Rollup merge of #66877 - skinny121:const-eval-entry-points, r=oli-obk

Add simpler entry points to const eval for common usages.

I found the `tcx.const_eval` API to be complex/awkward to work with, because of the inherent complexity from all of the different situations it is called from. Though it mainly used in one of the following ways:
- Evaluates the value of a constant without any substitutions, e.g. evaluating a static, discriminant, etc.
- Evaluates the value of a resolved instance of a constant. this happens when evaluating unevaluated constants or normalising trait constants.
- Evaluates a promoted constant.

This PR adds three new functions `const_eval_mono`, `const_eval_resolve`, and `const_eval_promoted` to `TyCtxt`, which each cater to one of the three ways `tcx.const_eval`
 is normally used.

7 files changed, 120 insertions, 57 deletions

diff --git a/src/librustc/mir/interpret/mod.rs b/src/librustc/mir/interpret/mod.rs
index fff876752db..80bac92d003 100644
--- a/src/librustc/mir/interpret/mod.rs
+++ b/src/librustc/mir/interpret/mod.rs
@@ -101,6 +101,7 @@ mod error;
 mod value;
 mod allocation;
 mod pointer;
+mod queries;
 
 pub use self::error::{
     InterpErrorInfo, InterpResult, InterpError, AssertMessage, ConstEvalErr, struct_error,
@@ -116,9 +117,10 @@ pub use self::pointer::{Pointer, PointerArithmetic, CheckInAllocMsg};
 
 use crate::mir;
 use crate::hir::def_id::DefId;
-use crate::ty::{self, TyCtxt, Instance, subst::GenericArgKind};
+use crate::ty::{self, TyCtxt, Instance};
 use crate::ty::codec::TyDecoder;
 use crate::ty::layout::{self, Size};
+use crate::ty::subst::GenericArgKind;
 use std::io;
 use std::fmt;
 use std::num::NonZeroU32;
diff --git a/src/librustc/mir/interpret/queries.rs b/src/librustc/mir/interpret/queries.rs
new file mode 100644
index 00000000000..5fd49a056d2
--- /dev/null
+++ b/src/librustc/mir/interpret/queries.rs
@@ -0,0 +1,89 @@
+use super::{ConstEvalResult, ErrorHandled, GlobalId};
+
+use crate::mir;
+use crate::hir::def_id::DefId;
+use crate::ty::{self, TyCtxt};
+use crate::ty::subst::{InternalSubsts, SubstsRef};
+use syntax_pos::Span;
+
+
+impl<'tcx> TyCtxt<'tcx> {
+
+    /// Evaluates a constant without providing any substitutions. This is useful to evaluate consts
+    /// that can't take any generic arguments like statics, const items or enum discriminants. If a
+    /// generic parameter is used within the constant `ErrorHandled::ToGeneric` will be returned.
+    pub fn const_eval_poly(self, def_id: DefId) -> ConstEvalResult<'tcx> {
+        // In some situations def_id will have substitutions within scope, but they aren't allowed
+        // to be used. So we can't use `Instance::mono`, instead we feed unresolved substitutions
+        // into `const_eval` which will return `ErrorHandled::ToGeneric` if any og them are
+        // encountered.
+        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);
+        self.const_eval_validated(param_env.and(cid))
+    }
+
+    /// Resolves and evaluates a constant.
+    ///
+    /// The constant can be located on a trait like `<A as B>::C`, in which case the given
+    /// substitutions and environment are used to resolve the constant. Alternatively if the
+    /// constant has generic parameters in scope the substitutions are used to evaluate the value of
+    /// the constant. For example in `fn foo<T>() { let _ = [0; bar::<T>()]; }` the repeat count
+    /// constant `bar::<T>()` requires a substitution for `T`, if the substitution for `T` is still
+    /// too generic for the constant to be evaluated then `Err(ErrorHandled::TooGeneric)` is
+    /// returned.
+    pub fn const_eval_resolve(
+        self,
+        param_env: ty::ParamEnv<'tcx>,
+        def_id: DefId,
+        substs: SubstsRef<'tcx>,
+        span: Option<Span>
+    ) -> ConstEvalResult<'tcx> {
+        let instance = ty::Instance::resolve(
+            self,
+            param_env,
+            def_id,
+            substs,
+        );
+        if let Some(instance) = instance {
+            self.const_eval_instance(param_env, instance, span)
+        } else {
+            Err(ErrorHandled::TooGeneric)
+        }
+    }
+
+    pub fn const_eval_instance(
+        self,
+        param_env: ty::ParamEnv<'tcx>,
+        instance: ty::Instance<'tcx>,
+        span: Option<Span>
+    ) -> ConstEvalResult<'tcx> {
+        let cid = GlobalId {
+            instance,
+            promoted: None,
+        };
+        if let Some(span) = span {
+            self.at(span).const_eval_validated(param_env.and(cid))
+        } else {
+            self.const_eval_validated(param_env.and(cid))
+        }
+    }
+
+    /// Evaluate a promoted constant.
+    pub fn const_eval_promoted(
+        self,
+        instance: ty::Instance<'tcx>,
+        promoted: mir::Promoted
+    ) -> ConstEvalResult<'tcx> {
+        let cid = GlobalId {
+            instance,
+            promoted: Some(promoted),
+        };
+        let param_env = ty::ParamEnv::reveal_all();
+        self.const_eval_validated(param_env.and(cid))
+    }
+}
diff --git a/src/librustc/query/mod.rs b/src/librustc/query/mod.rs
index cc02165f605..a9dd856e758 100644
--- a/src/librustc/query/mod.rs
+++ b/src/librustc/query/mod.rs
@@ -448,7 +448,8 @@ rustc_queries! {
         ///
         /// **Do not use this** outside const eval. Const eval uses this to break query cycles
         /// during validation. Please add a comment to every use site explaining why using
-        /// `const_eval` isn't sufficient.
+        /// `const_eval_validated` isn't sufficient. The returned constant also isn't in a suitable
+        /// form to be used outside of const eval.
         query const_eval_raw(key: ty::ParamEnvAnd<'tcx, GlobalId<'tcx>>)
             -> ConstEvalRawResult<'tcx> {
             no_force
@@ -460,7 +461,13 @@ rustc_queries! {
 
         /// Results of evaluating const items or constants embedded in
         /// other items (such as enum variant explicit discriminants).
-        query const_eval(key: ty::ParamEnvAnd<'tcx, GlobalId<'tcx>>)
+        ///
+        /// In contrast to `const_eval_raw` this performs some validation on the constant, and
+        /// returns a proper constant that is usable by the rest of the compiler.
+        ///
+        /// **Do not use this** directly, use one of the following wrappers: `tcx.const_eval_poly`,
+        /// `tcx.const_eval_resolve`, `tcx.const_eval_instance`, or `tcx.const_eval_promoted`.
+        query const_eval_validated(key: ty::ParamEnvAnd<'tcx, GlobalId<'tcx>>)
             -> ConstEvalResult<'tcx> {
             no_force
             desc { |tcx|
diff --git a/src/librustc/traits/fulfill.rs b/src/librustc/traits/fulfill.rs
index 27731990d2b..3fd2415c83c 100644
--- a/src/librustc/traits/fulfill.rs
+++ b/src/librustc/traits/fulfill.rs
@@ -1,5 +1,4 @@
 use crate::infer::{InferCtxt, ShallowResolver};
-use crate::mir::interpret::{GlobalId, ErrorHandled};
 use crate::ty::{self, Ty, TypeFoldable, ToPolyTraitRef};
 use crate::ty::error::ExpectedFound;
 use rustc_data_structures::obligation_forest::{DoCompleted, Error, ForestObligation};
@@ -501,27 +500,13 @@ impl<'a, 'b, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'b, 'tcx> {
                         ProcessResult::Unchanged
                 } else {
                     if !substs.has_local_value() {
-                        let instance = ty::Instance::resolve(
-                            self.selcx.tcx(),
-                            obligation.param_env,
-                            def_id,
-                            substs,
-                        );
-                        if let Some(instance) = instance {
-                            let cid = GlobalId {
-                                instance,
-                                promoted: None,
-                            };
-                            match self.selcx.tcx().at(obligation.cause.span)
-                                                    .const_eval(obligation.param_env.and(cid)) {
-                                Ok(_) => ProcessResult::Changed(vec![]),
-                                Err(err) => ProcessResult::Error(
-                                    CodeSelectionError(ConstEvalFailure(err)))
-                            }
-                        } else {
-                            ProcessResult::Error(CodeSelectionError(
-                                ConstEvalFailure(ErrorHandled::TooGeneric)
-                            ))
+                        match self.selcx.tcx().const_eval_resolve(obligation.param_env,
+                                                                  def_id,
+                                                                  substs,
+                                                                  Some(obligation.cause.span)) {
+                            Ok(_) => ProcessResult::Changed(vec![]),
+                            Err(err) => ProcessResult::Error(
+                                CodeSelectionError(ConstEvalFailure(err)))
                         }
                     } else {
                         pending_obligation.stalled_on =
diff --git a/src/librustc/traits/select.rs b/src/librustc/traits/select.rs
index f1cd002d19b..afc8a4d9e14 100644
--- a/src/librustc/traits/select.rs
+++ b/src/librustc/traits/select.rs
@@ -33,7 +33,6 @@ use crate::dep_graph::{DepKind, DepNodeIndex};
 use crate::hir::def_id::DefId;
 use crate::infer::{CombinedSnapshot, InferCtxt, InferOk, PlaceholderMap, TypeFreshener};
 use crate::middle::lang_items;
-use crate::mir::interpret::GlobalId;
 use crate::ty::fast_reject;
 use crate::ty::relate::TypeRelation;
 use crate::ty::subst::{Subst, SubstsRef};
@@ -820,22 +819,13 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             }
 
             ty::Predicate::ConstEvaluatable(def_id, substs) => {
-                let tcx = self.tcx();
                 if !(obligation.param_env, substs).has_local_value() {
-                    let param_env = obligation.param_env;
-                    let instance =
-                        ty::Instance::resolve(tcx, param_env, def_id, substs);
-                    if let Some(instance) = instance {
-                        let cid = GlobalId {
-                            instance,
-                            promoted: None,
-                        };
-                        match self.tcx().const_eval(param_env.and(cid)) {
-                            Ok(_) => Ok(EvaluatedToOk),
-                            Err(_) => Ok(EvaluatedToErr),
-                        }
-                    } else {
-                        Ok(EvaluatedToErr)
+                    match self.tcx().const_eval_resolve(obligation.param_env,
+                                                        def_id,
+                                                        substs,
+                                                        None) {
+                        Ok(_) => Ok(EvaluatedToOk),
+                        Err(_) => Ok(EvaluatedToErr),
                     }
                 } else {
                     // Inference variables still left in param_env or substs.
diff --git a/src/librustc/ty/mod.rs b/src/librustc/ty/mod.rs
index 37a443a2c74..30356d59a7c 100644
--- a/src/librustc/ty/mod.rs
+++ b/src/librustc/ty/mod.rs
@@ -19,7 +19,7 @@ use crate::middle::cstore::CrateStoreDyn;
 use crate::middle::lang_items::{FnTraitLangItem, FnMutTraitLangItem, FnOnceTraitLangItem};
 use crate::middle::resolve_lifetime::ObjectLifetimeDefault;
 use crate::mir::ReadOnlyBodyAndCache;
-use crate::mir::interpret::{GlobalId, ErrorHandled};
+use crate::mir::interpret::ErrorHandled;
 use crate::mir::GeneratorLayout;
 use crate::session::CrateDisambiguator;
 use crate::traits::{self, Reveal};
@@ -2344,13 +2344,7 @@ impl<'tcx> AdtDef {
     pub fn eval_explicit_discr(&self, tcx: TyCtxt<'tcx>, expr_did: DefId) -> Option<Discr<'tcx>> {
         let param_env = tcx.param_env(expr_did);
         let repr_type = self.repr.discr_type();
-        let substs = InternalSubsts::identity_for_item(tcx, expr_did);
-        let instance = ty::Instance::new(expr_did, substs);
-        let cid = GlobalId {
-            instance,
-            promoted: None
-        };
-        match tcx.const_eval(param_env.and(cid)) {
+        match tcx.const_eval_poly(expr_did) {
             Ok(val) => {
                 // FIXME: Find the right type and use it instead of `val.ty` here
                 if let Some(b) = val.try_eval_bits(tcx, param_env, val.ty) {
diff --git a/src/librustc/ty/sty.rs b/src/librustc/ty/sty.rs
index 02efe8df03d..604dc03ea17 100644
--- a/src/librustc/ty/sty.rs
+++ b/src/librustc/ty/sty.rs
@@ -15,7 +15,7 @@ use crate::ty::{self, AdtDef, Discr, DefIdTree, TypeFlags, Ty, TyCtxt, TypeFolda
 use crate::ty::{List, TyS, ParamEnvAnd, ParamEnv};
 use crate::ty::layout::VariantIdx;
 use crate::util::captures::Captures;
-use crate::mir::interpret::{Scalar, GlobalId};
+use crate::mir::interpret::Scalar;
 
 use polonius_engine::Atom;
 use rustc_index::vec::Idx;
@@ -2340,13 +2340,9 @@ impl<'tcx> Const<'tcx> {
 
             let (param_env, substs) = param_env_and_substs.into_parts();
 
-            // try to resolve e.g. associated constants to their definition on an impl
-            let instance = ty::Instance::resolve(tcx, param_env, did, substs)?;
-            let gid = GlobalId {
-                instance,
-                promoted: None,
-            };
-            tcx.const_eval(param_env.and(gid)).ok()
+            // try to resolve e.g. associated constants to their definition on an impl, and then
+            // evaluate the const.
+            tcx.const_eval_resolve(param_env, did, substs, None).ok()
         };
 
         match self.val {