diff --git a/src/librustc/infer/lexical_region_resolve/graphviz.rs b/src/librustc/infer/lexical_region_resolve/graphviz.rs
index 3b92d032bdc5a..a210d63f129e3 100644
--- a/src/librustc/infer/lexical_region_resolve/graphviz.rs
+++ b/src/librustc/infer/lexical_region_resolve/graphviz.rs
@@ -230,10 +230,7 @@ impl<'a, 'gcx, 'tcx> dot::GraphWalk<'a> for ConstraintGraph<'a, 'gcx, 'tcx> {
     type Node = Node;
     type Edge = Edge<'tcx>;
     fn nodes(&self) -> dot::Nodes<'_, Node> {
-        let mut set = FxHashSet::default();
-        for node in self.node_ids.keys() {
-            set.insert(*node);
-        }
+        let set = self.node_ids.keys().cloned().collect::<FxHashSet<_>>();
         debug!("constraint graph has {} nodes", set.len());
         set.into_iter().collect()
     }
diff --git a/src/librustc/middle/dependency_format.rs b/src/librustc/middle/dependency_format.rs
index 8b5c6d147d781..e3f9c51a1235e 100644
--- a/src/librustc/middle/dependency_format.rs
+++ b/src/librustc/middle/dependency_format.rs
@@ -94,12 +94,11 @@ pub enum Linkage {
 
 pub fn calculate<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>) {
     let sess = &tcx.sess;
-    let mut fmts = FxHashMap::default();
-    for &ty in sess.crate_types.borrow().iter() {
+    let fmts = sess.crate_types.borrow().iter().map(|&ty| {
         let linkage = calculate_type(tcx, ty);
         verify_ok(tcx, &linkage);
-        fmts.insert(ty, linkage);
-    }
+        (ty, linkage)
+    }).collect::<FxHashMap<_, _>>();
     sess.abort_if_errors();
     sess.dependency_formats.set(fmts);
 }
diff --git a/src/librustc/ty/query/on_disk_cache.rs b/src/librustc/ty/query/on_disk_cache.rs
index d6f7531855122..3dc31c517169f 100644
--- a/src/librustc/ty/query/on_disk_cache.rs
+++ b/src/librustc/ty/query/on_disk_cache.rs
@@ -174,10 +174,13 @@ impl<'sess> OnDiskCache<'sess> {
         tcx.dep_graph.with_ignore(|| {
             // Allocate SourceFileIndices
             let (file_to_file_index, file_index_to_stable_id) = {
-                let mut file_to_file_index = FxHashMap::default();
-                let mut file_index_to_stable_id = FxHashMap::default();
+                let files = tcx.sess.source_map().files();
+                let mut file_to_file_index = FxHashMap::with_capacity_and_hasher(
+                    files.len(), Default::default());
+                let mut file_index_to_stable_id = FxHashMap::with_capacity_and_hasher(
+                    files.len(), Default::default());
 
-                for (index, file) in tcx.sess.source_map().files().iter().enumerate() {
+                for (index, file) in files.iter().enumerate() {
                     let index = SourceFileIndex(index as u32);
                     let file_ptr: *const SourceFile = &**file as *const _;
                     file_to_file_index.insert(file_ptr, index);
diff --git a/src/librustc_typeck/check/mod.rs b/src/librustc_typeck/check/mod.rs
index 7e25694d5598f..4851938653b53 100644
--- a/src/librustc_typeck/check/mod.rs
+++ b/src/librustc_typeck/check/mod.rs
@@ -3510,10 +3510,9 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
             _ => span_bug!(span, "non-ADT passed to check_expr_struct_fields")
         };
 
-        let mut remaining_fields = FxHashMap::default();
-        for (i, field) in variant.fields.iter().enumerate() {
-            remaining_fields.insert(field.ident.modern(), (i, field));
-        }
+        let mut remaining_fields = variant.fields.iter().enumerate().map(|(i, field)|
+            (field.ident.modern(), (i, field))
+        ).collect::<FxHashMap<_, _>>();
 
         let mut seen_fields = FxHashMap::default();
 
@@ -5051,10 +5050,8 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
         // provided (if any) into their appropriate spaces. We'll also report
         // errors if type parameters are provided in an inappropriate place.
 
-        let mut generic_segs = FxHashSet::default();
-        for PathSeg(_, index) in &path_segs {
-            generic_segs.insert(index);
-        }
+        let generic_segs = path_segs.iter().map(|PathSeg(_, index)| index)
+            .collect::<FxHashSet<_>>();
         AstConv::prohibit_generics(self, segments.iter().enumerate().filter_map(|(index, seg)| {
             if !generic_segs.contains(&index) {
                 Some(seg)