Fix build and add comment about lifetimes

Author: aapoalas

compiler/rustc_builtin_macros/src/deriving/reborrow.rs

@@ -1,14 +1,12 @@
 use rustc_ast::{
-    self as ast, AngleBracketedArg, AttrArgs, DUMMY_NODE_ID, GenericArg, GenericBound,
-    GenericParam, GenericParamKind, Generics, ItemKind, WherePredicate, WherePredicateKind,
-    WhereRegionPredicate, token,
+    self as ast, AngleBracketedArg, AttrArgs, DUMMY_NODE_ID, GenericArg, Generics, ItemKind, token,
 };
 use rustc_data_structures::fx::FxHashSet;
 use rustc_errors::E0802;
 use rustc_expand::base::ExtCtxt;
 use rustc_macros::Diagnostic;
-use rustc_span::{DUMMY_SP, Ident, Span, Symbol, sym};
-use thin_vec::{ThinVec, thin_vec};
+use rustc_span::{Ident, Span, Symbol, sym};
+use thin_vec::ThinVec;
 
 use crate::deriving::generic::*;
 use crate::deriving::new_path;

compiler/rustc_hir_analysis/src/coherence/builtin/coerce_shared.rs

@@ -749,6 +749,12 @@ fn field_tys_satisfy_relation_after_normalization_and_resolution<'tcx>(
         return false;
     }
 
+    // impl<'a: 'b, 'b> CoerceShared<Target<'b>> for Source<'a> {}
+    //
+    // We expect all fields of `Source<'a>` to be one of:
+    // 1. `CoreceShared<TargetField<'b>>`: this is handled separately
+    // 2. assignable to target field: this is checked with a covariant .relate() call.
+    // 3. `&'a mut T` assigned to `&'b T`: this is checked below with .sub_regions() and .sup().
     match relation {
         FieldRelation::Equal => {
             if infcx
@@ -767,8 +773,11 @@ fn field_tys_satisfy_relation_after_normalization_and_resolution<'tcx>(
             else {
                 return false;
             };
+            // Here we're effectively checking assignability of `&'source_region mut T` to
+            // `&'target_region T`, so we require `'source_region: 'target_region` using
+            // `.sub_regions()` and the `T`'s to be unifiable with `.sup()`.
             infcx.sub_regions(
-                SubregionOrigin::RelateObjectBound(span),
+                SubregionOrigin::Reborrow(span),
                 target_region,
                 source_region,
                 ty::VisibleForLeakCheck::Yes,