Always point at index span on index obligation failure

compiler/rustc_hir_typeck/src/expr.rs

@@ -21,7 +21,7 @@ use crate::{
     TupleArgumentsFlag::DontTupleArguments,
 };
 use rustc_ast as ast;
-use rustc_data_structures::fx::FxHashMap;
+use rustc_data_structures::fx::{FxHashMap, FxHashSet};
 use rustc_data_structures::stack::ensure_sufficient_stack;
 use rustc_errors::{
     pluralize, struct_span_err, AddToDiagnostic, Applicability, Diagnostic, DiagnosticBuilder,
@@ -2877,7 +2877,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     // two-phase not needed because index_ty is never mutable
                     self.demand_coerce(idx, idx_t, index_ty, None, AllowTwoPhase::No);
                     self.select_obligations_where_possible(|errors| {
-                        self.point_at_index_if_possible(errors, idx.span)
+                        self.point_at_index(errors, idx.span);
                     });
                     element_ty
                 }
@@ -3036,16 +3036,28 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         .ok()
     }
 
-    fn point_at_index_if_possible(
-        &self,
-        errors: &mut Vec<traits::FulfillmentError<'tcx>>,
-        span: Span,
-    ) {
+    fn point_at_index(&self, errors: &mut Vec<traits::FulfillmentError<'tcx>>, span: Span) {
+        let mut seen_preds = FxHashSet::default();
+        // We re-sort here so that the outer most root obligations comes first, as we have the
+        // subsequent weird logic to identify *every* relevant obligation for proper deduplication
+        // of diagnostics.
+        errors.sort_by_key(|error| error.root_obligation.recursion_depth);
         for error in errors {
-            match error.obligation.predicate.kind().skip_binder() {
-                ty::PredicateKind::Clause(ty::ClauseKind::Trait(predicate))
-                    if self.tcx.is_diagnostic_item(sym::SliceIndex, predicate.trait_ref.def_id) => {
+            match (
+                error.root_obligation.predicate.kind().skip_binder(),
+                error.obligation.predicate.kind().skip_binder(),
+            ) {
+                (ty::PredicateKind::Clause(ty::ClauseKind::Trait(predicate)), _)
+                    if self.tcx.lang_items().index_trait() == Some(predicate.trait_ref.def_id) =>
+                {
+                    seen_preds.insert(error.obligation.predicate.kind().skip_binder());
+                }
+                (_, ty::PredicateKind::Clause(ty::ClauseKind::Trait(predicate)))
+                    if self.tcx.is_diagnostic_item(sym::SliceIndex, predicate.trait_ref.def_id) =>
+                {
+                    seen_preds.insert(error.obligation.predicate.kind().skip_binder());
                 }
+                (root, pred) if seen_preds.contains(&pred) || seen_preds.contains(&root) => {}
                 _ => continue,
             }
             error.obligation.cause.span = span;

tests/ui/indexing/indexing-requires-a-uint.stderr

@@ -8,6 +8,8 @@ LL |     [0][0u8];
    = help: the trait `SliceIndex<[{integer}]>` is implemented for `usize`
    = help: for that trait implementation, expected `usize`, found `u8`
    = note: required for `[{integer}]` to implement `Index<u8>`
+   = note: 1 redundant requirement hidden
+   = note: required for `[{integer}; 1]` to implement `Index<u8>`
 
 error[E0308]: mismatched types
   --> $DIR/indexing-requires-a-uint.rs:12:18

tests/ui/indexing/point-at-index-for-obligation-failure.rs

@@ -0,0 +1,7 @@
+fn main() {
+    let a = std::collections::HashMap::<String,String>::new();
+    let s = "hello";
+    let _b = a[
+        &s //~ ERROR E0277
+    ];
+}

tests/ui/indexing/point-at-index-for-obligation-failure.stderr

@@ -0,0 +1,13 @@
+error[E0277]: the trait bound `String: Borrow<&str>` is not satisfied
+  --> $DIR/point-at-index-for-obligation-failure.rs:5:9
+   |
+LL |         &s
+   |         ^^ the trait `Borrow<&str>` is not implemented for `String`
+   |
+   = help: the trait `Borrow<str>` is implemented for `String`
+   = help: for that trait implementation, expected `str`, found `&str`
+   = note: required for `HashMap<String, String>` to implement `Index<&&str>`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.

tests/ui/suggestions/suggest-dereferencing-index.stderr

@@ -8,6 +8,8 @@ LL |     let one_item_please: i32 = [1, 2, 3][i];
    = help: the trait `SliceIndex<[{integer}]>` is implemented for `usize`
    = help: for that trait implementation, expected `usize`, found `&usize`
    = note: required for `[{integer}]` to implement `Index<&usize>`
+   = note: 1 redundant requirement hidden
+   = note: required for `[{integer}; 3]` to implement `Index<&usize>`
 help: dereference this index
    |
 LL |     let one_item_please: i32 = [1, 2, 3][*i];