Rollup merge of rust-lang#53966 - ljedrz:mir_dataflow_cleanups, r=estebank

A few cleanups and minor improvements to mir/dataflow

- simplify `dot::GraphWalk::edges` and optimize its vector's allocation
- turn a `kill` loop into `kill_all`
- remove the `prepost` parameter from `dataflow_path` (it doesn't seem to do anything)
- a couple of other minor improvements

Author: kennytm

src/librustc_mir/dataflow/graphviz.rs

@@ -265,15 +265,12 @@ impl<'a, 'tcx, MWF, P> dot::GraphWalk<'a> for Graph<'a, 'tcx, MWF, P>
 
     fn edges(&self) -> dot::Edges<Edge> {
         let mir = self.mbcx.mir();
-        // base initial capacity on assumption every block has at
-        // least one outgoing edge (Which should be true for all
-        // blocks but one, the exit-block).
-        let mut edges = Vec::with_capacity(mir.basic_blocks().len());
-        for bb in mir.basic_blocks().indices() {
-            let outgoing = outgoing(mir, bb);
-            edges.extend(outgoing.into_iter());
-        }
-        edges.into_cow()
+
+        mir.basic_blocks()
+           .indices()
+           .flat_map(|bb| outgoing(mir, bb))
+           .collect::<Vec<_>>()
+           .into_cow()
     }
 
     fn source(&self, edge: &Edge) -> Node {

src/librustc_mir/dataflow/impls/borrows.rs

@@ -168,9 +168,7 @@ impl<'a, 'gcx, 'tcx> Borrows<'a, 'gcx, 'tcx> {
         // region, then setting that gen-bit will override any
         // potential kill introduced here.
         if let Some(indices) = self.borrows_out_of_scope_at_location.get(&location) {
-            for index in indices {
-                sets.kill(&index);
-            }
+            sets.kill_all(indices);
         }
     }
 

src/librustc_mir/dataflow/mod.rs

@@ -250,8 +250,7 @@ impl<'b, 'a: 'b, 'tcx: 'a, BD> PropagationContext<'b, 'a, 'tcx, BD> where BD: Bi
     }
 }
 
-fn dataflow_path(context: &str, prepost: &str, path: &str) -> PathBuf {
-    format!("{}_{}", context, prepost);
+fn dataflow_path(context: &str, path: &str) -> PathBuf {
     let mut path = PathBuf::from(path);
     let new_file_name = {
         let orig_file_name = path.file_name().unwrap().to_str().unwrap();
@@ -267,7 +266,7 @@ impl<'a, 'tcx: 'a, BD> DataflowBuilder<'a, 'tcx, BD> where BD: BitDenotation
         where P: Fn(&BD, BD::Idx) -> DebugFormatted
     {
         if let Some(ref path_str) = self.print_preflow_to {
-            let path = dataflow_path(BD::name(), "preflow", path_str);
+            let path = dataflow_path(BD::name(), path_str);
             graphviz::print_borrowck_graph_to(self, &path, p)
         } else {
             Ok(())
@@ -278,9 +277,9 @@ impl<'a, 'tcx: 'a, BD> DataflowBuilder<'a, 'tcx, BD> where BD: BitDenotation
         where P: Fn(&BD, BD::Idx) -> DebugFormatted
     {
         if let Some(ref path_str) = self.print_postflow_to {
-            let path = dataflow_path(BD::name(), "postflow", path_str);
+            let path = dataflow_path(BD::name(), path_str);
             graphviz::print_borrowck_graph_to(self, &path, p)
-        } else{
+        } else {
             Ok(())
         }
     }

src/librustc_mir/dataflow/move_paths/builder.rs

@@ -201,7 +201,7 @@ impl<'a, 'gcx, 'tcx> MoveDataBuilder<'a, 'gcx, 'tcx> {
             "done dumping moves"
         });
 
-        if self.errors.len() > 0 {
+        if !self.errors.is_empty() {
             Err((self.data, self.errors))
         } else {
             Ok(self.data)