Use nicer spans for deref_into_dyn_supertrait

Author: WaffleLapkin

compiler/rustc_lint/src/deref_into_dyn_supertrait.rs

@@ -3,6 +3,7 @@ use crate::{LateContext, LateLintPass, LintContext};
 use rustc_errors::DelayDm;
 use rustc_hir as hir;
 use rustc_middle::{traits::util::supertraits, ty};
+use rustc_span::sym;
 
 declare_lint! {
     /// The `deref_into_dyn_supertrait` lint is output whenever there is a use of the
@@ -72,13 +73,19 @@ impl<'tcx> LateLintPass<'tcx> for DerefIntoDynSupertrait {
         {
             cx.struct_span_lint(
                 DEREF_INTO_DYN_SUPERTRAIT,
-                item.span,
+                cx.tcx.def_span(item.owner_id.def_id),
                 DelayDm(|| {
                     format!(
-                        "`{t}` implements `Deref` with supertrait `{target_principal}` as output"
+                        "`{t}` implements `Deref` with supertrait `{target_principal}` as target"
                     )
                 }),
-                |lint| lint,
+                |lint| {
+                    if let Some(target_span) = impl_.items.iter().find_map(|i| (i.ident.name == sym::Target).then_some(i.span)) {
+                        lint.span_label(target_span, "target type is set here");
+                    }
+
+                    lint
+                },
             )
         }
     }

src/test/ui/traits/trait-upcasting/migrate-lint-deny.rs

@@ -9,7 +9,7 @@ trait A {}
 trait B: A {}
 
 impl<'a> Deref for dyn 'a + B {
-    //~^ ERROR `(dyn B + 'a)` implements `Deref` with supertrait `A` as output
+    //~^ ERROR `(dyn B + 'a)` implements `Deref` with supertrait `A` as target
     //~| WARN this was previously accepted by the compiler but is being phased out;
 
     type Target = dyn A;

src/test/ui/traits/trait-upcasting/migrate-lint-deny.stderr

@@ -1,14 +1,11 @@
-error: `(dyn B + 'a)` implements `Deref` with supertrait `A` as output
+error: `(dyn B + 'a)` implements `Deref` with supertrait `A` as target
   --> $DIR/migrate-lint-deny.rs:11:1
    |
-LL | / impl<'a> Deref for dyn 'a + B {
-LL | |
-LL | |
-LL | |
-...  |
-LL | |     }
-LL | | }
-   | |_^
+LL | impl<'a> Deref for dyn 'a + B {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+...
+LL |     type Target = dyn A;
+   |     -------------------- target type is set here
    |
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
    = note: for more information, see issue #89460 <https://github.com/rust-lang/rust/issues/89460>