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>

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),

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);

src/librustc_middle/ty/mod.rs

@@ -598,6 +598,10 @@ bitflags! {
         /// Does this have any [ReErased] regions?
         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.
@@ -618,7 +622,8 @@ bitflags! {
                                           | TypeFlags::HAS_TY_ERR.bits
                                           | TypeFlags::HAS_FREE_REGIONS.bits
                                           | TypeFlags::HAS_RE_LATE_BOUND.bits
-                                        | TypeFlags::HAS_RE_ERASED.bits;
+                                          | TypeFlags::HAS_RE_ERASED.bits
+                                          | TypeFlags::STILL_FURTHER_SPECIALIZABLE.bits;
     }
 }
 

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;

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: \

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 {