Add suggestion to borrow opaque Fn and FnMut instead of move

compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs

@@ -12,7 +12,7 @@ use rustc_middle::mir::{
     FakeReadCause, LocalDecl, LocalInfo, LocalKind, Location, Operand, Place, PlaceRef,
     ProjectionElem, Rvalue, Statement, StatementKind, Terminator, TerminatorKind, VarBindingForm,
 };
-use rustc_middle::ty::{self, suggest_constraining_type_params, PredicateKind, Ty};
+use rustc_middle::ty::{self, subst::Subst, suggest_constraining_type_params, PredicateKind, Ty};
 use rustc_mir_dataflow::move_paths::{InitKind, MoveOutIndex, MovePathIndex};
 use rustc_span::symbol::sym;
 use rustc_span::{BytePos, MultiSpan, Span};
@@ -276,22 +276,25 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                 }
             }
 
-            if needs_note {
-                let opt_name =
-                    self.describe_place_with_options(place.as_ref(), IncludingDowncast(true));
-                let note_msg = match opt_name {
-                    Some(ref name) => format!("`{}`", name),
-                    None => "value".to_owned(),
-                };
+            let opt_name =
+                self.describe_place_with_options(place.as_ref(), IncludingDowncast(true));
+            let note_msg = match opt_name {
+                Some(ref name) => format!("`{}`", name),
+                None => "value".to_owned(),
+            };
 
-                // Try to find predicates on *generic params* that would allow copying `ty`
-                let tcx = self.infcx.tcx;
-                let generics = tcx.generics_of(self.mir_def_id());
+            let tcx = self.infcx.tcx;
+            let generics = tcx.generics_of(self.mir_def_id());
+
+            if self.suggest_borrow_fn_like(&mut err, ty, &move_site_vec, &note_msg) {
+                // Suppress the next note, since we don't want to put more `Fn`-like bounds onto something that already has them
+            } else if needs_note {
                 if let Some(hir_generics) = tcx
                     .typeck_root_def_id(self.mir_def_id().to_def_id())
                     .as_local()
                     .and_then(|def_id| tcx.hir().get_generics(def_id))
                 {
+                    // Try to find predicates on *generic params* that would allow copying `ty`
                     let predicates: Result<Vec<_>, _> = tcx.infer_ctxt().enter(|infcx| {
                         let mut fulfill_cx =
                             <dyn rustc_infer::traits::TraitEngine<'_>>::new(infcx.tcx);
@@ -344,8 +347,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                 }
 
                 let span = if let Some(local) = place.as_local() {
-                    let decl = &self.body.local_decls[local];
-                    Some(decl.source_info.span)
+                    Some(self.body.local_decls[local].source_info.span)
                 } else {
                     None
                 };
@@ -373,6 +375,81 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
         }
     }
 
+    fn suggest_borrow_fn_like(
+        &self,
+        err: &mut DiagnosticBuilder<'tcx, ErrorGuaranteed>,
+        ty: Ty<'tcx>,
+        move_sites: &[MoveSite],
+        value_name: &str,
+    ) -> bool {
+        let tcx = self.infcx.tcx;
+
+        // Find out if the predicates show that the type is a Fn or FnMut
+        let find_fn_kind_from_did = |predicates: &[(ty::Predicate<'tcx>, Span)], substs| {
+            predicates.iter().find_map(|(pred, _)| {
+                let pred = if let Some(substs) = substs {
+                    pred.subst(tcx, substs).kind().skip_binder()
+                } else {
+                    pred.kind().skip_binder()
+                };
+                if let ty::PredicateKind::Trait(pred) = pred && pred.self_ty() == ty {
+                    if Some(pred.def_id()) == tcx.lang_items().fn_trait() {
+                        return Some(hir::Mutability::Not);
+                    } else if Some(pred.def_id()) == tcx.lang_items().fn_mut_trait() {
+                        return Some(hir::Mutability::Mut);
+                    }
+                }
+                None
+            })
+        };
+
+        // If the type is opaque/param/closure, and it is Fn or FnMut, let's suggest (mutably)
+        // borrowing the type, since `&mut F: FnMut` iff `F: FnMut` and similarly for `Fn`.
+        // These types seem reasonably opaque enough that they could be substituted with their
+        // borrowed variants in a function body when we see a move error.
+        let borrow_level = match ty.kind() {
+            ty::Param(_) => find_fn_kind_from_did(
+                tcx.explicit_predicates_of(self.mir_def_id().to_def_id()).predicates,
+                None,
+            ),
+            ty::Opaque(did, substs) => {
+                find_fn_kind_from_did(tcx.explicit_item_bounds(*did), Some(*substs))
+            }
+            ty::Closure(_, substs) => match substs.as_closure().kind() {
+                ty::ClosureKind::Fn => Some(hir::Mutability::Not),
+                ty::ClosureKind::FnMut => Some(hir::Mutability::Mut),
+                _ => None,
+            },
+            _ => None,
+        };
+
+        let Some(borrow_level) = borrow_level else { return false; };
+        let sugg = move_sites
+            .iter()
+            .map(|move_site| {
+                let move_out = self.move_data.moves[(*move_site).moi];
+                let moved_place = &self.move_data.move_paths[move_out.path].place;
+                let move_spans = self.move_spans(moved_place.as_ref(), move_out.source);
+                let move_span = move_spans.args_or_use();
+                let suggestion = if borrow_level == hir::Mutability::Mut {
+                    "&mut ".to_string()
+                } else {
+                    "&".to_string()
+                };
+                (move_span.shrink_to_lo(), suggestion)
+            })
+            .collect();
+        err.multipart_suggestion_verbose(
+            &format!(
+                "consider {}borrowing {value_name}",
+                if borrow_level == hir::Mutability::Mut { "mutably " } else { "" }
+            ),
+            sugg,
+            Applicability::MaybeIncorrect,
+        );
+        true
+    }
+
     pub(crate) fn report_move_out_while_borrowed(
         &mut self,
         location: Location,