Fix span for non-satisfied trivial trait bounds

The spans for "trait bound not satisfied" errors in trivial trait bounds referenced the entire item (fn, impl, struct) before.
Now they only reference the obligation itself (`String: Copy`)

Address #90869

compiler/rustc_infer/src/traits/util.rs

@@ -5,6 +5,7 @@ use crate::traits::{Obligation, ObligationCause, PredicateObligation};
 use rustc_data_structures::fx::{FxHashSet, FxIndexSet};
 use rustc_middle::ty::{self, ToPredicate, TyCtxt, WithConstness};
 use rustc_span::symbol::Ident;
+use rustc_span::Span;
 
 pub fn anonymize_predicate<'tcx>(
     tcx: TyCtxt<'tcx>,
@@ -97,6 +98,22 @@ pub fn elaborate_predicates<'tcx>(
     elaborate_obligations(tcx, obligations)
 }
 
+pub fn elaborate_predicates_with_span<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    predicates: impl Iterator<Item = (ty::Predicate<'tcx>, Span)>,
+) -> Elaborator<'tcx> {
+    let obligations = predicates
+        .map(|(predicate, span)| {
+            predicate_obligation(
+                predicate,
+                ty::ParamEnv::empty(),
+                ObligationCause::dummy_with_span(span),
+            )
+        })
+        .collect();
+    elaborate_obligations(tcx, obligations)
+}
+
 pub fn elaborate_obligations<'tcx>(
     tcx: TyCtxt<'tcx>,
     mut obligations: Vec<PredicateObligation<'tcx>>,

compiler/rustc_trait_selection/src/traits/mod.rs

@@ -65,7 +65,8 @@ pub use self::specialize::{specialization_graph, translate_substs, OverlapError}
 pub use self::structural_match::search_for_structural_match_violation;
 pub use self::structural_match::NonStructuralMatchTy;
 pub use self::util::{
-    elaborate_obligations, elaborate_predicates, elaborate_trait_ref, elaborate_trait_refs,
+    elaborate_obligations, elaborate_predicates, elaborate_predicates_with_span,
+    elaborate_trait_ref, elaborate_trait_refs,
 };
 pub use self::util::{expand_trait_aliases, TraitAliasExpander};
 pub use self::util::{

compiler/rustc_typeck/src/check/wfcheck.rs

@@ -1641,19 +1641,38 @@ fn report_bivariance(tcx: TyCtxt<'_>, param: &rustc_hir::GenericParam<'_>) {
 
 /// Feature gates RFC 2056 -- trivial bounds, checking for global bounds that
 /// aren't true.
-fn check_false_global_bounds(fcx: &FnCtxt<'_, '_>, span: Span, id: hir::HirId) {
+fn check_false_global_bounds(fcx: &FnCtxt<'_, '_>, mut span: Span, id: hir::HirId) {
     let empty_env = ty::ParamEnv::empty();
 
     let def_id = fcx.tcx.hir().local_def_id(id);
-    let predicates = fcx.tcx.predicates_of(def_id).predicates.iter().map(|(p, _)| *p);
+    let predicates_with_span =
+        fcx.tcx.predicates_of(def_id).predicates.iter().map(|(p, span)| (*p, *span));
     // Check elaborated bounds.
-    let implied_obligations = traits::elaborate_predicates(fcx.tcx, predicates);
+    let implied_obligations = traits::elaborate_predicates_with_span(fcx.tcx, predicates_with_span);
 
     for obligation in implied_obligations {
         let pred = obligation.predicate;
         // Match the existing behavior.
         if pred.is_global(fcx.tcx) && !pred.has_late_bound_regions() {
             let pred = fcx.normalize_associated_types_in(span, pred);
+            let hir_node = fcx.tcx.hir().find(id);
+
+            // only use the span of the predicate clause (#90869)
+
+            if let Some(hir::Generics { where_clause, .. }) =
+                hir_node.and_then(|node| node.generics())
+            {
+                let obligation_span = obligation.cause.span(fcx.tcx);
+
+                span = where_clause
+                    .predicates
+                    .iter()
+                    // There seems to be no better way to find out which predicate we are in
+                    .find(|pred| pred.span().contains(obligation_span))
+                    .map(|pred| pred.span())
+                    .unwrap_or(obligation_span);
+            }
+
             let obligation = traits::Obligation::new(
                 traits::ObligationCause::new(span, id, traits::TrivialBound),
                 empty_env,

src/test/ui/const-generics/issues/issue-67185-2.rs

@@ -9,11 +9,10 @@ trait Bar {}
 impl Bar for [u16; 4] {}
 impl Bar for [[u16; 3]; 3] {}
 
-trait Foo  //~ ERROR the trait bound `[u16; 3]: Bar` is not satisfied [E0277]
-           //~^ ERROR the trait bound `[[u16; 3]; 2]: Bar` is not satisfied [E0277]
-    where
-        [<u8 as Baz>::Quaks; 2]: Bar,
-        <u8 as Baz>::Quaks: Bar,
+trait Foo
+where
+    [<u8 as Baz>::Quaks; 2]: Bar, //~ ERROR the trait bound `[[u16; 3]; 2]: Bar` is not satisfied [E0277]
+    <u8 as Baz>::Quaks: Bar,  //~ ERROR the trait bound `[u16; 3]: Bar` is not satisfied [E0277]
 {
 }
 

src/test/ui/const-generics/issues/issue-67185-2.stderr

@@ -1,14 +1,8 @@
 error[E0277]: the trait bound `[u16; 3]: Bar` is not satisfied
-  --> $DIR/issue-67185-2.rs:12:1
+  --> $DIR/issue-67185-2.rs:15:5
    |
-LL | / trait Foo
-LL | |
-LL | |     where
-LL | |         [<u8 as Baz>::Quaks; 2]: Bar,
-LL | |         <u8 as Baz>::Quaks: Bar,
-LL | | {
-LL | | }
-   | |_^ the trait `Bar` is not implemented for `[u16; 3]`
+LL |     <u8 as Baz>::Quaks: Bar,
+   |     ^^^^^^^^^^^^^^^^^^^^^^^ the trait `Bar` is not implemented for `[u16; 3]`
    |
    = help: the following implementations were found:
              <[[u16; 3]; 3] as Bar>
@@ -17,16 +11,10 @@ LL | | }
    = help: add `#![feature(trivial_bounds)]` to the crate attributes to enable
 
 error[E0277]: the trait bound `[[u16; 3]; 2]: Bar` is not satisfied
-  --> $DIR/issue-67185-2.rs:12:1
+  --> $DIR/issue-67185-2.rs:14:5
    |
-LL | / trait Foo
-LL | |
-LL | |     where
-LL | |         [<u8 as Baz>::Quaks; 2]: Bar,
-LL | |         <u8 as Baz>::Quaks: Bar,
-LL | | {
-LL | | }
-   | |_^ the trait `Bar` is not implemented for `[[u16; 3]; 2]`
+LL |     [<u8 as Baz>::Quaks; 2]: Bar,
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Bar` is not implemented for `[[u16; 3]; 2]`
    |
    = help: the following implementations were found:
              <[[u16; 3]; 3] as Bar>
@@ -35,7 +23,7 @@ LL | | }
    = help: add `#![feature(trivial_bounds)]` to the crate attributes to enable
 
 error[E0277]: the trait bound `[u16; 3]: Bar` is not satisfied
-  --> $DIR/issue-67185-2.rs:22:6
+  --> $DIR/issue-67185-2.rs:21:6
    |
 LL | impl Foo for FooImpl {}
    |      ^^^ the trait `Bar` is not implemented for `[u16; 3]`
@@ -44,16 +32,16 @@ LL | impl Foo for FooImpl {}
              <[[u16; 3]; 3] as Bar>
              <[u16; 4] as Bar>
 note: required by a bound in `Foo`
-  --> $DIR/issue-67185-2.rs:16:29
+  --> $DIR/issue-67185-2.rs:15:25
    |
 LL | trait Foo
    |       --- required by a bound in this
 ...
-LL |         <u8 as Baz>::Quaks: Bar,
-   |                             ^^^ required by this bound in `Foo`
+LL |     <u8 as Baz>::Quaks: Bar,
+   |                         ^^^ required by this bound in `Foo`
 
 error[E0277]: the trait bound `[[u16; 3]; 2]: Bar` is not satisfied
-  --> $DIR/issue-67185-2.rs:22:6
+  --> $DIR/issue-67185-2.rs:21:6
    |
 LL | impl Foo for FooImpl {}
    |      ^^^ the trait `Bar` is not implemented for `[[u16; 3]; 2]`
@@ -62,16 +50,16 @@ LL | impl Foo for FooImpl {}
              <[[u16; 3]; 3] as Bar>
              <[u16; 4] as Bar>
 note: required by a bound in `Foo`
-  --> $DIR/issue-67185-2.rs:15:34
+  --> $DIR/issue-67185-2.rs:14:30
    |
 LL | trait Foo
    |       --- required by a bound in this
-...
-LL |         [<u8 as Baz>::Quaks; 2]: Bar,
-   |                                  ^^^ required by this bound in `Foo`
+LL | where
+LL |     [<u8 as Baz>::Quaks; 2]: Bar,
+   |                              ^^^ required by this bound in `Foo`
 
 error[E0277]: the trait bound `[[u16; 3]; 2]: Bar` is not satisfied
-  --> $DIR/issue-67185-2.rs:26:14
+  --> $DIR/issue-67185-2.rs:25:14
    |
 LL | fn f(_: impl Foo) {}
    |              ^^^ the trait `Bar` is not implemented for `[[u16; 3]; 2]`
@@ -80,16 +68,16 @@ LL | fn f(_: impl Foo) {}
              <[[u16; 3]; 3] as Bar>
              <[u16; 4] as Bar>
 note: required by a bound in `Foo`
-  --> $DIR/issue-67185-2.rs:15:34
+  --> $DIR/issue-67185-2.rs:14:30
    |
 LL | trait Foo
    |       --- required by a bound in this
-...
-LL |         [<u8 as Baz>::Quaks; 2]: Bar,
-   |                                  ^^^ required by this bound in `Foo`
+LL | where
+LL |     [<u8 as Baz>::Quaks; 2]: Bar,
+   |                              ^^^ required by this bound in `Foo`
 
 error[E0277]: the trait bound `[u16; 3]: Bar` is not satisfied
-  --> $DIR/issue-67185-2.rs:26:14
+  --> $DIR/issue-67185-2.rs:25:14
    |
 LL | fn f(_: impl Foo) {}
    |              ^^^ the trait `Bar` is not implemented for `[u16; 3]`
@@ -98,13 +86,13 @@ LL | fn f(_: impl Foo) {}
              <[[u16; 3]; 3] as Bar>
              <[u16; 4] as Bar>
 note: required by a bound in `Foo`
-  --> $DIR/issue-67185-2.rs:16:29
+  --> $DIR/issue-67185-2.rs:15:25
    |
 LL | trait Foo
    |       --- required by a bound in this
 ...
-LL |         <u8 as Baz>::Quaks: Bar,
-   |                             ^^^ required by this bound in `Foo`
+LL |     <u8 as Baz>::Quaks: Bar,
+   |                         ^^^ required by this bound in `Foo`
 
 error: aborting due to 6 previous errors
 

src/test/ui/cross/cross-fn-cache-hole.rs

@@ -12,8 +12,8 @@ trait Bar<X> { }
 
 // We don't always check where clauses for sanity, but in this case
 // wfcheck does report an error here:
-fn vacuous<A>() //~ ERROR the trait bound `i32: Bar<u32>` is not satisfied
-    where i32: Foo<u32, A>
+fn vacuous<A>()
+    where i32: Foo<u32, A> //~ ERROR the trait bound `i32: Bar<u32>` is not satisfied
 {
     // ... the original intention was to check that we don't use that
     // vacuous where clause (which could never be satisfied) to accept

src/test/ui/cross/cross-fn-cache-hole.stderr

@@ -1,14 +1,8 @@
 error[E0277]: the trait bound `i32: Bar<u32>` is not satisfied
-  --> $DIR/cross-fn-cache-hole.rs:15:1
+  --> $DIR/cross-fn-cache-hole.rs:16:11
    |
-LL | / fn vacuous<A>()
-LL | |     where i32: Foo<u32, A>
-LL | | {
-LL | |     // ... the original intention was to check that we don't use that
-...  |
-LL | |     require::<i32, u32>();
-LL | | }
-   | |_^ the trait `Bar<u32>` is not implemented for `i32`
+LL |     where i32: Foo<u32, A>
+   |           ^^^^^^^^^^^^^^^^ the trait `Bar<u32>` is not implemented for `i32`
    |
    = help: see issue #48214
    = help: add `#![feature(trivial_bounds)]` to the crate attributes to enable