const_evaluatable_checked: Stop eagerly erroring in is_const_evaluatable

compiler/rustc_middle/src/mir/abstract_const.rs

@@ -18,3 +18,20 @@ pub enum Node<'tcx> {
     UnaryOp(mir::UnOp, NodeId),
     FunctionCall(NodeId, &'tcx [NodeId]),
 }
+
+#[derive(Debug, Copy, Clone, PartialEq, Eq, HashStable, TyEncodable, TyDecodable)]
+pub enum NotConstEvaluatable {
+    Error(rustc_errors::ErrorReported),
+    MentionsInfer,
+    MentionsParam,
+}
+
+impl From<rustc_errors::ErrorReported> for NotConstEvaluatable {
+    fn from(e: rustc_errors::ErrorReported) -> NotConstEvaluatable {
+        NotConstEvaluatable::Error(e)
+    }
+}
+
+TrivialTypeFoldableAndLiftImpls! {
+    NotConstEvaluatable,
+}

compiler/rustc_middle/src/traits/mod.rs

@@ -9,7 +9,7 @@ pub mod specialization_graph;
 mod structural_impls;
 
 use crate::infer::canonical::Canonical;
-use crate::mir::interpret::ErrorHandled;
+use crate::mir::abstract_const::NotConstEvaluatable;
 use crate::ty::subst::SubstsRef;
 use crate::ty::{self, AdtKind, Ty, TyCtxt};
 
@@ -398,7 +398,7 @@ pub enum SelectionError<'tcx> {
         ty::error::TypeError<'tcx>,
     ),
     TraitNotObjectSafe(DefId),
-    ConstEvalFailure(ErrorHandled),
+    NotConstEvaluatable(NotConstEvaluatable),
     Overflow,
 }
 

compiler/rustc_trait_selection/src/traits/const_evaluatable.rs

@@ -13,7 +13,7 @@ use rustc_hir::def::DefKind;
 use rustc_index::bit_set::BitSet;
 use rustc_index::vec::IndexVec;
 use rustc_infer::infer::InferCtxt;
-use rustc_middle::mir::abstract_const::{Node, NodeId};
+use rustc_middle::mir::abstract_const::{Node, NodeId, NotConstEvaluatable};
 use rustc_middle::mir::interpret::ErrorHandled;
 use rustc_middle::mir::{self, Rvalue, StatementKind, TerminatorKind};
 use rustc_middle::ty::subst::{Subst, SubstsRef};
@@ -32,7 +32,7 @@ pub fn is_const_evaluatable<'cx, 'tcx>(
     substs: SubstsRef<'tcx>,
     param_env: ty::ParamEnv<'tcx>,
     span: Span,
-) -> Result<(), ErrorHandled> {
+) -> Result<(), NotConstEvaluatable> {
     debug!("is_const_evaluatable({:?}, {:?})", def, substs);
     if infcx.tcx.features().const_evaluatable_checked {
         let tcx = infcx.tcx;
@@ -103,29 +103,10 @@ pub fn is_const_evaluatable<'cx, 'tcx>(
 
                 match failure_kind {
                     FailureKind::MentionsInfer => {
-                        return Err(ErrorHandled::TooGeneric);
+                        return Err(NotConstEvaluatable::MentionsInfer);
                     }
                     FailureKind::MentionsParam => {
-                        // FIXME(const_evaluatable_checked): Better error message.
-                        let mut err =
-                            infcx.tcx.sess.struct_span_err(span, "unconstrained generic constant");
-                        let const_span = tcx.def_span(def.did);
-                        // FIXME(const_evaluatable_checked): Update this suggestion once
-                        // explicit const evaluatable bounds are implemented.
-                        if let Ok(snippet) = infcx.tcx.sess.source_map().span_to_snippet(const_span)
-                        {
-                            err.span_help(
-                                tcx.def_span(def.did),
-                                &format!("try adding a `where` bound using this expression: `where [u8; {}]: Sized`", snippet),
-                            );
-                        } else {
-                            err.span_help(
-                                const_span,
-                                "consider adding a `where` bound for this expression",
-                            );
-                        }
-                        err.emit();
-                        return Err(ErrorHandled::Reported(ErrorReported));
+                        return Err(NotConstEvaluatable::MentionsParam);
                     }
                     FailureKind::Concrete => {
                         // Dealt with below by the same code which handles this
@@ -180,34 +161,18 @@ pub fn is_const_evaluatable<'cx, 'tcx>(
 
     debug!(?concrete, "is_const_evaluatable");
     match concrete {
-        Err(ErrorHandled::TooGeneric) if !substs.has_infer_types_or_consts() => {
-            // FIXME(const_evaluatable_checked): We really should move
-            // emitting this error message to fulfill instead. For
-            // now this is easier.
-            //
-            // This is not a problem without `const_evaluatable_checked` as
-            // all `ConstEvaluatable` predicates have to be fulfilled for compilation
-            // to succeed.
-            //
-            // @lcnr: We already emit an error for things like
-            // `fn test<const N: usize>() -> [0 - N]` eagerly here,
-            // so until we fix this I don't really care.
-
-            let mut err = infcx
-                .tcx
-                .sess
-                .struct_span_err(span, "constant expression depends on a generic parameter");
-            // FIXME(const_generics): we should suggest to the user how they can resolve this
-            // issue. However, this is currently not actually possible
-            // (see https://github.com/rust-lang/rust/issues/66962#issuecomment-575907083).
-            //
-            // Note that with `feature(const_evaluatable_checked)` this case should not
-            // be reachable.
-            err.note("this may fail depending on what value the parameter takes");
-            err.emit();
-            Err(ErrorHandled::Reported(ErrorReported))
+        Err(ErrorHandled::TooGeneric) => {
+            return Err(match substs.has_infer_types_or_consts() {
+                true => NotConstEvaluatable::MentionsInfer,
+                false => NotConstEvaluatable::MentionsParam,
+            });
+        }
+        Err(ErrorHandled::Linted) => {
+            infcx.tcx.sess.delay_span_bug(span, "constant in type had error reported as lint");
+            Err(NotConstEvaluatable::Error(ErrorReported))
         }
-        c => c.map(drop),
+        Err(ErrorHandled::Reported(e)) => Err(NotConstEvaluatable::Error(e)),
+        Ok(_) => Ok(()),
     }
 }
 

compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs

@@ -2,10 +2,10 @@ pub mod on_unimplemented;
 pub mod suggestions;
 
 use super::{
-    ConstEvalFailure, EvaluationResult, FulfillmentError, FulfillmentErrorCode,
-    MismatchedProjectionTypes, Obligation, ObligationCause, ObligationCauseCode,
-    OnUnimplementedDirective, OnUnimplementedNote, OutputTypeParameterMismatch, Overflow,
-    PredicateObligation, SelectionContext, SelectionError, TraitNotObjectSafe,
+    EvaluationResult, FulfillmentError, FulfillmentErrorCode, MismatchedProjectionTypes,
+    Obligation, ObligationCause, ObligationCauseCode, OnUnimplementedDirective,
+    OnUnimplementedNote, OutputTypeParameterMismatch, Overflow, PredicateObligation,
+    SelectionContext, SelectionError, TraitNotObjectSafe,
 };
 
 use crate::infer::error_reporting::{TyCategory, TypeAnnotationNeeded as ErrorCode};
@@ -17,7 +17,7 @@ use rustc_hir as hir;
 use rustc_hir::def_id::{DefId, LOCAL_CRATE};
 use rustc_hir::intravisit::Visitor;
 use rustc_hir::Node;
-use rustc_middle::mir::interpret::ErrorHandled;
+use rustc_middle::mir::abstract_const::NotConstEvaluatable;
 use rustc_middle::ty::error::ExpectedFound;
 use rustc_middle::ty::fold::TypeFolder;
 use rustc_middle::ty::{
@@ -738,24 +738,59 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                 let violations = self.tcx.object_safety_violations(did);
                 report_object_safety_error(self.tcx, span, did, violations)
             }
-            ConstEvalFailure(ErrorHandled::TooGeneric) => {
-                bug!("too generic should have been handled in `is_const_evaluatable`");
+
+            SelectionError::NotConstEvaluatable(NotConstEvaluatable::MentionsInfer) => {
+                bug!(
+                    "MentionsInfer should have been handled in `traits/fulfill.rs` or `traits/select/mod.rs`"
+                )
+            }
+            SelectionError::NotConstEvaluatable(NotConstEvaluatable::MentionsParam) => {
+                if !self.tcx.features().const_evaluatable_checked {
+                    let mut err = self.tcx.sess.struct_span_err(
+                        span,
+                        "constant expression depends on a generic parameter",
+                    );
+                    // FIXME(const_generics): we should suggest to the user how they can resolve this
+                    // issue. However, this is currently not actually possible
+                    // (see https://github.com/rust-lang/rust/issues/66962#issuecomment-575907083).
+                    //
+                    // Note that with `feature(const_evaluatable_checked)` this case should not
+                    // be reachable.
+                    err.note("this may fail depending on what value the parameter takes");
+                    err.emit();
+                    return;
+                }
+
+                match obligation.predicate.kind().skip_binder() {
+                    ty::PredicateKind::ConstEvaluatable(def, _) => {
+                        let mut err =
+                            self.tcx.sess.struct_span_err(span, "unconstrained generic constant");
+                        let const_span = self.tcx.def_span(def.did);
+                        match self.tcx.sess.source_map().span_to_snippet(const_span) {
+                            Ok(snippet) => err.help(&format!(
+                                "try adding a `where` bound using this expression: `where [(); {}]:`",
+                                snippet
+                            )),
+                            _ => err.help("consider adding a `where` bound using this expression"),
+                        };
+                        err
+                    }
+                    _ => {
+                        span_bug!(
+                            span,
+                            "unexpected non-ConstEvaluatable predicate, this should not be reachable"
+                        )
+                    }
+                }
             }
+
             // Already reported in the query.
-            ConstEvalFailure(ErrorHandled::Reported(ErrorReported)) => {
+            SelectionError::NotConstEvaluatable(NotConstEvaluatable::Error(ErrorReported)) => {
                 // FIXME(eddyb) remove this once `ErrorReported` becomes a proof token.
                 self.tcx.sess.delay_span_bug(span, "`ErrorReported` without an error");
                 return;
             }
 
-            // Already reported in the query, but only as a lint.
-            // This shouldn't actually happen for constants used in types, modulo
-            // bugs. The `delay_span_bug` here ensures it won't be ignored.
-            ConstEvalFailure(ErrorHandled::Linted) => {
-                self.tcx.sess.delay_span_bug(span, "constant in type had error reported as lint");
-                return;
-            }
-
             Overflow => {
                 bug!("overflow should be handled before the `report_selection_error` path");
             }

compiler/rustc_trait_selection/src/traits/fulfill.rs

@@ -3,7 +3,8 @@ use rustc_data_structures::obligation_forest::ProcessResult;
 use rustc_data_structures::obligation_forest::{Error, ForestObligation, Outcome};
 use rustc_data_structures::obligation_forest::{ObligationForest, ObligationProcessor};
 use rustc_errors::ErrorReported;
-use rustc_infer::traits::{TraitEngine, TraitEngineExt as _, TraitObligation};
+use rustc_infer::traits::{SelectionError, TraitEngine, TraitEngineExt as _, TraitObligation};
+use rustc_middle::mir::abstract_const::NotConstEvaluatable;
 use rustc_middle::mir::interpret::ErrorHandled;
 use rustc_middle::ty::error::ExpectedFound;
 use rustc_middle::ty::subst::SubstsRef;
@@ -18,7 +19,7 @@ use super::wf;
 use super::CodeAmbiguity;
 use super::CodeProjectionError;
 use super::CodeSelectionError;
-use super::{ConstEvalFailure, Unimplemented};
+use super::Unimplemented;
 use super::{FulfillmentError, FulfillmentErrorCode};
 use super::{ObligationCause, PredicateObligation};
 
@@ -498,14 +499,23 @@ impl<'a, 'b, 'tcx> FulfillProcessor<'a, 'b, 'tcx> {
                         obligation.cause.span,
                     ) {
                         Ok(()) => ProcessResult::Changed(vec![]),
-                        Err(ErrorHandled::TooGeneric) => {
+                        Err(NotConstEvaluatable::MentionsInfer) => {
                             pending_obligation.stalled_on.clear();
                             pending_obligation.stalled_on.extend(
                                 substs.iter().filter_map(TyOrConstInferVar::maybe_from_generic_arg),
                             );
                             ProcessResult::Unchanged
                         }
-                        Err(e) => ProcessResult::Error(CodeSelectionError(ConstEvalFailure(e))),
+                        Err(NotConstEvaluatable::MentionsParam) => ProcessResult::Error(
+                            CodeSelectionError(SelectionError::NotConstEvaluatable(
+                                NotConstEvaluatable::MentionsParam,
+                            )),
+                        ),
+                        Err(NotConstEvaluatable::Error(e)) => {
+                            ProcessResult::Error(CodeSelectionError(
+                                SelectionError::NotConstEvaluatable(NotConstEvaluatable::Error(e)),
+                            ))
+                        }
                     }
                 }
 
@@ -576,11 +586,11 @@ impl<'a, 'b, 'tcx> FulfillProcessor<'a, 'b, 'tcx> {
                             }
                         }
                         (Err(ErrorHandled::Reported(ErrorReported)), _)
-                        | (_, Err(ErrorHandled::Reported(ErrorReported))) => {
-                            ProcessResult::Error(CodeSelectionError(ConstEvalFailure(
-                                ErrorHandled::Reported(ErrorReported),
-                            )))
-                        }
+                        | (_, Err(ErrorHandled::Reported(ErrorReported))) => ProcessResult::Error(
+                            CodeSelectionError(SelectionError::NotConstEvaluatable(
+                                NotConstEvaluatable::Error(ErrorReported),
+                            )),
+                        ),
                         (Err(ErrorHandled::Linted), _) | (_, Err(ErrorHandled::Linted)) => {
                             span_bug!(
                                 obligation.cause.span(self.selcx.tcx()),

compiler/rustc_trait_selection/src/traits/select/mod.rs

@@ -34,6 +34,7 @@ use rustc_hir::def_id::DefId;
 use rustc_hir::Constness;
 use rustc_infer::infer::LateBoundRegionConversionTime;
 use rustc_middle::dep_graph::{DepKind, DepNodeIndex};
+use rustc_middle::mir::abstract_const::NotConstEvaluatable;
 use rustc_middle::mir::interpret::ErrorHandled;
 use rustc_middle::ty::fast_reject;
 use rustc_middle::ty::print::with_no_trimmed_paths;
@@ -547,7 +548,8 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                         obligation.cause.span,
                     ) {
                         Ok(()) => Ok(EvaluatedToOk),
-                        Err(ErrorHandled::TooGeneric) => Ok(EvaluatedToAmbig),
+                        Err(NotConstEvaluatable::MentionsInfer) => Ok(EvaluatedToAmbig),
+                        Err(NotConstEvaluatable::MentionsParam) => Ok(EvaluatedToErr),
                         Err(_) => Ok(EvaluatedToErr),
                     }
                 }

src/test/ui/const-generics/const_evaluatable_checked/cross_crate_predicate.stderr

@@ -3,48 +3,52 @@ error: unconstrained generic constant
    |
 LL |     let _ = const_evaluatable_lib::test1::<T>();
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   |
-help: try adding a `where` bound using this expression: `where [u8; std::mem::size_of::<T>() - 1]: Sized`
-  --> $DIR/auxiliary/const_evaluatable_lib.rs:6:10
+   | 
+  ::: $DIR/auxiliary/const_evaluatable_lib.rs:6:10
    |
 LL |     [u8; std::mem::size_of::<T>() - 1]: Sized,
-   |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |          ---------------------------- required by this bound in `test1`
+   |
+   = help: try adding a `where` bound using this expression: `where [(); std::mem::size_of::<T>() - 1]:`
 
 error: unconstrained generic constant
   --> $DIR/cross_crate_predicate.rs:7:13
    |
 LL |     let _ = const_evaluatable_lib::test1::<T>();
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   |
-help: try adding a `where` bound using this expression: `where [u8; std::mem::size_of::<T>() - 1]: Sized`
-  --> $DIR/auxiliary/const_evaluatable_lib.rs:4:27
+   | 
+  ::: $DIR/auxiliary/const_evaluatable_lib.rs:4:27
    |
 LL | pub fn test1<T>() -> [u8; std::mem::size_of::<T>() - 1]
-   |                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |                           ---------------------------- required by this bound in `test1`
+   |
+   = help: try adding a `where` bound using this expression: `where [(); std::mem::size_of::<T>() - 1]:`
 
 error: unconstrained generic constant
   --> $DIR/cross_crate_predicate.rs:7:13
    |
 LL |     let _ = const_evaluatable_lib::test1::<T>();
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   |
-help: try adding a `where` bound using this expression: `where [u8; std::mem::size_of::<T>() - 1]: Sized`
-  --> $DIR/auxiliary/const_evaluatable_lib.rs:6:10
+   | 
+  ::: $DIR/auxiliary/const_evaluatable_lib.rs:6:10
    |
 LL |     [u8; std::mem::size_of::<T>() - 1]: Sized,
-   |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |          ---------------------------- required by this bound in `test1`
+   |
+   = help: try adding a `where` bound using this expression: `where [(); std::mem::size_of::<T>() - 1]:`
 
 error: unconstrained generic constant
   --> $DIR/cross_crate_predicate.rs:7:13
    |
 LL |     let _ = const_evaluatable_lib::test1::<T>();
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   |
-help: try adding a `where` bound using this expression: `where [u8; std::mem::size_of::<T>() - 1]: Sized`
-  --> $DIR/auxiliary/const_evaluatable_lib.rs:4:27
+   | 
+  ::: $DIR/auxiliary/const_evaluatable_lib.rs:4:27
    |
 LL | pub fn test1<T>() -> [u8; std::mem::size_of::<T>() - 1]
-   |                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |                           ---------------------------- required by this bound in `test1`
+   |
+   = help: try adding a `where` bound using this expression: `where [(); std::mem::size_of::<T>() - 1]:`
 
 error: aborting due to 4 previous errors
 

src/test/ui/const-generics/const_evaluatable_checked/different-fn.stderr

@@ -4,11 +4,7 @@ error: unconstrained generic constant
 LL |     [0; size_of::<Foo<T>>()]
    |         ^^^^^^^^^^^^^^^^^^^
    |
-help: try adding a `where` bound using this expression: `where [u8; size_of::<Foo<T>>()]: Sized`
-  --> $DIR/different-fn.rs:10:9
-   |
-LL |     [0; size_of::<Foo<T>>()]
-   |         ^^^^^^^^^^^^^^^^^^^
+   = help: try adding a `where` bound using this expression: `where [(); size_of::<Foo<T>>()]:`
 
 error: aborting due to previous error