Clarify implicit captures for RPITIT

compiler/rustc_hir_analysis/messages.ftl

@@ -259,6 +259,9 @@ hir_analysis_late_bound_lifetime_in_apit = `impl Trait` can only mention lifetim
 hir_analysis_late_bound_type_in_apit = `impl Trait` can only mention type parameters from an fn or impl
     .label = type parameter declared here
 
+hir_analysis_lifetime_implicitly_captured = `impl Trait` captures lifetime parameter, but it is not mentioned in `use<...>` precise captures list
+    .param_label = all lifetime parameters originating from a trait are captured implicitly
+
 hir_analysis_lifetime_must_be_first = lifetime parameter `{$name}` must be listed before non-lifetime parameters
     .label = move the lifetime before this parameter
 

compiler/rustc_hir_analysis/src/check/check.rs

@@ -588,6 +588,12 @@ fn check_opaque_precise_captures<'tcx>(tcx: TyCtxt<'tcx>, opaque_def_id: LocalDe
                                 use_span,
                                 param_span: tcx.def_span(def_id),
                             });
+                        } else {
+                            if tcx.def_kind(tcx.parent(param.def_id)) == DefKind::Trait {
+                                tcx.dcx().emit_err(errors::LifetimeImplicitlyCaptured {
+                                    opaque_span,
+                                    param_span: tcx.def_span(param.def_id),
+                                });
                             } else {
                                 // If the `use_span` is actually just the param itself, then we must
                                 // have not duplicated the lifetime but captured the original.
@@ -599,6 +605,7 @@ fn check_opaque_precise_captures<'tcx>(tcx: TyCtxt<'tcx>, opaque_def_id: LocalDe
                                     param_span: use_span,
                                 });
                             }
+                        }
                         continue;
                     }
                 }

compiler/rustc_hir_analysis/src/errors/precise_captures.rs

@@ -34,6 +34,15 @@ pub(crate) struct LifetimeNotCaptured {
     pub opaque_span: Span,
 }
 
+#[derive(Diagnostic)]
+#[diag(hir_analysis_lifetime_implicitly_captured)]
+pub(crate) struct LifetimeImplicitlyCaptured {
+    #[primary_span]
+    pub opaque_span: Span,
+    #[label(hir_analysis_param_label)]
+    pub param_span: Span,
+}
+
 #[derive(Diagnostic)]
 #[diag(hir_analysis_bad_precise_capture)]
 pub(crate) struct BadPreciseCapture {

tests/ui/impl-trait/precise-capturing/rpitit.stderr

@@ -2,9 +2,9 @@ error: `impl Trait` captures lifetime parameter, but it is not mentioned in `use
   --> $DIR/rpitit.rs:11:19
    |
 LL | trait TraitLt<'a: 'a> {
-   |               -- this lifetime parameter is captured
+   |               -- all lifetime parameters originating from a trait are captured implicitly
 LL |     fn hello() -> impl Sized + use<Self>;
-   |                   ^^^^^^^^^^^^^^^^^^^^^^ lifetime captured due to being mentioned in the bounds of the `impl Trait`
+   |                   ^^^^^^^^^^^^^^^^^^^^^^
 
 error: lifetime may not live long enough
   --> $DIR/rpitit.rs:15:5