point out unblamed constraints from Copy/Sized bounds in region errors

compiler/rustc_borrowck/src/diagnostics/explain_borrow.rs

@@ -331,6 +331,7 @@ impl<'tcx> BorrowExplanation<'tcx> {
                 };
 
                 cx.add_placeholder_from_predicate_note(err, &path);
+                cx.add_sized_or_copy_bound_info(err, category, &path);
 
                 if let ConstraintCategory::Cast {
                     is_implicit_coercion: true,

compiler/rustc_borrowck/src/diagnostics/mod.rs

@@ -37,6 +37,7 @@ use super::MirBorrowckCtxt;
 use super::borrow_set::BorrowData;
 use crate::constraints::OutlivesConstraint;
 use crate::fluent_generated as fluent;
+use crate::nll::ConstraintDescription;
 use crate::session_diagnostics::{
     CaptureArgLabel, CaptureReasonLabel, CaptureReasonNote, CaptureReasonSuggest, CaptureVarCause,
     CaptureVarKind, CaptureVarPathUseCause, OnClosureNote,
@@ -647,6 +648,27 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
             err.span_note(span, "due to current limitations in the borrow checker, this implies a `'static` lifetime");
         }
     }
+
+    /// Add a label to region errors and borrow explanations when outlives constraints arise from
+    /// proving a type implements `Sized` or `Copy`.
+    fn add_sized_or_copy_bound_info(
+        &self,
+        err: &mut Diag<'_>,
+        blamed_category: ConstraintCategory<'tcx>,
+        path: &[OutlivesConstraint<'tcx>],
+    ) {
+        for sought_category in [ConstraintCategory::SizedBound, ConstraintCategory::CopyBound] {
+            if sought_category != blamed_category
+                && let Some(sought_constraint) = path.iter().find(|c| c.category == sought_category)
+            {
+                let label = format!(
+                    "requirement occurs due to {}",
+                    sought_category.description().trim_end()
+                );
+                err.span_label(sought_constraint.span, label);
+            }
+        }
+    }
 }
 
 /// The span(s) associated to a use of a place.

compiler/rustc_borrowck/src/diagnostics/region_errors.rs

@@ -554,6 +554,7 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
         }
 
         self.add_placeholder_from_predicate_note(&mut diag, &path);
+        self.add_sized_or_copy_bound_info(&mut diag, category, &path);
 
         self.buffer_error(diag);
     }