Auto merge of #48557 - matthewjasper:allow-trvial-bounds, r=nikomatsakis

Implement RFC 2056 trivial constraints in where clauses

This is an implementation of the new behaviour for #48214. Tests are mostly updated to show the effects of this. Feature gate hasn't been added yet.

Some things that are worth noting and are maybe not want we want

* `&mut T: Copy` doesn't allow as much as someone might expect because there is often an implicit reborrow.
* ~There isn't a check that a where clause is well-formed any more, so `where Vec<str>: Debug` is now allowed (without a `str: Sized` bound).~

r? @nikomatsakis

src/librustc/traits/error_reporting.rs

@@ -1479,6 +1479,14 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
             }
             ObligationCauseCode::ReturnType(_) |
             ObligationCauseCode::BlockTailExpression(_) => (),
+            ObligationCauseCode::TrivialBound => {
+                err.help("see issue #48214");
+                if tcx.sess.opts.unstable_features.is_nightly_build() {
+                    err.help("add #![feature(trivial_bounds)] to the \
+                              crate attributes to enable",
+                    );
+                }
+            }
         }
     }
 

src/librustc/traits/fulfill.rs

@@ -330,7 +330,7 @@ fn process_predicate<'a, 'gcx, 'tcx>(
         ty::Predicate::Trait(ref data) => {
             let trait_obligation = obligation.with(data.clone());
 
-            if data.is_global() {
+            if data.is_global() && !data.has_late_bound_regions() {
                 // no type variables present, can use evaluation for better caching.
                 // FIXME: consider caching errors too.
                 if selcx.infcx().predicate_must_hold(&obligation) {

src/librustc/traits/mod.rs

@@ -243,6 +243,9 @@ pub enum ObligationCauseCode<'tcx> {
 
     /// Block implicit return
     BlockTailExpression(ast::NodeId),
+
+    /// #[feature(trivial_bounds)] is not enabled
+    TrivialBound,
 }
 
 #[derive(Clone, Debug, PartialEq, Eq, Hash)]
@@ -641,17 +644,8 @@ pub fn normalize_param_env_or_error<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 
     let predicates: Vec<_> =
         util::elaborate_predicates(tcx, unnormalized_env.caller_bounds.to_vec())
-        .filter(|p| !p.is_global()) // (*)
         .collect();
 
-    // (*) Any predicate like `i32: Trait<u32>` or whatever doesn't
-    // need to be in the *environment* to be proven, so screen those
-    // out. This is important for the soundness of inter-fn
-    // caching. Note though that we should probably check that these
-    // predicates hold at the point where the environment is
-    // constructed, but I am not currently doing so out of laziness.
-    // -nmatsakis
-
     debug!("normalize_param_env_or_error: elaborated-predicates={:?}",
            predicates);
 

src/librustc/traits/select.rs

@@ -305,9 +305,6 @@ enum BuiltinImplConditions<'tcx> {
     /// There is no built-in impl. There may be some other
     /// candidate (a where-clause or user-defined impl).
     None,
-    /// There is *no* impl for this, builtin or not. Ignore
-    /// all where-clauses.
-    Never,
     /// It is unknown whether there is an impl.
     Ambiguous
 }
@@ -781,13 +778,13 @@ impl<'cx, 'gcx, 'tcx> SelectionContext<'cx, 'gcx, 'tcx> {
                                                 mut obligation: TraitObligation<'tcx>)
                                                 -> Result<EvaluationResult, OverflowError>
     {
-        debug!("evaluate_trait_predicate_recursively({:?})",
-               obligation);
+        debug!("evaluate_trait_predicate_recursively({:?})", obligation);
 
-        if !self.intercrate.is_some() && obligation.is_global() {
-            // If a param env is consistent, global obligations do not depend on its particular
-            // value in order to work, so we can clear out the param env and get better
-            // caching. (If the current param env is inconsistent, we don't care what happens).
+        if self.intercrate.is_none() && obligation.is_global()
+            && obligation.param_env.caller_bounds.iter().all(|bound| bound.needs_subst()) {
+            // If a param env has no global bounds, global obligations do not
+            // depend on its particular value in order to work, so we can clear
+            // out the param env and get better caching.
             debug!("evaluate_trait_predicate_recursively({:?}) - in global", obligation);
             obligation.param_env = obligation.param_env.without_caller_bounds();
         }
@@ -1451,22 +1448,22 @@ impl<'cx, 'gcx, 'tcx> SelectionContext<'cx, 'gcx, 'tcx> {
             let sized_conditions = self.sized_conditions(obligation);
             self.assemble_builtin_bound_candidates(sized_conditions,
                                                    &mut candidates)?;
-         } else if lang_items.unsize_trait() == Some(def_id) {
-             self.assemble_candidates_for_unsizing(obligation, &mut candidates);
-         } else {
-             if lang_items.clone_trait() == Some(def_id) {
-                 // Same builtin conditions as `Copy`, i.e. every type which has builtin support
-                 // for `Copy` also has builtin support for `Clone`, + tuples and arrays of `Clone`
-                 // types have builtin support for `Clone`.
-                 let clone_conditions = self.copy_clone_conditions(obligation);
-                 self.assemble_builtin_bound_candidates(clone_conditions, &mut candidates)?;
-             }
-
-             self.assemble_generator_candidates(obligation, &mut candidates)?;
-             self.assemble_closure_candidates(obligation, &mut candidates)?;
-             self.assemble_fn_pointer_candidates(obligation, &mut candidates)?;
-             self.assemble_candidates_from_impls(obligation, &mut candidates)?;
-             self.assemble_candidates_from_object_ty(obligation, &mut candidates);
+        } else if lang_items.unsize_trait() == Some(def_id) {
+            self.assemble_candidates_for_unsizing(obligation, &mut candidates);
+        } else {
+            if lang_items.clone_trait() == Some(def_id) {
+                // Same builtin conditions as `Copy`, i.e. every type which has builtin support
+                // for `Copy` also has builtin support for `Clone`, + tuples and arrays of `Clone`
+                // types have builtin support for `Clone`.
+                let clone_conditions = self.copy_clone_conditions(obligation);
+                self.assemble_builtin_bound_candidates(clone_conditions, &mut candidates)?;
+            }
+
+            self.assemble_generator_candidates(obligation, &mut candidates)?;
+            self.assemble_closure_candidates(obligation, &mut candidates)?;
+            self.assemble_fn_pointer_candidates(obligation, &mut candidates)?;
+            self.assemble_candidates_from_impls(obligation, &mut candidates)?;
+            self.assemble_candidates_from_object_ty(obligation, &mut candidates);
         }
 
         self.assemble_candidates_from_projected_tys(obligation, &mut candidates);
@@ -2081,13 +2078,8 @@ impl<'cx, 'gcx, 'tcx> SelectionContext<'cx, 'gcx, 'tcx> {
     // BUILTIN BOUNDS
     //
     // These cover the traits that are built-in to the language
-    // itself.  This includes `Copy` and `Sized` for sure. For the
-    // moment, it also includes `Send` / `Sync` and a few others, but
-    // those will hopefully change to library-defined traits in the
-    // future.
+    // itself: `Copy`, `Clone` and `Sized`.
 
-    // HACK: if this returns an error, selection exits without considering
-    // other impls.
     fn assemble_builtin_bound_candidates<'o>(&mut self,
                                              conditions: BuiltinImplConditions<'tcx>,
                                              candidates: &mut SelectionCandidateSet<'tcx>)
@@ -2106,14 +2098,13 @@ impl<'cx, 'gcx, 'tcx> SelectionContext<'cx, 'gcx, 'tcx> {
                 debug!("assemble_builtin_bound_candidates: ambiguous builtin");
                 Ok(candidates.ambiguous = true)
             }
-            BuiltinImplConditions::Never => { Err(Unimplemented) }
         }
     }
 
     fn sized_conditions(&mut self, obligation: &TraitObligation<'tcx>)
                      -> BuiltinImplConditions<'tcx>
     {
-        use self::BuiltinImplConditions::{Ambiguous, None, Never, Where};
+        use self::BuiltinImplConditions::{Ambiguous, None, Where};
 
         // NOTE: binder moved to (*)
         let self_ty = self.infcx.shallow_resolve(
@@ -2130,7 +2121,7 @@ impl<'cx, 'gcx, 'tcx> SelectionContext<'cx, 'gcx, 'tcx> {
                 Where(ty::Binder::dummy(Vec::new()))
             }
 
-            ty::TyStr | ty::TySlice(_) | ty::TyDynamic(..) | ty::TyForeign(..) => Never,
+            ty::TyStr | ty::TySlice(_) | ty::TyDynamic(..) | ty::TyForeign(..) => None,
 
             ty::TyTuple(tys) => {
                 Where(ty::Binder::bind(tys.last().into_iter().cloned().collect()))
@@ -2164,7 +2155,7 @@ impl<'cx, 'gcx, 'tcx> SelectionContext<'cx, 'gcx, 'tcx> {
         let self_ty = self.infcx.shallow_resolve(
             obligation.predicate.skip_binder().self_ty());
 
-        use self::BuiltinImplConditions::{Ambiguous, None, Never, Where};
+        use self::BuiltinImplConditions::{Ambiguous, None, Where};
 
         match self_ty.sty {
             ty::TyInfer(ty::IntVar(_)) | ty::TyInfer(ty::FloatVar(_)) |
@@ -2182,7 +2173,7 @@ impl<'cx, 'gcx, 'tcx> SelectionContext<'cx, 'gcx, 'tcx> {
             ty::TyDynamic(..) | ty::TyStr | ty::TySlice(..) |
             ty::TyGenerator(..) | ty::TyGeneratorWitness(..) | ty::TyForeign(..) |
             ty::TyRef(_, _, hir::MutMutable) => {
-                Never
+                None
             }
 
             ty::TyArray(element_ty, _) => {
@@ -2202,7 +2193,7 @@ impl<'cx, 'gcx, 'tcx> SelectionContext<'cx, 'gcx, 'tcx> {
                 if is_copy_trait || is_clone_trait {
                     Where(ty::Binder::bind(substs.upvar_tys(def_id, self.tcx()).collect()))
                 } else {
-                    Never
+                    None
                 }
             }
 

src/librustc/traits/structural_impls.rs

@@ -243,6 +243,7 @@ impl<'a, 'tcx> Lift<'tcx> for traits::ObligationCauseCode<'a> {
             super::IntrinsicType => Some(super::IntrinsicType),
             super::MethodReceiver => Some(super::MethodReceiver),
             super::BlockTailExpression(id) => Some(super::BlockTailExpression(id)),
+            super::TrivialBound => Some(super::TrivialBound),
         }
     }
 }

src/librustc/ty/flags.rs

@@ -79,7 +79,7 @@ impl FlagComputation {
             }
 
             &ty::TyParam(ref p) => {
-                self.add_flags(TypeFlags::HAS_LOCAL_NAMES);
+                self.add_flags(TypeFlags::HAS_FREE_LOCAL_NAMES);
                 if p.is_self() {
                     self.add_flags(TypeFlags::HAS_SELF);
                 } else {
@@ -89,7 +89,7 @@ impl FlagComputation {
 
             &ty::TyGenerator(_, ref substs, _) => {
                 self.add_flags(TypeFlags::HAS_TY_CLOSURE);
-                self.add_flags(TypeFlags::HAS_LOCAL_NAMES);
+                self.add_flags(TypeFlags::HAS_FREE_LOCAL_NAMES);
                 self.add_substs(&substs.substs);
             }
 
@@ -101,12 +101,12 @@ impl FlagComputation {
 
             &ty::TyClosure(_, ref substs) => {
                 self.add_flags(TypeFlags::HAS_TY_CLOSURE);
-                self.add_flags(TypeFlags::HAS_LOCAL_NAMES);
+                self.add_flags(TypeFlags::HAS_FREE_LOCAL_NAMES);
                 self.add_substs(&substs.substs);
             }
 
             &ty::TyInfer(infer) => {
-                self.add_flags(TypeFlags::HAS_LOCAL_NAMES); // it might, right?
+                self.add_flags(TypeFlags::HAS_FREE_LOCAL_NAMES); // it might, right?
                 self.add_flags(TypeFlags::HAS_TY_INFER);
                 match infer {
                     ty::FreshTy(_) |

src/librustc/ty/fold.rs

@@ -116,10 +116,14 @@ pub trait TypeFoldable<'tcx>: fmt::Debug + Clone {
 
     /// Indicates whether this value references only 'global'
     /// types/lifetimes that are the same regardless of what fn we are
-    /// in. This is used for caching. Errs on the side of returning
-    /// false.
+    /// in. This is used for caching.
     fn is_global(&self) -> bool {
-        !self.has_type_flags(TypeFlags::HAS_LOCAL_NAMES)
+        !self.has_type_flags(TypeFlags::HAS_FREE_LOCAL_NAMES)
+    }
+
+    /// True if there are any late-bound regions
+    fn has_late_bound_regions(&self) -> bool {
+        self.has_type_flags(TypeFlags::HAS_RE_LATE_BOUND)
     }
 }
 

src/librustc/ty/mod.rs

@@ -441,7 +441,7 @@ bitflags! {
 
         // true if there are "names" of types and regions and so forth
         // that are local to a particular fn
-        const HAS_LOCAL_NAMES    = 1 << 10;
+        const HAS_FREE_LOCAL_NAMES    = 1 << 10;
 
         // Present if the type belongs in a local type context.
         // Only set for TyInfer other than Fresh.
@@ -455,6 +455,10 @@ bitflags! {
         // ought to be true only for the results of canonicalization.
         const HAS_CANONICAL_VARS = 1 << 13;
 
+        /// Does this have any `ReLateBound` regions? Used to check
+        /// if a global bound is safe to evaluate.
+        const HAS_RE_LATE_BOUND = 1 << 14;
+
         const NEEDS_SUBST        = TypeFlags::HAS_PARAMS.bits |
                                    TypeFlags::HAS_SELF.bits |
                                    TypeFlags::HAS_RE_EARLY_BOUND.bits;
@@ -472,9 +476,10 @@ bitflags! {
                                   TypeFlags::HAS_TY_ERR.bits |
                                   TypeFlags::HAS_PROJECTION.bits |
                                   TypeFlags::HAS_TY_CLOSURE.bits |
-                                  TypeFlags::HAS_LOCAL_NAMES.bits |
+                                  TypeFlags::HAS_FREE_LOCAL_NAMES.bits |
                                   TypeFlags::KEEP_IN_LOCAL_TCX.bits |
-                                  TypeFlags::HAS_CANONICAL_VARS.bits;
+                                  TypeFlags::HAS_CANONICAL_VARS.bits |
+                                  TypeFlags::HAS_RE_LATE_BOUND.bits;
     }
 }
 

src/librustc/ty/sty.rs

@@ -1268,7 +1268,9 @@ impl RegionKind {
                 flags = flags | TypeFlags::HAS_FREE_REGIONS;
                 flags = flags | TypeFlags::HAS_RE_SKOL;
             }
-            ty::ReLateBound(..) => { }
+            ty::ReLateBound(..) => {
+                flags = flags | TypeFlags::HAS_RE_LATE_BOUND;
+            }
             ty::ReEarlyBound(..) => {
                 flags = flags | TypeFlags::HAS_FREE_REGIONS;
                 flags = flags | TypeFlags::HAS_RE_EARLY_BOUND;
@@ -1291,8 +1293,8 @@ impl RegionKind {
         }
 
         match *self {
-            ty::ReStatic | ty::ReEmpty | ty::ReErased => (),
-            _ => flags = flags | TypeFlags::HAS_LOCAL_NAMES,
+            ty::ReStatic | ty::ReEmpty | ty::ReErased | ty::ReLateBound(..) => (),
+            _ => flags = flags | TypeFlags::HAS_FREE_LOCAL_NAMES,
         }
 
         debug!("type_flags({:?}) = {:?}", self, flags);

src/librustc_lint/builtin.rs

@@ -1591,3 +1591,61 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for ExternCrate {
         self.0 += 1;
     }
 }
+
+/// Lint for trait and lifetime bounds that don't depend on type parameters
+/// which either do nothing, or stop the item from being used.
+pub struct TrivialConstraints;
+
+declare_lint! {
+    TRIVIAL_BOUNDS,
+    Warn,
+    "these bounds don't depend on an type parameters"
+}
+
+impl LintPass for TrivialConstraints {
+    fn get_lints(&self) -> LintArray {
+        lint_array!(TRIVIAL_BOUNDS)
+    }
+}
+
+impl<'a, 'tcx> LateLintPass<'a, 'tcx> for TrivialConstraints {
+    fn check_item(
+        &mut self,
+        cx: &LateContext<'a, 'tcx>,
+        item: &'tcx hir::Item,
+    ) {
+        use rustc::ty::fold::TypeFoldable;
+        use rustc::ty::Predicate::*;
+
+
+        if cx.tcx.features().trivial_bounds {
+            let def_id = cx.tcx.hir.local_def_id(item.id);
+            let predicates = cx.tcx.predicates_of(def_id);
+            for predicate in &predicates.predicates {
+                let predicate_kind_name = match *predicate {
+                    Trait(..) => "Trait",
+                    TypeOutlives(..) |
+                    RegionOutlives(..) => "Lifetime",
+
+                    // Ignore projections, as they can only be global
+                    // if the trait bound is global
+                    Projection(..) |
+                    // Ignore bounds that a user can't type
+                    WellFormed(..) |
+                    ObjectSafe(..) |
+                    ClosureKind(..) |
+                    Subtype(..) |
+                    ConstEvaluatable(..) => continue,
+                };
+                if predicate.is_global() {
+                    cx.span_lint(
+                        TRIVIAL_BOUNDS,
+                        item.span,
+                        &format!("{} bound {} does not depend on any type \
+                                or lifetime parameters", predicate_kind_name, predicate),
+                    );
+                }
+            }
+        }
+    }
+}

src/librustc_lint/lib.rs

@@ -137,6 +137,7 @@ pub fn register_builtins(store: &mut lint::LintStore, sess: Option<&Session>) {
                  UnreachablePub,
                  TypeAliasBounds,
                  UnusedBrokenConst,
+                 TrivialConstraints,
                  );
 
     add_builtin_with_new!(sess,