivesProjectionEnv,
+                    // OutlivesProjectionTraitDef, and
+                    // OutlivesProjectionComponents to regionck.
+                    out.push(Component::Projection(*data));
+                } else {
+                    // fallback case: hard code
+                    // OutlivesProjectionComponents.  Continue walking
+                    // through and constrain Pi.
+                    let mut subcomponents = smallvec![];
+                    let mut subvisited = SsoHashSet::new();
+                    compute_components_recursive(tcx, ty.into(), &mut subcomponents, &mut subvisited);
+                    out.push(Component::EscapingProjection(subcomponents.into_iter().collect()));
+                }
+            }
+
+            // We assume that inference variables are fully resolved.
+            // So, if we encounter an inference variable, just record
+            // the unresolved variable as a component.
+            ty::Infer(infer_ty) => {
+                out.push(Component::UnresolvedInferenceVariable(infer_ty));
+            }
+
+            // Most types do not introduce any region binders, nor
+            // involve any other subtle cases, and so the WF relation
+            // simply constraints any regions referenced directly by
+            // the type and then visits the types that are lexically
+            // contained within. (The comments refer to relevant rules
+            // from RFC1214.)
+            ty::Bool |            // OutlivesScalar
+            ty::Char |            // OutlivesScalar
+            ty::Int(..) |         // OutlivesScalar
+            ty::Uint(..) |        // OutlivesScalar
+            ty::Float(..) |       // OutlivesScalar
+            ty::Never |           // ...
+            ty::Adt(..) |         // OutlivesNominalType
+            ty::Opaque(..) |      // OutlivesNominalType (ish)
+            ty::Foreign(..) |     // OutlivesNominalType
+            ty::Str |             // OutlivesScalar (ish)
+            ty::Slice(..) |       // ...
+            ty::RawPtr(..) |      // ...
+            ty::Ref(..) |         // OutlivesReference
+            ty::Tuple(..) |       // ...
+            ty::FnPtr(_) |        // OutlivesFunction (*)
+            ty::Dynamic(..) |     // OutlivesObject, OutlivesFragment (*)
+            ty::Placeholder(..) |
+            ty::Bound(..) |
+            ty::Error(_) => {
+                // (*) Function pointers and trait objects are both binders.
+                // In the RFC, this means we would add the bound regions to
+                // the "bound regions list".  In our representation, no such
+                // list is maintained explicitly, because bound regions
+                // themselves can be readily identified.
+                compute_components_recursive(tcx, ty.into(), out, visited);
+            }
+        }
+}
+
+fn compute_components_recursive(
+    tcx: TyCtxt<'tcx>,
+    parent: GenericArg<'tcx>,
+    out: &mut SmallVec<[Component<'tcx>; 4]>,
+    visited: &mut SsoHashSet<GenericArg<'tcx>>,
+) {
+    for child in parent.walk_shallow(tcx, visited) {
+        match child.unpack() {
+            GenericArgKind::Type(ty) => {
+                compute_components(tcx, ty, out, visited);
+            }
+            GenericArgKind::Lifetime(lt) => {
+                // Ignore late-bound regions.
+                if !lt.is_late_bound() {
+                    out.push(Component::Region(lt));
+                }
+            }
+            GenericArgKind::Const(_) => {
+                compute_components_recursive(tcx, child, out, visited);
+            }
+        }
+    }
+}
diff --git a/compiler/rustc_infer/src/infer/outlives/mod.rs b/compiler/rustc_infer/src/infer/outlives/mod.rs
index 4dd5e8ba545..03d6c45a653 100644
--- a/compiler/rustc_infer/src/infer/outlives/mod.rs
+++ b/compiler/rustc_infer/src/infer/outlives/mod.rs
@@ -1,5 +1,6 @@
 //! Various code related to computing outlives relations.
 
+pub mod components;
 pub mod env;
 pub mod obligations;
 pub mod verify;
diff --git a/compiler/rustc_infer/src/infer/outlives/obligations.rs b/compiler/rustc_infer/src/infer/outlives/obligations.rs
index 437083c68dc..91a22ecc5a9 100644
--- a/compiler/rustc_infer/src/infer/outlives/obligations.rs
+++ b/compiler/rustc_infer/src/infer/outlives/obligations.rs
@@ -1,5 +1,5 @@
 //! Code that handles "type-outlives" constraints like `T: 'a`. This
-//! is based on the `push_outlives_components` function defined on the tcx,
+//! is based on the `push_outlives_components` function defined in rustc_infer,
 //! but it adds a bit of heuristics on top, in particular to deal with
 //! associated types and projections.
 //!
@@ -59,13 +59,13 @@
 //! might later infer `?U` to something like `&'b u32`, which would
 //! imply that `'b: 'a`.
 
+use crate::infer::outlives::components::{push_outlives_components, Component};
 use crate::infer::outlives::env::RegionBoundPairs;
 use crate::infer::outlives::verify::VerifyBoundCx;
 use crate::infer::{
     self, GenericKind, InferCtxt, RegionObligation, SubregionOrigin, UndoLog, VerifyBound,
 };
 use crate::traits::{ObligationCause, ObligationCauseCode};
-use rustc_middle::ty::outlives::Component;
 use rustc_middle::ty::subst::GenericArgKind;
 use rustc_middle::ty::{self, Region, Ty, TyCtxt, TypeFoldable};
 
@@ -271,7 +271,7 @@ where
         assert!(!ty.has_escaping_bound_vars());
 
         let mut components = smallvec![];
-        self.tcx.push_outlives_components(ty, &mut components);
+        push_outlives_components(self.tcx, ty, &mut components);
         self.components_must_outlive(origin, &components, region);
     }