Make trivial dropck outlives a query

This allows caching some recursive types and getting to an error much
more quickly.

Author: Mark-Simulacrum

src/librustc/query/mod.rs

@@ -231,6 +231,12 @@ rustc_queries! {
             cycle_delay_bug
         }
 
+        query trivial_dropck_outlives(ty: Ty<'tcx>) -> bool {
+            anon
+            no_force
+            desc { "checking if `{:?}` has trivial dropck", ty }
+        }
+
         query adt_dtorck_constraint(
             _: DefId
         ) -> Result<DtorckConstraint<'tcx>, NoSolution> {}

src/librustc/traits/query/dropck_outlives.rs

@@ -5,6 +5,7 @@ use std::iter::FromIterator;
 use syntax::source_map::Span;
 use crate::ty::subst::GenericArg;
 use crate::ty::{self, Ty, TyCtxt};
+use crate::ty::query::Providers;
 
 impl<'cx, 'tcx> At<'cx, 'tcx> {
     /// Given a type `ty` of some value being dropped, computes a set
@@ -33,7 +34,7 @@ impl<'cx, 'tcx> At<'cx, 'tcx> {
         // Quick check: there are a number of cases that we know do not require
         // any destructor.
         let tcx = self.infcx.tcx;
-        if trivial_dropck_outlives(tcx, ty) {
+        if tcx.trivial_dropck_outlives(ty) {
             return InferOk {
                 value: vec![],
                 obligations: vec![],
@@ -207,15 +208,15 @@ pub fn trivial_dropck_outlives<'tcx>(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>) -> bool {
         | ty::Error => true,
 
         // [T; N] and [T] have same properties as T.
-        ty::Array(ty, _) | ty::Slice(ty) => trivial_dropck_outlives(tcx, ty),
+        ty::Array(ty, _) | ty::Slice(ty) => tcx.trivial_dropck_outlives(ty),
 
         // (T1..Tn) and closures have same properties as T1..Tn --
         // check if *any* of those are trivial.
-        ty::Tuple(ref tys) => tys.iter().all(|t| trivial_dropck_outlives(tcx, t.expect_ty())),
+        ty::Tuple(ref tys) => tys.iter().all(|t| tcx.trivial_dropck_outlives(t.expect_ty())),
         ty::Closure(def_id, ref substs) => substs
             .as_closure()
             .upvar_tys(def_id, tcx)
-            .all(|t| trivial_dropck_outlives(tcx, t)),
+            .all(|t| tcx.trivial_dropck_outlives(t)),
 
         ty::Adt(def, _) => {
             if Some(def.did) == tcx.lang_items().manually_drop() {
@@ -243,3 +244,10 @@ pub fn trivial_dropck_outlives<'tcx>(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>) -> bool {
         ty::UnnormalizedProjection(..) => bug!("only used with chalk-engine"),
     }
 }
+
+crate fn provide(p: &mut Providers<'_>) {
+    *p = Providers {
+        trivial_dropck_outlives,
+        ..*p
+    };
+}

src/librustc/traits/query/type_op/outlives.rs

@@ -1,5 +1,4 @@
 use crate::infer::canonical::{Canonicalized, CanonicalizedQueryResponse};
-use crate::traits::query::dropck_outlives::trivial_dropck_outlives;
 use crate::traits::query::dropck_outlives::DropckOutlivesResult;
 use crate::traits::query::Fallible;
 use crate::ty::{ParamEnvAnd, Ty, TyCtxt};
@@ -22,7 +21,7 @@ impl super::QueryTypeOp<'tcx> for DropckOutlives<'tcx> {
         tcx: TyCtxt<'tcx>,
         key: &ParamEnvAnd<'tcx, Self>,
     ) -> Option<Self::QueryResponse> {
-        if trivial_dropck_outlives(tcx, key.value.dropped_ty) {
+        if tcx.trivial_dropck_outlives(key.value.dropped_ty) {
             Some(DropckOutlivesResult::default())
         } else {
             None

src/librustc/ty/mod.rs

@@ -3394,6 +3394,7 @@ pub fn provide(providers: &mut ty::query::Providers<'_>) {
     layout::provide(providers);
     util::provide(providers);
     constness::provide(providers);
+    crate::traits::query::dropck_outlives::provide(providers);
     *providers = ty::query::Providers {
         asyncness,
         associated_item,

src/librustc_traits/dropck_outlives.rs

@@ -166,6 +166,10 @@ fn dtorck_constraint_for_ty<'tcx>(
         });
     }
 
+    if tcx.trivial_dropck_outlives(ty) {
+        return Ok(DtorckConstraint::empty());
+    }
+
     let result = match ty.kind {
         ty::Bool
         | ty::Char