Fix Instance::resolve() incorrectly returning specialized instances

We only want to return specializations when `Reveal::All` is passed, not
when `Reveal::UserFacing` is. Resolving this fixes several issues with
the `ConstProp`, `SimplifyBranches`, and `Inline` MIR optimization
passes.

Fixes #66901

src/librustc/mir/interpret/queries.rs

@@ -18,7 +18,7 @@ impl<'tcx> TyCtxt<'tcx> {
         let substs = InternalSubsts::identity_for_item(self, def_id);
         let instance = ty::Instance::new(def_id, substs);
         let cid = GlobalId { instance, promoted: None };
-        let param_env = self.param_env(def_id);
+        let param_env = self.param_env(def_id).with_reveal_all();
         self.const_eval_validated(param_env.and(cid))
     }
 

src/librustc/traits/project.rs

@@ -1028,6 +1028,9 @@ fn assemble_candidates_from_impls<'cx, 'tcx>(
                 // In either case, we handle this by not adding a
                 // candidate for an impl if it contains a `default`
                 // type.
+                //
+                // NOTE: This should be kept in sync with the similar code in
+                // `rustc::ty::instance::resolve_associated_item()`.
                 let node_item =
                     assoc_ty_def(selcx, impl_data.impl_def_id, obligation.predicate.item_def_id);
 

src/librustc/ty/instance.rs

@@ -346,6 +346,25 @@ fn resolve_associated_item<'tcx>(
         traits::VtableImpl(impl_data) => {
             let (def_id, substs) =
                 traits::find_associated_item(tcx, param_env, trait_item, rcvr_substs, &impl_data);
+
+            let resolved_item = tcx.associated_item(def_id);
+
+            // Since this is a trait item, we need to see if the item is either a trait default item
+            // or a specialization because we can't resolve those unless we can `Reveal::All`.
+            // NOTE: This should be kept in sync with the similar code in
+            // `rustc::traits::project::assemble_candidates_from_impls()`.
+            let eligible = if !resolved_item.defaultness.is_default() {
+                true
+            } else if param_env.reveal == traits::Reveal::All {
+                !trait_ref.needs_subst()
+            } else {
+                false
+            };
+
+            if !eligible {
+                return None;
+            }
+
             let substs = tcx.erase_regions(&substs);
             Some(ty::Instance::new(def_id, substs))
         }

src/librustc_mir/const_eval/eval_queries.rs

@@ -212,11 +212,7 @@ pub fn const_eval_validated_provider<'tcx>(
         key.param_env.reveal = Reveal::UserFacing;
         match tcx.const_eval_validated(key) {
             // try again with reveal all as requested
-            Err(ErrorHandled::TooGeneric) => {
-                // Promoteds should never be "too generic" when getting evaluated.
-                // They either don't get evaluated, or we are in a monomorphic context
-                assert!(key.value.promoted.is_none());
-            }
+            Err(ErrorHandled::TooGeneric) => {}
             // dedupliate calls
             other => return other,
         }
@@ -301,10 +297,18 @@ pub fn const_eval_raw_provider<'tcx>(
                 // Ensure that if the above error was either `TooGeneric` or `Reported`
                 // an error must be reported.
                 let v = err.report_as_error(ecx.tcx, "could not evaluate static initializer");
-                tcx.sess.delay_span_bug(
-                    err.span,
-                    &format!("static eval failure did not emit an error: {:#?}", v),
-                );
+
+                // If this is `Reveal:All`, then we need to make sure an error is reported but if
+                // this is `Reveal::UserFacing`, then it's expected that we could get a
+                // `TooGeneric` error. When we fall back to `Reveal::All`, then it will either
+                // succeed or we'll report this error then.
+                if key.param_env.reveal == Reveal::All {
+                    tcx.sess.delay_span_bug(
+                        err.span,
+                        &format!("static eval failure did not emit an error: {:#?}", v),
+                    );
+                }
+
                 v
             } else if def_id.is_local() {
                 // constant defined in this crate, we can figure out a lint level!

src/librustc_mir/hair/pattern/mod.rs

@@ -742,7 +742,13 @@ impl<'a, 'tcx> PatCtxt<'a, 'tcx> {
         let kind = match res {
             Res::Def(DefKind::Const, def_id) | Res::Def(DefKind::AssocConst, def_id) => {
                 let substs = self.tables.node_substs(id);
-                match self.tcx.const_eval_resolve(self.param_env, def_id, substs, Some(span)) {
+                // Use `Reveal::All` here because patterns are always monomorphic even if their function isn't.
+                match self.tcx.const_eval_resolve(
+                    self.param_env.with_reveal_all(),
+                    def_id,
+                    substs,
+                    Some(span),
+                ) {
                     Ok(value) => {
                         let pattern = self.const_to_pat(value, id, span);
                         if !is_associated_const {

src/librustc_mir/transform/const_prop.rs

@@ -33,6 +33,7 @@ use crate::interpret::{
     ScalarMaybeUndef, StackPopCleanup,
 };
 use crate::rustc::ty::subst::Subst;
+use crate::rustc::ty::TypeFoldable;
 use crate::transform::{MirPass, MirSource};
 
 /// The maximum number of bytes that we'll allocate space for a return value.
@@ -293,13 +294,20 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
         source: MirSource<'tcx>,
     ) -> ConstPropagator<'mir, 'tcx> {
         let def_id = source.def_id();
-        let param_env = tcx.param_env(def_id);
+        let substs = &InternalSubsts::identity_for_item(tcx, def_id);
+        let mut param_env = tcx.param_env(def_id);
+
+        // If we're evaluating inside a monomorphic function, then use `Reveal::All` because
+        // we want to see the same instances that codegen will see. This allows us to `resolve()`
+        // specializations.
+        if !substs.needs_subst() {
+            param_env = param_env.with_reveal_all();
+        }
+
         let span = tcx.def_span(def_id);
         let mut ecx = InterpCx::new(tcx.at(span), param_env, ConstPropMachine, ());
         let can_const_prop = CanConstProp::check(body);
 
-        let substs = &InternalSubsts::identity_for_item(tcx, def_id);
-
         let ret = ecx
             .layout_of(body.return_ty().subst(tcx, substs))
             .ok()

src/librustc_mir/transform/inline.rs

@@ -8,8 +8,8 @@ use rustc_index::vec::{Idx, IndexVec};
 
 use rustc::mir::visit::*;
 use rustc::mir::*;
-use rustc::ty::subst::{Subst, SubstsRef};
-use rustc::ty::{self, Instance, InstanceDef, ParamEnv, Ty, TyCtxt};
+use rustc::ty::subst::{InternalSubsts, Subst, SubstsRef};
+use rustc::ty::{self, Instance, InstanceDef, ParamEnv, Ty, TyCtxt, TypeFoldable};
 
 use super::simplify::{remove_dead_blocks, CfgSimplifier};
 use crate::transform::{MirPass, MirSource};
@@ -66,7 +66,14 @@ impl Inliner<'tcx> {
 
         let mut callsites = VecDeque::new();
 
-        let param_env = self.tcx.param_env(self.source.def_id());
+        let mut param_env = self.tcx.param_env(self.source.def_id());
+
+        let substs = &InternalSubsts::identity_for_item(self.tcx, self.source.def_id());
+
+        // For monomorphic functions, we can use `Reveal::All` to resolve specialized instances.
+        if !substs.needs_subst() {
+            param_env = param_env.with_reveal_all();
+        }
 
         // Only do inlining into fn bodies.
         let id = self.tcx.hir().as_local_hir_id(self.source.def_id()).unwrap();

src/test/mir-opt/inline/inline-specialization.rs

@@ -0,0 +1,48 @@
+#![feature(specialization)]
+
+fn main() {
+    let x = <Vec::<()> as Foo>::bar();
+}
+
+trait Foo {
+    fn bar() -> u32;
+}
+
+impl<T> Foo for Vec<T> {
+    #[inline(always)]
+    default fn bar() -> u32 { 123 }
+}
+
+// END RUST SOURCE
+// START rustc.main.Inline.before.mir
+// let mut _0: ();
+// let _1: u32;
+// scope 1 {
+//   debug x => _1;
+// }
+// bb0: {
+//   StorageLive(_1);
+//   _1 = const <std::vec::Vec<()> as Foo>::bar() -> bb1;
+// }
+// bb1: {
+//   _0 = ();
+//   StorageDead(_1);
+//   return;
+// }
+// END rustc.main.Inline.before.mir
+// START rustc.main.Inline.after.mir
+// let mut _0: ();
+// let _1: u32;
+// scope 1 {
+//   debug x => _1;
+// }
+// scope 2 {
+// }
+// bb0: {
+//   StorageLive(_1);
+//   _1 = const 123u32;
+//   _0 = ();
+//   StorageDead(_1);
+//   return;
+// }
+// END rustc.main.Inline.after.mir

src/test/ui/consts/trait_specialization.rs

@@ -0,0 +1,65 @@
+// ignore-wasm32-bare which doesn't support `std::process:exit()`
+// compile-flags: -Zmir-opt-level=2
+// run-pass
+
+// Tests that specialization does not cause optimizations running on polymorphic MIR to resolve
+// to a `default` implementation.
+
+#![feature(specialization)]
+
+trait Marker {}
+
+trait SpecializedTrait {
+    const CONST_BOOL: bool;
+    const CONST_STR: &'static str;
+    fn method() -> &'static str;
+}
+impl <T> SpecializedTrait for T {
+    default const CONST_BOOL: bool = false;
+    default const CONST_STR: &'static str = "in default impl";
+    #[inline(always)]
+    default fn method() -> &'static str {
+        "in default impl"
+    }
+}
+impl <T: Marker> SpecializedTrait for T {
+    const CONST_BOOL: bool = true;
+    const CONST_STR: &'static str = "in specialized impl";
+    fn method() -> &'static str {
+        "in specialized impl"
+    }
+}
+
+fn const_bool<T>() -> &'static str {
+    if <T as SpecializedTrait>::CONST_BOOL {
+        "in specialized impl"
+    } else {
+        "in default impl"
+    }
+}
+fn const_str<T>() -> &'static str {
+    <T as SpecializedTrait>::CONST_STR
+}
+fn run_method<T>() -> &'static str {
+    <T as SpecializedTrait>::method()
+}
+
+struct TypeA;
+impl Marker for TypeA {}
+struct TypeB;
+
+#[inline(never)]
+fn exit_if_not_eq(left: &str, right: &str) {
+    if left != right {
+        std::process::exit(1);
+    }
+}
+
+pub fn main() {
+    exit_if_not_eq("in specialized impl", const_bool::<TypeA>());
+    exit_if_not_eq("in default impl", const_bool::<TypeB>());
+    exit_if_not_eq("in specialized impl", const_str::<TypeA>());
+    exit_if_not_eq("in default impl", const_str::<TypeB>());
+    exit_if_not_eq("in specialized impl", run_method::<TypeA>());
+    exit_if_not_eq("in default impl", run_method::<TypeB>());
+}

src/test/ui/type-alias-enum-variants/self-in-enum-definition.stderr

@@ -4,6 +4,11 @@ error[E0391]: cycle detected when const-evaluating + checking `Alpha::V3::{{cons
 LL |     V3 = Self::V1 {} as u8 + 2,
    |          ^^^^^^^^
    |
+note: ...which requires const-evaluating + checking `Alpha::V3::{{constant}}#0`...
+  --> $DIR/self-in-enum-definition.rs:5:10
+   |
+LL |     V3 = Self::V1 {} as u8 + 2,
+   |          ^^^^^^^^
 note: ...which requires const-evaluating `Alpha::V3::{{constant}}#0`...
   --> $DIR/self-in-enum-definition.rs:5:10
    |