Auto merge of rust-lang#9635 - smoelius:fix-9386-bug, r=Jarcho

Fix bug introduced by rust-lang#9386

rust-lang#9386 introduced a potential out-of-bounds array access. Specifically, a location returned by `local_assignments` could have  [`location.statement_index` equal to `mir.basic_blocks[location.block].statements.len()`](https://github.com/rust-lang/rust-clippy/blob/b8a9a507bf9e3149d287841454842116c72d66c4/clippy_utils/src/mir/mod.rs#L129), in which case the location would refer to the block terminator:
https://github.com/rust-lang/rust-clippy/blob/b8a9a507bf9e3149d287841454842116c72d66c4/clippy_lints/src/dereference.rs#L1204-L1206
I suspect the bug is not triggerable now, because of checks leading up to where it occurs. But a future code change could make it triggerable. Hence, it should be fixed.

r? `@Jarcho`

changelog: none

clippy_lints/src/dereference.rs

@@ -1192,16 +1192,16 @@ fn has_ref_mut_self_method(cx: &LateContext<'_>, trait_def_id: DefId) -> bool {
         })
 }
 
-fn referent_used_exactly_once<'a, 'tcx>(
-    cx: &'a LateContext<'tcx>,
+fn referent_used_exactly_once<'tcx>(
+    cx: &LateContext<'tcx>,
     possible_borrowers: &mut Vec<(LocalDefId, PossibleBorrowerMap<'tcx, 'tcx>)>,
     reference: &Expr<'tcx>,
 ) -> bool {
     let mir = enclosing_mir(cx.tcx, reference.hir_id);
     if let Some(local) = expr_local(cx.tcx, reference)
         && let [location] = *local_assignments(mir, local).as_slice()
-        && let StatementKind::Assign(box (_, Rvalue::Ref(_, _, place))) =
-            mir.basic_blocks[location.block].statements[location.statement_index].kind
+        && let Some(statement) = mir.basic_blocks[location.block].statements.get(location.statement_index)
+        && let StatementKind::Assign(box (_, Rvalue::Ref(_, _, place))) = statement.kind
         && !place.has_deref()
     {
         let body_owner_local_def_id = cx.tcx.hir().enclosing_body_owner(reference.hir_id);

clippy_utils/src/mir/mod.rs

@@ -121,8 +121,7 @@ pub fn expr_local(tcx: TyCtxt<'_>, expr: &Expr<'_>) -> Option<Local> {
     })
 }
 
-/// Returns a vector of `mir::Location` where `local` is assigned. Each statement referred to has
-/// kind `StatementKind::Assign`.
+/// Returns a vector of `mir::Location` where `local` is assigned.
 pub fn local_assignments(mir: &Body<'_>, local: Local) -> Vec<Location> {
     let mut locations = Vec::new();
     for (block, data) in mir.basic_blocks.iter_enumerated() {