Allow MIR borrowck to catch unused mutable locals

src/librustc/ich/impls_mir.rs

@@ -563,6 +563,11 @@ impl<'a, 'gcx> HashStable<StableHashingContext<'a>> for mir::Literal<'gcx> {
 
 impl_stable_hash_for!(struct mir::Location { block, statement_index });
 
+impl_stable_hash_for!(struct mir::BorrowCheckResult<'tcx> {
+    closure_requirements,
+    used_mut_upvars
+});
+
 impl_stable_hash_for!(struct mir::ClosureRegionRequirements<'tcx> {
     num_external_vids,
     outlives_requirements

src/librustc/mir/mod.rs

@@ -21,6 +21,7 @@ use rustc_data_structures::indexed_vec::{IndexVec, Idx};
 use rustc_data_structures::control_flow_graph::dominators::{Dominators, dominators};
 use rustc_data_structures::control_flow_graph::{GraphPredecessors, GraphSuccessors};
 use rustc_data_structures::control_flow_graph::ControlFlowGraph;
+use rustc_data_structures::small_vec::SmallVec;
 use rustc_serialize as serialize;
 use hir::def::CtorKind;
 use hir::def_id::DefId;
@@ -247,6 +248,22 @@ impl<'tcx> Mir<'tcx> {
         })
     }
 
+    /// Returns an iterator over all user-declared mutable arguments and locals.
+    #[inline]
+    pub fn mut_vars_and_args_iter<'a>(&'a self) -> impl Iterator<Item=Local> + 'a {
+        (1..self.local_decls.len()).filter_map(move |index| {
+            let local = Local::new(index);
+            let decl = &self.local_decls[local];
+            if (decl.is_user_variable || index < self.arg_count + 1)
+               && decl.mutability == Mutability::Mut
+            {
+                Some(local)
+            } else {
+                None
+            }
+        })
+    }
+
     /// Returns an iterator over all function arguments.
     #[inline]
     pub fn args_iter(&self) -> impl Iterator<Item=Local> {
@@ -2029,6 +2046,12 @@ pub struct GeneratorLayout<'tcx> {
     pub fields: Vec<LocalDecl<'tcx>>,
 }
 
+#[derive(Clone, Debug, RustcEncodable, RustcDecodable)]
+pub struct BorrowCheckResult<'gcx> {
+    pub closure_requirements: Option<ClosureRegionRequirements<'gcx>>,
+    pub used_mut_upvars: SmallVec<[Field; 8]>,
+}
+
 /// After we borrow check a closure, we are left with various
 /// requirements that we have inferred between the free regions that
 /// appear in the closure's signature or on its field types.  These

src/librustc/ty/maps/mod.rs

@@ -211,7 +211,7 @@ define_maps! { <'tcx>
 
     /// Borrow checks the function body. If this is a closure, returns
     /// additional requirements that the closure's creator must verify.
-    [] fn mir_borrowck: MirBorrowCheck(DefId) -> Option<mir::ClosureRegionRequirements<'tcx>>,
+    [] fn mir_borrowck: MirBorrowCheck(DefId) -> mir::BorrowCheckResult<'tcx>,
 
     /// Gets a complete map from all types to their inherent impls.
     /// Not meant to be used directly outside of coherence.

src/librustc_borrowck/borrowck/mod.rs

@@ -144,7 +144,10 @@ fn borrowck<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, owner_def_id: DefId)
     {
         check_loans::check_loans(&mut bccx, &loan_dfcx, &flowed_moves, &all_loans, body);
     }
-    unused::check(&mut bccx, body);
+
+    if !tcx.use_mir_borrowck() {
+        unused::check(&mut bccx, body);
+    }
 
     Lrc::new(BorrowCheckResult {
         used_mut_nodes: bccx.used_mut_nodes.into_inner(),