Auto merge of #69255 - estebank:e0599-details, r=varkor

Add more context to E0599 errors

Point at the intermediary unfulfilled trait bounds.

Fix #52523, fix #61661, cc #36513, fix #68131, fix #64417, fix #61768, cc #57457, cc #9082, fix #57994, cc #64934, cc #65149.

Author: bors

src/librustc/traits/mod.rs

@@ -581,6 +581,20 @@ impl<'tcx, N> Vtable<'tcx, N> {
         }
     }
 
+    pub fn borrow_nested_obligations(&self) -> &[N] {
+        match &self {
+            VtableImpl(i) => &i.nested[..],
+            VtableParam(n) => &n[..],
+            VtableBuiltin(i) => &i.nested[..],
+            VtableAutoImpl(d) => &d.nested[..],
+            VtableClosure(c) => &c.nested[..],
+            VtableGenerator(c) => &c.nested[..],
+            VtableObject(d) => &d.nested[..],
+            VtableFnPointer(d) => &d.nested[..],
+            VtableTraitAlias(d) => &d.nested[..],
+        }
+    }
+
     pub fn map<M, F>(self, f: F) -> Vtable<'tcx, M>
     where
         F: FnMut(N) -> M,

src/librustc_infer/infer/error_reporting/note.rs

@@ -645,8 +645,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                     self.tcx.sess,
                     span,
                     E0491,
-                    "in type `{}`, reference has a longer lifetime \
-                                                than the data it references",
+                    "in type `{}`, reference has a longer lifetime than the data it references",
                     self.ty_to_string(ty)
                 );
                 note_and_explain_region(

src/librustc_parse/parser/generics.rs

@@ -1,7 +1,6 @@
 use super::Parser;
 
 use rustc_errors::PResult;
-use rustc_span::source_map::DUMMY_SP;
 use rustc_span::symbol::{kw, sym};
 use syntax::ast::{self, Attribute, GenericBounds, GenericParam, GenericParamKind, WhereClause};
 use syntax::token;
@@ -157,7 +156,10 @@ impl<'a> Parser<'a> {
         };
         Ok(ast::Generics {
             params,
-            where_clause: WhereClause { predicates: Vec::new(), span: DUMMY_SP },
+            where_clause: WhereClause {
+                predicates: Vec::new(),
+                span: self.prev_span.shrink_to_hi(),
+            },
             span,
         })
     }

src/librustc_typeck/check/method/mod.rs

@@ -14,7 +14,7 @@ use crate::check::FnCtxt;
 use rustc::ty::subst::Subst;
 use rustc::ty::subst::{InternalSubsts, SubstsRef};
 use rustc::ty::GenericParamDefKind;
-use rustc::ty::{self, ToPolyTraitRef, ToPredicate, TraitRef, Ty, TypeFoldable, WithConstness};
+use rustc::ty::{self, ToPolyTraitRef, ToPredicate, Ty, TypeFoldable, WithConstness};
 use rustc_data_structures::sync::Lrc;
 use rustc_errors::{Applicability, DiagnosticBuilder};
 use rustc_hir as hir;
@@ -67,7 +67,7 @@ pub enum MethodError<'tcx> {
 // could lead to matches if satisfied, and a list of not-in-scope traits which may work.
 pub struct NoMatchData<'tcx> {
     pub static_candidates: Vec<CandidateSource>,
-    pub unsatisfied_predicates: Vec<TraitRef<'tcx>>,
+    pub unsatisfied_predicates: Vec<(ty::Predicate<'tcx>, Option<ty::Predicate<'tcx>>)>,
     pub out_of_scope_traits: Vec<DefId>,
     pub lev_candidate: Option<ty::AssocItem>,
     pub mode: probe::Mode,
@@ -76,7 +76,7 @@ pub struct NoMatchData<'tcx> {
 impl<'tcx> NoMatchData<'tcx> {
     pub fn new(
         static_candidates: Vec<CandidateSource>,
-        unsatisfied_predicates: Vec<TraitRef<'tcx>>,
+        unsatisfied_predicates: Vec<(ty::Predicate<'tcx>, Option<ty::Predicate<'tcx>>)>,
         out_of_scope_traits: Vec<DefId>,
         lev_candidate: Option<ty::AssocItem>,
         mode: probe::Mode,

src/librustc_typeck/check/method/probe.rs

@@ -14,8 +14,7 @@ use rustc::session::config::nightly_options;
 use rustc::ty::subst::{InternalSubsts, Subst, SubstsRef};
 use rustc::ty::GenericParamDefKind;
 use rustc::ty::{
-    self, ParamEnvAnd, ToPolyTraitRef, ToPredicate, TraitRef, Ty, TyCtxt, TypeFoldable,
-    WithConstness,
+    self, ParamEnvAnd, ToPolyTraitRef, ToPredicate, Ty, TyCtxt, TypeFoldable, WithConstness,
 };
 use rustc_data_structures::fx::FxHashSet;
 use rustc_data_structures::sync::Lrc;
@@ -78,7 +77,7 @@ struct ProbeContext<'a, 'tcx> {
 
     /// Collects near misses when trait bounds for type parameters are unsatisfied and is only used
     /// for error reporting
-    unsatisfied_predicates: Vec<TraitRef<'tcx>>,
+    unsatisfied_predicates: Vec<(ty::Predicate<'tcx>, Option<ty::Predicate<'tcx>>)>,
 
     is_suggestion: IsSuggestion,
 }
@@ -1224,7 +1223,10 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
         &self,
         self_ty: Ty<'tcx>,
         probes: ProbesIter,
-        possibly_unsatisfied_predicates: &mut Vec<TraitRef<'tcx>>,
+        possibly_unsatisfied_predicates: &mut Vec<(
+            ty::Predicate<'tcx>,
+            Option<ty::Predicate<'tcx>>,
+        )>,
         unstable_candidates: Option<&mut Vec<(&'b Candidate<'tcx>, Symbol)>>,
     ) -> Option<PickResult<'tcx>>
     where
@@ -1343,7 +1345,10 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
         &self,
         self_ty: Ty<'tcx>,
         probe: &Candidate<'tcx>,
-        possibly_unsatisfied_predicates: &mut Vec<TraitRef<'tcx>>,
+        possibly_unsatisfied_predicates: &mut Vec<(
+            ty::Predicate<'tcx>,
+            Option<ty::Predicate<'tcx>>,
+        )>,
     ) -> ProbeResult {
         debug!("consider_probe: self_ty={:?} probe={:?}", self_ty, probe);
 
@@ -1398,21 +1403,45 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
                     let predicate = trait_ref.without_const().to_predicate();
                     let obligation = traits::Obligation::new(cause, self.param_env, predicate);
                     if !self.predicate_may_hold(&obligation) {
-                        if self.probe(|_| self.select_trait_candidate(trait_ref).is_err()) {
+                        if self.probe(|_| {
+                            match self.select_trait_candidate(trait_ref) {
+                                Err(_) => return true,
+                                Ok(Some(vtable))
+                                    if !vtable.borrow_nested_obligations().is_empty() =>
+                                {
+                                    for obligation in vtable.borrow_nested_obligations() {
+                                        // Determine exactly which obligation wasn't met, so
+                                        // that we can give more context in the error.
+                                        if !self.predicate_may_hold(&obligation) {
+                                            result = ProbeResult::NoMatch;
+                                            let o = self.resolve_vars_if_possible(obligation);
+                                            let predicate =
+                                                self.resolve_vars_if_possible(&predicate);
+                                            let p = if predicate == o.predicate {
+                                                // Avoid "`MyStruct: Foo` which is required by
+                                                // `MyStruct: Foo`" in E0599.
+                                                None
+                                            } else {
+                                                Some(predicate)
+                                            };
+                                            possibly_unsatisfied_predicates.push((o.predicate, p));
+                                        }
+                                    }
+                                }
+                                _ => {
+                                    // Some nested subobligation of this predicate
+                                    // failed.
+                                    result = ProbeResult::NoMatch;
+                                    let predicate = self.resolve_vars_if_possible(&predicate);
+                                    possibly_unsatisfied_predicates.push((predicate, None));
+                                }
+                            }
+                            false
+                        }) {
                             // This candidate's primary obligation doesn't even
                             // select - don't bother registering anything in
                             // `potentially_unsatisfied_predicates`.
                             return ProbeResult::NoMatch;
-                        } else {
-                            // Some nested subobligation of this predicate
-                            // failed.
-                            //
-                            // FIXME: try to find the exact nested subobligation
-                            // and point at it rather than reporting the entire
-                            // trait-ref?
-                            result = ProbeResult::NoMatch;
-                            let trait_ref = self.resolve_vars_if_possible(&trait_ref);
-                            possibly_unsatisfied_predicates.push(trait_ref);
                         }
                     }
                     vec![]
@@ -1429,9 +1458,7 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
                 let o = self.resolve_vars_if_possible(&o);
                 if !self.predicate_may_hold(&o) {
                     result = ProbeResult::NoMatch;
-                    if let &ty::Predicate::Trait(ref pred, _) = &o.predicate {
-                        possibly_unsatisfied_predicates.push(pred.skip_binder().trait_ref);
-                    }
+                    possibly_unsatisfied_predicates.push((o.predicate, None));
                 }
             }