Better account for FnOnce in move errors

```
error[E0382]: use of moved value: `blk`
  --> $DIR/once-cant-call-twice-on-heap.rs:8:5
   |
LL | fn foo<F:FnOnce()>(blk: F) {
   |                    --- move occurs because `blk` has type `F`, which does not implement the `Copy` trait
LL |     blk();
   |     ----- `blk` moved due to this call
LL |     blk();
   |     ^^^ value used here after move
   |
note: `FnOnce` closures can only be called once
  --> $DIR/once-cant-call-twice-on-heap.rs:6:10
   |
LL | fn foo<F:FnOnce()>(blk: F) {
   |          ^^^^^^^^ `F` is made to be an `FnOnce` closure here
LL |     blk();
   |     ----- this value implements `FnOnce`, which causes it to be moved when called
```

Author: estebank

compiler/rustc_borrowck/messages.ftl

@@ -87,6 +87,12 @@ borrowck_move_unsized =
 borrowck_moved_a_fn_once_in_call =
     this value implements `FnOnce`, which causes it to be moved when called
 
+borrowck_moved_a_fn_once_in_call_call =
+    `FnOnce` closures can only be called once
+
+borrowck_moved_a_fn_once_in_call_def =
+    `{$ty}` is made to be an `FnOnce` closure here
+
 borrowck_moved_due_to_await =
     {$place_name} {$is_partial ->
         [true] partially moved

compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs

@@ -282,7 +282,19 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                 Some(name) => format!("`{name}`"),
                 None => "value".to_owned(),
             };
-            if self.suggest_borrow_fn_like(&mut err, ty, &move_site_vec, &note_msg) {
+            if self.suggest_borrow_fn_like(&mut err, ty, &move_site_vec, &note_msg)
+                || if let UseSpans::FnSelfUse { kind, .. } = use_spans
+                    && let CallKind::FnCall { fn_trait_id, self_ty } = kind
+                    && let ty::Param(_) = self_ty.kind()
+                    && ty == self_ty
+                    && Some(fn_trait_id) == self.infcx.tcx.lang_items().fn_once_trait()
+                {
+                    // this is a type parameter `T: FnOnce()`, don't suggest `T: FnOnce() + Clone`.
+                    true
+                } else {
+                    false
+                }
+            {
                 // Suppress the next suggestion since we don't want to put more bounds onto
                 // something that already has `Fn`-like bounds (or is a closure), so we can't
                 // restrict anyways.
@@ -776,7 +788,6 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                         None
                     }
                 })
-            && { true }
             && parent_binop == other_parent_binop
         {
             // Explicitly look for `expr += other_expr;` and avoid suggesting

compiler/rustc_borrowck/src/diagnostics/mod.rs

@@ -6,6 +6,7 @@ use crate::session_diagnostics::{
 };
 use itertools::Itertools;
 use rustc_errors::{Applicability, Diag};
+use rustc_errors::{DiagCtxt, MultiSpan};
 use rustc_hir as hir;
 use rustc_hir::def::{CtorKind, Namespace};
 use rustc_hir::CoroutineKind;
@@ -30,6 +31,8 @@ use rustc_trait_selection::infer::InferCtxtExt;
 use rustc_trait_selection::traits::error_reporting::suggestions::TypeErrCtxtExt as _;
 use rustc_trait_selection::traits::type_known_to_meet_bound_modulo_regions;
 
+use crate::fluent_generated as fluent;
+
 use super::borrow_set::BorrowData;
 use super::MirBorrowckCtxt;
 
@@ -589,7 +592,7 @@ impl UseSpans<'_> {
     #[allow(rustc::diagnostic_outside_of_impl)]
     pub(super) fn args_subdiag(
         self,
-        dcx: &rustc_errors::DiagCtxt,
+        dcx: &DiagCtxt,
         err: &mut Diag<'_>,
         f: impl FnOnce(Span) -> CaptureArgLabel,
     ) {
@@ -603,7 +606,7 @@ impl UseSpans<'_> {
     #[allow(rustc::diagnostic_outside_of_impl)]
     pub(super) fn var_path_only_subdiag(
         self,
-        dcx: &rustc_errors::DiagCtxt,
+        dcx: &DiagCtxt,
         err: &mut Diag<'_>,
         action: crate::InitializationRequiringAction,
     ) {
@@ -641,7 +644,7 @@ impl UseSpans<'_> {
     #[allow(rustc::diagnostic_outside_of_impl)]
     pub(super) fn var_subdiag(
         self,
-        dcx: &rustc_errors::DiagCtxt,
+        dcx: &DiagCtxt,
         err: &mut Diag<'_>,
         kind: Option<rustc_middle::mir::BorrowKind>,
         f: impl FnOnce(hir::ClosureKind, Span) -> CaptureVarCause,
@@ -1036,7 +1039,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                 .map(|n| format!("`{n}`"))
                 .unwrap_or_else(|| "value".to_owned());
             match kind {
-                CallKind::FnCall { fn_trait_id, .. }
+                CallKind::FnCall { fn_trait_id, self_ty }
                     if Some(fn_trait_id) == self.infcx.tcx.lang_items().fn_once_trait() =>
                 {
                     err.subdiagnostic(
@@ -1048,7 +1051,58 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                             is_loop_message,
                         },
                     );
-                    err.subdiagnostic(self.dcx(), CaptureReasonNote::FnOnceMoveInCall { var_span });
+                    if let ty::Param(param_ty) = self_ty.kind()
+                        && let generics = self.infcx.tcx.generics_of(self.mir_def_id())
+                        && let param = generics.type_param(param_ty, self.infcx.tcx)
+                        && let Some(hir_generics) = self
+                            .infcx
+                            .tcx
+                            .typeck_root_def_id(self.mir_def_id().to_def_id())
+                            .as_local()
+                            .and_then(|def_id| self.infcx.tcx.hir().get_generics(def_id))
+                        && let spans = hir_generics
+                            .predicates
+                            .iter()
+                            .filter_map(|pred| match pred {
+                                hir::WherePredicate::BoundPredicate(pred) => Some(pred),
+                                _ => None,
+                            })
+                            .filter(|pred| {
+                                if let Some((id, _)) = pred.bounded_ty.as_generic_param() {
+                                    id == param.def_id
+                                } else {
+                                    false
+                                }
+                            })
+                            .flat_map(|pred| pred.bounds)
+                            .filter_map(|bound| {
+                                if let Some(trait_ref) = bound.trait_ref()
+                                    && let Some(trait_def_id) = trait_ref.trait_def_id()
+                                    && trait_def_id == fn_trait_id
+                                {
+                                    Some(bound.span())
+                                } else {
+                                    None
+                                }
+                            })
+                            .collect::<Vec<Span>>()
+                        && !spans.is_empty()
+                    {
+                        let mut span: MultiSpan = spans.clone().into();
+                        for sp in spans {
+                            span.push_span_label(sp, fluent::borrowck_moved_a_fn_once_in_call_def);
+                        }
+                        span.push_span_label(
+                            fn_call_span,
+                            fluent::borrowck_moved_a_fn_once_in_call,
+                        );
+                        err.span_note(span, fluent::borrowck_moved_a_fn_once_in_call_call);
+                    } else {
+                        err.subdiagnostic(
+                            self.dcx(),
+                            CaptureReasonNote::FnOnceMoveInCall { var_span },
+                        );
+                    }
                 }
                 CallKind::Operator { self_arg, trait_id, .. } => {
                     let self_arg = self_arg.unwrap();

tests/ui/borrowck/borrowck-unboxed-closures.stderr

@@ -29,15 +29,13 @@ LL |     f(1, 2);
 LL |     f(1, 2);
    |     ^ value used here after move
    |
-note: this value implements `FnOnce`, which causes it to be moved when called
-  --> $DIR/borrowck-unboxed-closures.rs:11:5
+note: `FnOnce` closures can only be called once
+  --> $DIR/borrowck-unboxed-closures.rs:10:8
    |
+LL | fn c<F:FnOnce(isize, isize) -> isize>(f: F) {
+   |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `F` is made to be an `FnOnce` closure here
 LL |     f(1, 2);
-   |     ^
-help: consider further restricting this bound
-   |
-LL | fn c<F:FnOnce(isize, isize) -> isize + Copy>(f: F) {
-   |                                      ++++++
+   |     ------- this value implements `FnOnce`, which causes it to be moved when called
 
 error: aborting due to 3 previous errors
 

tests/ui/once-cant-call-twice-on-heap.stderr

@@ -8,15 +8,13 @@ LL |     blk();
 LL |     blk();
    |     ^^^ value used here after move
    |
-note: this value implements `FnOnce`, which causes it to be moved when called
-  --> $DIR/once-cant-call-twice-on-heap.rs:7:5
+note: `FnOnce` closures can only be called once
+  --> $DIR/once-cant-call-twice-on-heap.rs:6:10
    |
+LL | fn foo<F:FnOnce()>(blk: F) {
+   |          ^^^^^^^^ `F` is made to be an `FnOnce` closure here
 LL |     blk();
-   |     ^^^
-help: consider further restricting this bound
-   |
-LL | fn foo<F:FnOnce() + Copy>(blk: F) {
-   |                   ++++++
+   |     ----- this value implements `FnOnce`, which causes it to be moved when called
 
 error: aborting due to 1 previous error