add `STILL_FURTHER_SPECIALIZABLE` flag

This commit adds a STILL_FURTHER_SPECIALIZABLE flag to `TypeFlags`
which replaces `needs_infer` and `needs_subst` in `Instance::resolve`
and `assemble_candidates_from_impls.`

Signed-off-by: David Wood <david@davidtw.co>

6 files changed, 86 insertions, 57 deletions

diff --git a/src/librustc_middle/ty/flags.rs b/src/librustc_middle/ty/flags.rs
index 5243e1fbf57..99a6511b297 100644
--- a/src/librustc_middle/ty/flags.rs
+++ b/src/librustc_middle/ty/flags.rs
@@ -1,4 +1,4 @@
-use crate::ty::subst::{GenericArgKind, SubstsRef};
+use crate::ty::subst::{GenericArg, GenericArgKind};
 use crate::ty::{self, InferConst, Ty, TypeFlags};
 
 #[derive(Debug)]
@@ -81,6 +81,7 @@ impl FlagComputation {
 
             &ty::Param(_) => {
                 self.add_flags(TypeFlags::HAS_TY_PARAM);
+                self.add_flags(TypeFlags::STILL_FURTHER_SPECIALIZABLE);
             }
 
             &ty::Generator(_, ref substs, _) => {
@@ -99,14 +100,17 @@ impl FlagComputation {
 
             &ty::Bound(debruijn, _) => {
                 self.add_binder(debruijn);
+                self.add_flags(TypeFlags::STILL_FURTHER_SPECIALIZABLE);
             }
 
             &ty::Placeholder(..) => {
                 self.add_flags(TypeFlags::HAS_TY_PLACEHOLDER);
+                self.add_flags(TypeFlags::STILL_FURTHER_SPECIALIZABLE);
             }
 
             &ty::Infer(infer) => {
                 self.add_flags(TypeFlags::HAS_TY_INFER);
+                self.add_flags(TypeFlags::STILL_FURTHER_SPECIALIZABLE);
                 match infer {
                     ty::FreshTy(_) | ty::FreshIntTy(_) | ty::FreshFloatTy(_) => {}
 
@@ -218,17 +222,23 @@ impl FlagComputation {
             }
             ty::ConstKind::Infer(infer) => {
                 self.add_flags(TypeFlags::HAS_CT_INFER);
+                self.add_flags(TypeFlags::STILL_FURTHER_SPECIALIZABLE);
                 match infer {
                     InferConst::Fresh(_) => {}
                     InferConst::Var(_) => self.add_flags(TypeFlags::KEEP_IN_LOCAL_TCX),
                 }
             }
-            ty::ConstKind::Bound(debruijn, _) => self.add_binder(debruijn),
+            ty::ConstKind::Bound(debruijn, _) => {
+                self.add_binder(debruijn);
+                self.add_flags(TypeFlags::STILL_FURTHER_SPECIALIZABLE);
+            }
             ty::ConstKind::Param(_) => {
                 self.add_flags(TypeFlags::HAS_CT_PARAM);
+                self.add_flags(TypeFlags::STILL_FURTHER_SPECIALIZABLE);
             }
             ty::ConstKind::Placeholder(_) => {
                 self.add_flags(TypeFlags::HAS_CT_PLACEHOLDER);
+                self.add_flags(TypeFlags::STILL_FURTHER_SPECIALIZABLE);
             }
             ty::ConstKind::Value(_) => {}
         }
@@ -243,7 +253,7 @@ impl FlagComputation {
         self.add_substs(projection_ty.substs);
     }
 
-    fn add_substs(&mut self, substs: SubstsRef<'_>) {
+    fn add_substs(&mut self, substs: &[GenericArg<'_>]) {
         for kind in substs {
             match kind.unpack() {
                 GenericArgKind::Type(ty) => self.add_ty(ty),
diff --git a/src/librustc_middle/ty/fold.rs b/src/librustc_middle/ty/fold.rs
index 3f4f2407f1e..a3d611a1325 100644
--- a/src/librustc_middle/ty/fold.rs
+++ b/src/librustc_middle/ty/fold.rs
@@ -142,6 +142,13 @@ pub trait TypeFoldable<'tcx>: fmt::Debug + Clone {
         self.has_type_flags(TypeFlags::HAS_RE_LATE_BOUND)
     }
 
+    /// Indicates whether this value still has parameters/placeholders/inference variables
+    /// which could be replaced later, in a way that would change the results of `impl`
+    /// specialization.
+    fn still_further_specializable(&self) -> bool {
+        self.has_type_flags(TypeFlags::STILL_FURTHER_SPECIALIZABLE)
+    }
+
     /// A visitor that does not recurse into types, works like `fn walk_shallow` in `Ty`.
     fn visit_tys_shallow(&self, visit: impl FnMut(Ty<'tcx>) -> bool) -> bool {
         pub struct Visitor<F>(F);
diff --git a/src/librustc_middle/ty/mod.rs b/src/librustc_middle/ty/mod.rs
index 43982439d47..2585de07a5b 100644
--- a/src/librustc_middle/ty/mod.rs
+++ b/src/librustc_middle/ty/mod.rs
@@ -524,101 +524,106 @@ bitflags! {
         // Does this have parameters? Used to determine whether substitution is
         // required.
         /// Does this have [Param]?
-        const HAS_TY_PARAM              = 1 << 0;
+        const HAS_TY_PARAM                = 1 << 0;
         /// Does this have [ReEarlyBound]?
-        const HAS_RE_PARAM              = 1 << 1;
+        const HAS_RE_PARAM                = 1 << 1;
         /// Does this have [ConstKind::Param]?
-        const HAS_CT_PARAM              = 1 << 2;
+        const HAS_CT_PARAM                = 1 << 2;
 
-        const NEEDS_SUBST               = TypeFlags::HAS_TY_PARAM.bits
-                                        | TypeFlags::HAS_RE_PARAM.bits
-                                        | TypeFlags::HAS_CT_PARAM.bits;
+        const NEEDS_SUBST                 = TypeFlags::HAS_TY_PARAM.bits
+                                          | TypeFlags::HAS_RE_PARAM.bits
+                                          | TypeFlags::HAS_CT_PARAM.bits;
 
         /// Does this have [Infer]?
-        const HAS_TY_INFER              = 1 << 3;
+        const HAS_TY_INFER                = 1 << 3;
         /// Does this have [ReVar]?
-        const HAS_RE_INFER              = 1 << 4;
+        const HAS_RE_INFER                = 1 << 4;
         /// Does this have [ConstKind::Infer]?
-        const HAS_CT_INFER              = 1 << 5;
+        const HAS_CT_INFER                = 1 << 5;
 
         /// Does this have inference variables? Used to determine whether
         /// inference is required.
-        const NEEDS_INFER               = TypeFlags::HAS_TY_INFER.bits
-                                        | TypeFlags::HAS_RE_INFER.bits
-                                        | TypeFlags::HAS_CT_INFER.bits;
+        const NEEDS_INFER                 = TypeFlags::HAS_TY_INFER.bits
+                                          | TypeFlags::HAS_RE_INFER.bits
+                                          | TypeFlags::HAS_CT_INFER.bits;
 
         /// Does this have [Placeholder]?
-        const HAS_TY_PLACEHOLDER        = 1 << 6;
+        const HAS_TY_PLACEHOLDER          = 1 << 6;
         /// Does this have [RePlaceholder]?
-        const HAS_RE_PLACEHOLDER        = 1 << 7;
+        const HAS_RE_PLACEHOLDER          = 1 << 7;
         /// Does this have [ConstKind::Placeholder]?
-        const HAS_CT_PLACEHOLDER        = 1 << 8;
+        const HAS_CT_PLACEHOLDER          = 1 << 8;
 
         /// `true` if there are "names" of regions and so forth
         /// that are local to a particular fn/inferctxt
-        const HAS_FREE_LOCAL_REGIONS    = 1 << 9;
+        const HAS_FREE_LOCAL_REGIONS      = 1 << 9;
 
         /// `true` if there are "names" of types and regions and so forth
         /// that are local to a particular fn
-        const HAS_FREE_LOCAL_NAMES      = TypeFlags::HAS_TY_PARAM.bits
-                                        | TypeFlags::HAS_CT_PARAM.bits
-                                        | TypeFlags::HAS_TY_INFER.bits
-                                        | TypeFlags::HAS_CT_INFER.bits
-                                        | TypeFlags::HAS_TY_PLACEHOLDER.bits
-                                        | TypeFlags::HAS_CT_PLACEHOLDER.bits
-                                        | TypeFlags::HAS_FREE_LOCAL_REGIONS.bits;
+        const HAS_FREE_LOCAL_NAMES        = TypeFlags::HAS_TY_PARAM.bits
+                                          | TypeFlags::HAS_CT_PARAM.bits
+                                          | TypeFlags::HAS_TY_INFER.bits
+                                          | TypeFlags::HAS_CT_INFER.bits
+                                          | TypeFlags::HAS_TY_PLACEHOLDER.bits
+                                          | TypeFlags::HAS_CT_PLACEHOLDER.bits
+                                          | TypeFlags::HAS_FREE_LOCAL_REGIONS.bits;
 
         /// Does this have [Projection] or [UnnormalizedProjection]?
-        const HAS_TY_PROJECTION         = 1 << 10;
+        const HAS_TY_PROJECTION           = 1 << 10;
         /// Does this have [Opaque]?
-        const HAS_TY_OPAQUE             = 1 << 11;
+        const HAS_TY_OPAQUE               = 1 << 11;
         /// Does this have [ConstKind::Unevaluated]?
-        const HAS_CT_PROJECTION         = 1 << 12;
+        const HAS_CT_PROJECTION           = 1 << 12;
 
         /// Could this type be normalized further?
-        const HAS_PROJECTION            = TypeFlags::HAS_TY_PROJECTION.bits
-                                        | TypeFlags::HAS_TY_OPAQUE.bits
-                                        | TypeFlags::HAS_CT_PROJECTION.bits;
+        const HAS_PROJECTION              = TypeFlags::HAS_TY_PROJECTION.bits
+                                          | TypeFlags::HAS_TY_OPAQUE.bits
+                                          | TypeFlags::HAS_CT_PROJECTION.bits;
 
         /// Present if the type belongs in a local type context.
         /// Set for placeholders and inference variables that are not "Fresh".
-        const KEEP_IN_LOCAL_TCX         = 1 << 13;
+        const KEEP_IN_LOCAL_TCX           = 1 << 13;
 
         /// Is an error type reachable?
-        const HAS_TY_ERR                = 1 << 14;
+        const HAS_TY_ERR                  = 1 << 14;
 
         /// Does this have any region that "appears free" in the type?
         /// Basically anything but [ReLateBound] and [ReErased].
-        const HAS_FREE_REGIONS          = 1 << 15;
+        const HAS_FREE_REGIONS            = 1 << 15;
 
         /// Does this have any [ReLateBound] regions? Used to check
         /// if a global bound is safe to evaluate.
-        const HAS_RE_LATE_BOUND         = 1 << 16;
+        const HAS_RE_LATE_BOUND           = 1 << 16;
 
         /// Does this have any [ReErased] regions?
-        const HAS_RE_ERASED             = 1 << 17;
+        const HAS_RE_ERASED               = 1 << 17;
+
+        /// Does this value have parameters/placeholders/inference variables which could be
+        /// replaced later, in a way that would change the results of `impl` specialization?
+        const STILL_FURTHER_SPECIALIZABLE = 1 << 18;
 
         /// Flags representing the nominal content of a type,
         /// computed by FlagsComputation. If you add a new nominal
         /// flag, it should be added here too.
-        const NOMINAL_FLAGS             = TypeFlags::HAS_TY_PARAM.bits
-                                        | TypeFlags::HAS_RE_PARAM.bits
-                                        | TypeFlags::HAS_CT_PARAM.bits
-                                        | TypeFlags::HAS_TY_INFER.bits
-                                        | TypeFlags::HAS_RE_INFER.bits
-                                        | TypeFlags::HAS_CT_INFER.bits
-                                        | TypeFlags::HAS_TY_PLACEHOLDER.bits
-                                        | TypeFlags::HAS_RE_PLACEHOLDER.bits
-                                        | TypeFlags::HAS_CT_PLACEHOLDER.bits
-                                        | TypeFlags::HAS_FREE_LOCAL_REGIONS.bits
-                                        | TypeFlags::HAS_TY_PROJECTION.bits
-                                        | TypeFlags::HAS_TY_OPAQUE.bits
-                                        | TypeFlags::HAS_CT_PROJECTION.bits
-                                        | TypeFlags::KEEP_IN_LOCAL_TCX.bits
-                                        | TypeFlags::HAS_TY_ERR.bits
-                                        | TypeFlags::HAS_FREE_REGIONS.bits
-                                        | TypeFlags::HAS_RE_LATE_BOUND.bits
-                                        | TypeFlags::HAS_RE_ERASED.bits;
+        const NOMINAL_FLAGS               = TypeFlags::HAS_TY_PARAM.bits
+                                          | TypeFlags::HAS_RE_PARAM.bits
+                                          | TypeFlags::HAS_CT_PARAM.bits
+                                          | TypeFlags::HAS_TY_INFER.bits
+                                          | TypeFlags::HAS_RE_INFER.bits
+                                          | TypeFlags::HAS_CT_INFER.bits
+                                          | TypeFlags::HAS_TY_PLACEHOLDER.bits
+                                          | TypeFlags::HAS_RE_PLACEHOLDER.bits
+                                          | TypeFlags::HAS_CT_PLACEHOLDER.bits
+                                          | TypeFlags::HAS_FREE_LOCAL_REGIONS.bits
+                                          | TypeFlags::HAS_TY_PROJECTION.bits
+                                          | TypeFlags::HAS_TY_OPAQUE.bits
+                                          | TypeFlags::HAS_CT_PROJECTION.bits
+                                          | TypeFlags::KEEP_IN_LOCAL_TCX.bits
+                                          | TypeFlags::HAS_TY_ERR.bits
+                                          | TypeFlags::HAS_FREE_REGIONS.bits
+                                          | TypeFlags::HAS_RE_LATE_BOUND.bits
+                                          | TypeFlags::HAS_RE_ERASED.bits
+                                          | TypeFlags::STILL_FURTHER_SPECIALIZABLE.bits;
     }
 }
 
diff --git a/src/librustc_middle/ty/sty.rs b/src/librustc_middle/ty/sty.rs
index 57df50dc1a0..e98ea097c90 100644
--- a/src/librustc_middle/ty/sty.rs
+++ b/src/librustc_middle/ty/sty.rs
@@ -1623,16 +1623,19 @@ impl RegionKind {
                 flags = flags | TypeFlags::HAS_FREE_LOCAL_REGIONS;
                 flags = flags | TypeFlags::HAS_RE_INFER;
                 flags = flags | TypeFlags::KEEP_IN_LOCAL_TCX;
+                flags = flags | TypeFlags::STILL_FURTHER_SPECIALIZABLE;
             }
             ty::RePlaceholder(..) => {
                 flags = flags | TypeFlags::HAS_FREE_REGIONS;
                 flags = flags | TypeFlags::HAS_FREE_LOCAL_REGIONS;
                 flags = flags | TypeFlags::HAS_RE_PLACEHOLDER;
+                flags = flags | TypeFlags::STILL_FURTHER_SPECIALIZABLE;
             }
             ty::ReEarlyBound(..) => {
                 flags = flags | TypeFlags::HAS_FREE_REGIONS;
                 flags = flags | TypeFlags::HAS_FREE_LOCAL_REGIONS;
                 flags = flags | TypeFlags::HAS_RE_PARAM;
+                flags = flags | TypeFlags::STILL_FURTHER_SPECIALIZABLE;
             }
             ty::ReFree { .. } | ty::ReScope { .. } => {
                 flags = flags | TypeFlags::HAS_FREE_REGIONS;
diff --git a/src/librustc_trait_selection/traits/project.rs b/src/librustc_trait_selection/traits/project.rs
index aae0d467563..3057b79547d 100644
--- a/src/librustc_trait_selection/traits/project.rs
+++ b/src/librustc_trait_selection/traits/project.rs
@@ -1028,7 +1028,7 @@ fn assemble_candidates_from_impls<'cx, 'tcx>(
                         // assume `poly_trait_ref` isn't monomorphic, if it contains any.
                         let poly_trait_ref =
                             selcx.infcx().resolve_vars_if_possible(&poly_trait_ref);
-                        !poly_trait_ref.needs_infer() && !poly_trait_ref.needs_subst()
+                        !poly_trait_ref.still_further_specializable()
                     } else {
                         debug!(
                             "assemble_candidates_from_impls: not eligible due to default: \
diff --git a/src/librustc_ty/instance.rs b/src/librustc_ty/instance.rs
index 47c4b1c41cd..e845cc9a158 100644
--- a/src/librustc_ty/instance.rs
+++ b/src/librustc_ty/instance.rs
@@ -127,7 +127,11 @@ fn resolve_associated_item<'tcx>(
                 // and the obligation is monomorphic, otherwise passes such as
                 // transmute checking and polymorphic MIR optimizations could
                 // get a result which isn't correct for all monomorphizations.
-                if param_env.reveal == Reveal::All { !trait_ref.needs_subst() } else { false }
+                if param_env.reveal == Reveal::All {
+                    !trait_ref.still_further_specializable()
+                } else {
+                    false
+                }
             };
 
             if !eligible {