Rollup merge of rust-lang#101431 - compiler-errors:move-place-ty-for-move-place-sugg, r=cjgillot

Look at move place's type when suggesting mutable reborrow

Not sure why we are looking at the use site's ty instead of the move site's ty in order to suggest reborrowing the move site, but it was suppressing a perfectly valid reborrow suggestion.

r? `@estebank` who i think touched this last in 520461f, though that was quite a while ago so feel free to reassign.

Author: fee1-dead

compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs

@@ -198,7 +198,6 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                     move_span,
                     move_spans,
                     *moved_place,
-                    Some(used_place),
                     partially_str,
                     loop_message,
                     move_msg,

compiler/rustc_borrowck/src/diagnostics/mod.rs

@@ -972,7 +972,6 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
         move_span: Span,
         move_spans: UseSpans<'tcx>,
         moved_place: Place<'tcx>,
-        used_place: Option<PlaceRef<'tcx>>,
         partially_str: &str,
         loop_message: &str,
         move_msg: &str,
@@ -1060,9 +1059,11 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                                 place_name, partially_str, loop_message
                             ),
                         );
-                        // If we have a `&mut` ref, we need to reborrow.
-                        if let Some(ty::Ref(_, _, hir::Mutability::Mut)) = used_place
-                            .map(|used_place| used_place.ty(self.body, self.infcx.tcx).ty.kind())
+                        // If the moved place was a `&mut` ref, then we can
+                        // suggest to reborrow it where it was moved, so it
+                        // will still be valid by the time we get to the usage.
+                        if let ty::Ref(_, _, hir::Mutability::Mut) =
+                            moved_place.ty(self.body, self.infcx.tcx).ty.kind()
                         {
                             // If we are in a loop this will be suggested later.
                             if !is_loop_move {

compiler/rustc_borrowck/src/diagnostics/move_errors.rs

@@ -401,7 +401,7 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
         };
         if let Some(use_spans) = use_spans {
             self.explain_captures(
-                &mut err, span, span, use_spans, move_place, None, "", "", "", false, true,
+                &mut err, span, span, use_spans, move_place, "", "", "", false, true,
             );
         }
         err

src/test/ui/borrowck/reborrow-sugg-move-then-borrow.rs

@@ -0,0 +1,26 @@
+// Tests the suggestion to reborrow the first move site
+// when we move then borrow a `&mut` ref.
+
+struct State;
+
+impl IntoIterator for &mut State {
+    type IntoIter = std::vec::IntoIter<()>;
+    type Item = ();
+
+    fn into_iter(self) -> Self::IntoIter {
+        vec![].into_iter()
+    }
+}
+
+fn once(f: impl FnOnce()) {}
+
+fn fill_memory_blocks_mt(state: &mut State) {
+    for _ in state {}
+    //~^ HELP consider creating a fresh reborrow of `state` here
+    fill_segment(state);
+    //~^ ERROR borrow of moved value: `state`
+}
+
+fn fill_segment(state: &mut State) {}
+
+fn main() {}

src/test/ui/borrowck/reborrow-sugg-move-then-borrow.stderr

@@ -0,0 +1,24 @@
+error[E0382]: borrow of moved value: `state`
+  --> $DIR/reborrow-sugg-move-then-borrow.rs:20:18
+   |
+LL | fn fill_memory_blocks_mt(state: &mut State) {
+   |                          ----- move occurs because `state` has type `&mut State`, which does not implement the `Copy` trait
+LL |     for _ in state {}
+   |              ----- `state` moved due to this implicit call to `.into_iter()`
+LL |
+LL |     fill_segment(state);
+   |                  ^^^^^ value borrowed here after move
+   |
+note: this function takes ownership of the receiver `self`, which moves `state`
+  --> $SRC_DIR/core/src/iter/traits/collect.rs:LL:COL
+   |
+LL |     fn into_iter(self) -> Self::IntoIter;
+   |                  ^^^^
+help: consider creating a fresh reborrow of `state` here
+   |
+LL |     for _ in &mut *state {}
+   |              ++++++
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0382`.