Auto merge of #69793 - estebank:predicate-obligations-4, r=nikomatsakis

 Maintain chain of derived obligations

When evaluating the derived obligations from super traits, maintain a
reference to the original obligation in order to give more actionable
context in the output.

Continuation (and built on) #69745, subset of #69709.

r? @eddyb

src/librustc_middle/traits/mod.rs

@@ -191,6 +191,8 @@ pub enum ObligationCauseCode<'tcx> {
 
     ImplDerivedObligation(DerivedObligationCause<'tcx>),
 
+    DerivedObligation(DerivedObligationCause<'tcx>),
+
     /// Error derived when matching traits/impls; see ObligationCause for more details
     CompareImplMethodObligation {
         item_name: ast::Name,
@@ -257,28 +259,22 @@ pub enum ObligationCauseCode<'tcx> {
 
     /// #[feature(trivial_bounds)] is not enabled
     TrivialBound,
-
-    AssocTypeBound(Box<AssocTypeBoundData>),
 }
 
 impl ObligationCauseCode<'_> {
     // Return the base obligation, ignoring derived obligations.
     pub fn peel_derives(&self) -> &Self {
         let mut base_cause = self;
-        while let BuiltinDerivedObligation(cause) | ImplDerivedObligation(cause) = base_cause {
+        while let BuiltinDerivedObligation(cause)
+        | ImplDerivedObligation(cause)
+        | DerivedObligation(cause) = base_cause
+        {
             base_cause = &cause.parent_code;
         }
         base_cause
     }
 }
 
-#[derive(Clone, Debug, PartialEq, Eq, Hash)]
-pub struct AssocTypeBoundData {
-    pub impl_span: Option<Span>,
-    pub original: Span,
-    pub bounds: Vec<Span>,
-}
-
 // `ObligationCauseCode` is used a lot. Make sure it doesn't unintentionally get bigger.
 #[cfg(target_arch = "x86_64")]
 static_assert_size!(ObligationCauseCode<'_>, 32);

src/librustc_middle/traits/structural_impls.rs

@@ -456,6 +456,7 @@ impl<'a, 'tcx> Lift<'tcx> for traits::ObligationCauseCode<'a> {
             super::ImplDerivedObligation(ref cause) => {
                 tcx.lift(cause).map(super::ImplDerivedObligation)
             }
+            super::DerivedObligation(ref cause) => tcx.lift(cause).map(super::DerivedObligation),
             super::CompareImplMethodObligation {
                 item_name,
                 impl_item_def_id,
@@ -501,7 +502,6 @@ impl<'a, 'tcx> Lift<'tcx> for traits::ObligationCauseCode<'a> {
             super::MethodReceiver => Some(super::MethodReceiver),
             super::BlockTailExpression(id) => Some(super::BlockTailExpression(id)),
             super::TrivialBound => Some(super::TrivialBound),
-            super::AssocTypeBound(ref data) => Some(super::AssocTypeBound(data.clone())),
         }
     }
 }

src/librustc_trait_selection/traits/error_reporting/on_unimplemented.rs

@@ -134,7 +134,8 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
 
         match obligation.cause.code {
             ObligationCauseCode::BuiltinDerivedObligation(..)
-            | ObligationCauseCode::ImplDerivedObligation(..) => {}
+            | ObligationCauseCode::ImplDerivedObligation(..)
+            | ObligationCauseCode::DerivedObligation(..) => {}
             _ => {
                 // this is a "direct", user-specified, rather than derived,
                 // obligation.

src/librustc_trait_selection/traits/error_reporting/suggestions.rs

@@ -1135,7 +1135,8 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
         while let Some(code) = next_code {
             debug!("maybe_note_obligation_cause_for_async_await: code={:?}", code);
             match code {
-                ObligationCauseCode::BuiltinDerivedObligation(derived_obligation)
+                ObligationCauseCode::DerivedObligation(derived_obligation)
+                | ObligationCauseCode::BuiltinDerivedObligation(derived_obligation)
                 | ObligationCauseCode::ImplDerivedObligation(derived_obligation) => {
                     let ty = derived_obligation.parent_trait_ref.self_ty();
                     debug!(
@@ -1531,14 +1532,14 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                 let item_name = tcx.def_path_str(item_def_id);
                 let msg = format!("required by this bound in `{}`", item_name);
                 if let Some(ident) = tcx.opt_item_name(item_def_id) {
-                    let sm = self.tcx.sess.source_map();
+                    let sm = tcx.sess.source_map();
                     let same_line =
                         match (sm.lookup_line(ident.span.hi()), sm.lookup_line(span.lo())) {
                             (Ok(l), Ok(r)) => l.line == r.line,
                             _ => true,
                         };
                     if !ident.span.overlaps(span) && !same_line {
-                        err.span_label(ident.span, "");
+                        err.span_label(ident.span, "required by a bound in this");
                     }
                 }
                 if span != DUMMY_SP {
@@ -1661,6 +1662,16 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                     obligated_types,
                 );
             }
+            ObligationCauseCode::DerivedObligation(ref data) => {
+                let parent_trait_ref = self.resolve_vars_if_possible(&data.parent_trait_ref);
+                let parent_predicate = parent_trait_ref.without_const().to_predicate();
+                self.note_obligation_cause_code(
+                    err,
+                    &parent_predicate,
+                    &data.parent_code,
+                    obligated_types,
+                );
+            }
             ObligationCauseCode::CompareImplMethodObligation { .. } => {
                 err.note(&format!(
                     "the requirement `{}` appears on the impl method \
@@ -1684,15 +1695,6 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                     err.help("add `#![feature(trivial_bounds)]` to the crate attributes to enable");
                 }
             }
-            ObligationCauseCode::AssocTypeBound(ref data) => {
-                err.span_label(data.original, "associated type defined here");
-                if let Some(sp) = data.impl_span {
-                    err.span_label(sp, "in this `impl` item");
-                }
-                for sp in &data.bounds {
-                    err.span_label(*sp, "restricted in this bound");
-                }
-            }
         }
     }