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

Author: dianne

Diff for: 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,

Diff for: 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.

Diff for: 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);
     }

Diff for: tests/ui/impl-trait/multiple-lifetimes/error-handling.stderr

@@ -6,6 +6,9 @@ LL | fn foo<'a, 'b, 'c>(x: &'static i32, mut y: &'a i32) -> E<'b, 'c> {
    |        |
    |        lifetime `'a` defined here
 ...
+LL |     let u = v;
+   |             - requirement occurs due to copying this value
+...
 LL |         let _: &'b i32 = *u.0;
    |                ^^^^^^^ type annotation requires that `'a` must outlive `'b`
    |

Diff for: tests/ui/lifetimes/copy_modulo_regions.stderr

@@ -4,7 +4,10 @@ error: lifetime may not live long enough
 LL | fn foo<'a>() -> [Foo<'a>; 100] {
    |        -- lifetime `'a` defined here
 LL |     [mk_foo::<'a>(); 100]
-   |     ^^^^^^^^^^^^^^^^^^^^^ returning this value requires that `'a` must outlive `'static`
+   |     ^^^^^^^^^^^^^^^^^^^^^
+   |     |
+   |     returning this value requires that `'a` must outlive `'static`
+   |     requirement occurs due to copying this value
    |
    = note: requirement occurs because of the type `Foo<'_>`, which makes the generic argument `'_` invariant
    = note: the struct `Foo<'a>` is invariant over the parameter `'a`