diff --git a/src/bootstrap/util.rs b/src/bootstrap/util.rs
index 9a2b9e90440..be03796921a 100644
--- a/src/bootstrap/util.rs
+++ b/src/bootstrap/util.rs
@@ -92,10 +92,7 @@ pub fn push_exe_path(mut buf: PathBuf, components: &[&str]) -> PathBuf {
         file.push_str(".exe");
     }
 
-    for c in components {
-        buf.push(c);
-    }
-
+    buf.extend(components);
     buf.push(file);
 
     buf
diff --git a/src/librustc/cfg/construct.rs b/src/librustc/cfg/construct.rs
index f1e27946915..98cfa094c16 100644
--- a/src/librustc/cfg/construct.rs
+++ b/src/librustc/cfg/construct.rs
@@ -567,12 +567,12 @@ impl<'a, 'tcx> CFGBuilder<'a, 'tcx> {
     fn add_returning_edge(&mut self,
                           _from_expr: &hir::Expr,
                           from_index: CFGIndex) {
-        let mut data = CFGEdgeData {
-            exiting_scopes: vec![],
+        let data = CFGEdgeData {
+            exiting_scopes: self.loop_scopes.iter()
+                                            .rev()
+                                            .map(|&LoopScope { loop_id: id, .. }| id)
+                                            .collect()
         };
-        for &LoopScope { loop_id: id, .. } in self.loop_scopes.iter().rev() {
-            data.exiting_scopes.push(id);
-        }
         self.graph.add_edge(from_index, self.fn_exit, data);
     }
 
diff --git a/src/librustc/infer/outlives/obligations.rs b/src/librustc/infer/outlives/obligations.rs
index b8991a0366a..c74783f5e4d 100644
--- a/src/librustc/infer/outlives/obligations.rs
+++ b/src/librustc/infer/outlives/obligations.rs
@@ -151,13 +151,12 @@ impl<'cx, 'gcx, 'tcx> InferCtxt<'cx, 'gcx, 'tcx> {
         debug!("process_registered_region_obligations()");
 
         // pull out the region obligations with the given `body_id` (leaving the rest)
-        let mut my_region_obligations = Vec::with_capacity(self.region_obligations.borrow().len());
-        {
+        let my_region_obligations = {
             let mut r_o = self.region_obligations.borrow_mut();
-            for (_, obligation) in r_o.drain_filter(|(ro_body_id, _)| *ro_body_id == body_id) {
-                my_region_obligations.push(obligation);
-            }
-        }
+            let my_r_o = r_o.drain_filter(|(ro_body_id, _)| *ro_body_id == body_id)
+                            .map(|(_, obligation)| obligation).collect::<Vec<_>>();
+            my_r_o
+        };
 
         let outlives = &mut TypeOutlives::new(
             self,
diff --git a/src/librustc/middle/reachable.rs b/src/librustc/middle/reachable.rs
index 7f4b0bb126b..19c82d7d27c 100644
--- a/src/librustc/middle/reachable.rs
+++ b/src/librustc/middle/reachable.rs
@@ -367,9 +367,7 @@ impl<'a, 'tcx: 'a> ItemLikeVisitor<'tcx> for CollectPrivateImplItemsVisitor<'a,
         // We need only trait impls here, not inherent impls, and only non-exported ones
         if let hir::ItemKind::Impl(.., Some(ref trait_ref), _, ref impl_item_refs) = item.node {
             if !self.access_levels.is_reachable(item.id) {
-                for impl_item_ref in impl_item_refs {
-                    self.worklist.push(impl_item_ref.id.node_id);
-                }
+                self.worklist.extend(impl_item_refs.iter().map(|r| r.id.node_id));
 
                 let trait_def_id = match trait_ref.path.def {
                     Def::Trait(def_id) => def_id,
@@ -426,9 +424,7 @@ fn reachable_set<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, crate_num: CrateNum) ->
     //         If other crates link to us, they're going to expect to be able to
     //         use the lang items, so we need to be sure to mark them as
     //         exported.
-    for (id, _) in &access_levels.map {
-        reachable_context.worklist.push(*id);
-    }
+    reachable_context.worklist.extend(access_levels.map.iter().map(|(id, _)| *id));
     for item in tcx.lang_items().items().iter() {
         if let Some(did) = *item {
             if let Some(node_id) = tcx.hir.as_local_node_id(did) {
diff --git a/src/librustc/mir/traversal.rs b/src/librustc/mir/traversal.rs
index 87ba3420eec..86fd825850f 100644
--- a/src/librustc/mir/traversal.rs
+++ b/src/librustc/mir/traversal.rs
@@ -64,9 +64,7 @@ impl<'a, 'tcx> Iterator for Preorder<'a, 'tcx> {
             let data = &self.mir[idx];
 
             if let Some(ref term) = data.terminator {
-                for &succ in term.successors() {
-                    self.worklist.push(succ);
-                }
+                self.worklist.extend(term.successors());
             }
 
             return Some((idx, data));
diff --git a/src/librustc/session/config.rs b/src/librustc/session/config.rs
index 8f3ed3106d5..e0532a3320b 100644
--- a/src/librustc/session/config.rs
+++ b/src/librustc/session/config.rs
@@ -899,9 +899,7 @@ macro_rules! options {
                       -> bool {
             match v {
                 Some(s) => {
-                    for s in s.split_whitespace() {
-                        slot.push(s.to_string());
-                    }
+                    slot.extend(s.split_whitespace().map(|s| s.to_string()));
                     true
                 },
                 None => false,
diff --git a/src/librustc/traits/specialize/mod.rs b/src/librustc/traits/specialize/mod.rs
index f151f3b2531..1ed9ae4392f 100644
--- a/src/librustc/traits/specialize/mod.rs
+++ b/src/librustc/traits/specialize/mod.rs
@@ -438,9 +438,9 @@ fn to_pretty_impl_header(tcx: TyCtxt, impl_def_id: DefId) -> Option<String> {
         }
         pretty_predicates.push(p.to_string());
     }
-    for ty in types_without_default_bounds {
-        pretty_predicates.push(format!("{}: ?Sized", ty));
-    }
+    pretty_predicates.extend(
+        types_without_default_bounds.iter().map(|ty| format!("{}: ?Sized", ty))
+    );
     if !pretty_predicates.is_empty() {
         write!(w, "\n  where {}", pretty_predicates.join(", ")).unwrap();
     }
diff --git a/src/librustc_codegen_llvm/back/rpath.rs b/src/librustc_codegen_llvm/back/rpath.rs
index e73073cfad0..2c3a143646c 100644
--- a/src/librustc_codegen_llvm/back/rpath.rs
+++ b/src/librustc_codegen_llvm/back/rpath.rs
@@ -152,9 +152,7 @@ fn path_relative_from(path: &Path, base: &Path) -> Option<PathBuf> {
                 (Some(_), Some(b)) if b == Component::ParentDir => return None,
                 (Some(a), Some(_)) => {
                     comps.push(Component::ParentDir);
-                    for _ in itb {
-                        comps.push(Component::ParentDir);
-                    }
+                    comps.extend(itb.map(|_| Component::ParentDir));
                     comps.push(a);
                     comps.extend(ita.by_ref());
                     break;
diff --git a/src/librustc_codegen_llvm/debuginfo/metadata.rs b/src/librustc_codegen_llvm/debuginfo/metadata.rs
index 6d727f7b048..2f4dd5a7ce5 100644
--- a/src/librustc_codegen_llvm/debuginfo/metadata.rs
+++ b/src/librustc_codegen_llvm/debuginfo/metadata.rs
@@ -39,6 +39,7 @@ use rustc::util::common::path2cstr;
 use libc::{c_uint, c_longlong};
 use std::ffi::CString;
 use std::fmt::Write;
+use std::iter;
 use std::ptr;
 use std::path::{Path, PathBuf};
 use syntax::ast;
@@ -364,18 +365,16 @@ fn subroutine_type_metadata<'a, 'tcx>(cx: &CodegenCx<'a, 'tcx>,
         &signature,
     );
 
-    let mut signature_metadata: Vec<DIType> = Vec::with_capacity(signature.inputs().len() + 1);
-
-    // return type
-    signature_metadata.push(match signature.output().sty {
-        ty::TyTuple(ref tys) if tys.is_empty() => ptr::null_mut(),
-        _ => type_metadata(cx, signature.output(), span)
-    });
-
-    // regular arguments
-    for &argument_type in signature.inputs() {
-        signature_metadata.push(type_metadata(cx, argument_type, span));
-    }
+    let signature_metadata: Vec<DIType> = iter::once(
+        // return type
+        match signature.output().sty {
+            ty::TyTuple(ref tys) if tys.is_empty() => ptr::null_mut(),
+            _ => type_metadata(cx, signature.output(), span)
+        }
+    ).chain(
+        // regular arguments
+        signature.inputs().iter().map(|argument_type| type_metadata(cx, argument_type, span))
+    ).collect();
 
     return_if_metadata_created_in_meantime!(cx, unique_type_id);
 
diff --git a/src/librustc_codegen_llvm/debuginfo/mod.rs b/src/librustc_codegen_llvm/debuginfo/mod.rs
index 068dd9821ac..9671db75baf 100644
--- a/src/librustc_codegen_llvm/debuginfo/mod.rs
+++ b/src/librustc_codegen_llvm/debuginfo/mod.rs
@@ -352,9 +352,10 @@ pub fn create_function_debug_context<'a, 'tcx>(cx: &CodegenCx<'a, 'tcx>,
 
         if sig.abi == Abi::RustCall && !sig.inputs().is_empty() {
             if let ty::TyTuple(args) = sig.inputs()[sig.inputs().len() - 1].sty {
-                for &argument_type in args {
-                    signature.push(type_metadata(cx, argument_type, syntax_pos::DUMMY_SP));
-                }
+                signature.extend(
+                    args.iter().map(|argument_type|
+                        type_metadata(cx, argument_type, syntax_pos::DUMMY_SP))
+                );
             }
         }
 
diff --git a/src/librustc_driver/lib.rs b/src/librustc_driver/lib.rs
index 29b9990de34..e25da0632c0 100644
--- a/src/librustc_driver/lib.rs
+++ b/src/librustc_driver/lib.rs
@@ -1588,10 +1588,7 @@ pub fn in_rustc_thread<F, R>(f: F) -> Result<R, Box<dyn Any + Send>>
 /// debugging, since some ICEs only happens with non-default compiler flags
 /// (and the users don't always report them).
 fn extra_compiler_flags() -> Option<(Vec<String>, bool)> {
-    let mut args = Vec::new();
-    for arg in env::args_os() {
-        args.push(arg.to_string_lossy().to_string());
-    }
+    let args = env::args_os().map(|arg| arg.to_string_lossy().to_string()).collect::<Vec<_>>();
 
     // Avoid printing help because of empty args. This can suggest the compiler
     // itself is not the program root (consider RLS).
diff --git a/src/librustc_driver/profile/trace.rs b/src/librustc_driver/profile/trace.rs
index 797ed4505b4..3d4f4b029e5 100644
--- a/src/librustc_driver/profile/trace.rs
+++ b/src/librustc_driver/profile/trace.rs
@@ -204,10 +204,9 @@ pub fn write_counts(count_file: &mut File, counts: &mut HashMap<String,QueryMetr
     use rustc::util::common::duration_to_secs_str;
     use std::cmp::Reverse;
 
-    let mut data = vec![];
-    for (ref cons, ref qm) in counts.iter() {
-        data.push((cons.clone(), qm.count.clone(), qm.dur_total.clone(), qm.dur_self.clone()));
-    };
+    let mut data = counts.iter().map(|(ref cons, ref qm)|
+        (cons.clone(), qm.count.clone(), qm.dur_total.clone(), qm.dur_self.clone())
+    ).collect::<Vec<_>>();
     data.sort_by_key(|k| Reverse(k.3));
     for (cons, count, dur_total, dur_self) in data {
         write!(count_file, "{}, {}, {}, {}\n",
diff --git a/src/librustc_llvm/lib.rs b/src/librustc_llvm/lib.rs
index 741758cb954..91f9b4ac03b 100644
--- a/src/librustc_llvm/lib.rs
+++ b/src/librustc_llvm/lib.rs
@@ -275,12 +275,8 @@ pub fn get_param(llfn: ValueRef, index: c_uint) -> ValueRef {
 fn get_params(llfn: ValueRef) -> Vec<ValueRef> {
     unsafe {
         let num_params = LLVMCountParams(llfn);
-        let mut params = Vec::with_capacity(num_params as usize);
-        for idx in 0..num_params {
-            params.push(LLVMGetParam(llfn, idx));
-        }
 
-        params
+        (0..num_params).map(|idx| LLVMGetParam(llfn, idx)).collect()
     }
 }
 
diff --git a/src/librustc_mir/transform/inline.rs b/src/librustc_mir/transform/inline.rs
index a0a5980d141..46fab544aaf 100644
--- a/src/librustc_mir/transform/inline.rs
+++ b/src/librustc_mir/transform/inline.rs
@@ -406,10 +406,9 @@ impl<'a, 'tcx> Inliner<'a, 'tcx> {
                     local_map.push(idx);
                 }
 
-                for p in callee_mir.promoted.iter().cloned() {
-                    let idx = caller_mir.promoted.push(p);
-                    promoted_map.push(idx);
-                }
+                promoted_map.extend(
+                    callee_mir.promoted.iter().cloned().map(|p| caller_mir.promoted.push(p))
+                );
 
                 // If the call is something like `a[*i] = f(i)`, where
                 // `i : &mut usize`, then just duplicating the `a[*i]`
diff --git a/src/librustc_resolve/lib.rs b/src/librustc_resolve/lib.rs
index 725b3b4f150..b6f0ff291d4 100644
--- a/src/librustc_resolve/lib.rs
+++ b/src/librustc_resolve/lib.rs
@@ -3831,9 +3831,9 @@ impl<'a> Resolver<'a> {
             }
             // Add primitive types to the mix
             if filter_fn(Def::PrimTy(TyBool)) {
-                for (name, _) in &self.primitive_type_table.primitive_types {
-                    names.push(*name);
-                }
+                names.extend(
+                    self.primitive_type_table.primitive_types.iter().map(|(name, _)| name)
+                )
             }
         } else {
             // Search in module.
diff --git a/src/librustc_save_analysis/dump_visitor.rs b/src/librustc_save_analysis/dump_visitor.rs
index ac7085a55cf..04a4bca4ffb 100644
--- a/src/librustc_save_analysis/dump_visitor.rs
+++ b/src/librustc_save_analysis/dump_visitor.rs
@@ -1318,14 +1318,13 @@ impl<'l, 'tcx: 'l, 'll, O: DumpOutput + 'll> DumpVisitor<'l, 'tcx, 'll, O> {
                 };
 
                 // Make a comma-separated list of names of imported modules.
-                let mut names = vec![];
                 let glob_map = &self.save_ctxt.analysis.glob_map;
                 let glob_map = glob_map.as_ref().unwrap();
-                if glob_map.contains_key(&id) {
-                    for n in glob_map.get(&id).unwrap() {
-                        names.push(n.to_string());
-                    }
-                }
+                let names = if glob_map.contains_key(&id) {
+                    glob_map.get(&id).unwrap().iter().map(|n| n.to_string()).collect()
+                } else {
+                    Vec::new()
+                };
 
                 let sub_span = self.span.sub_span_of_token(use_tree.span,
                                                            token::BinOp(token::Star));
diff --git a/src/librustc_typeck/collect.rs b/src/librustc_typeck/collect.rs
index cd84a61bb2c..77b02d9ff5b 100644
--- a/src/librustc_typeck/collect.rs
+++ b/src/librustc_typeck/collect.rs
@@ -962,19 +962,21 @@ fn generics_of<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
             &["<closure_kind>", "<closure_signature>"][..]
         };
 
-        for (i, &arg) in dummy_args.iter().enumerate() {
-            params.push(ty::GenericParamDef {
-                index: type_start + i as u32,
-                name: Symbol::intern(arg).as_interned_str(),
-                def_id,
-                pure_wrt_drop: false,
-                kind: ty::GenericParamDefKind::Type {
-                    has_default: false,
-                    object_lifetime_default: rl::Set1::Empty,
-                    synthetic: None,
-                },
-            });
-        }
+        params.extend(
+            dummy_args.iter().enumerate().map(|(i, &arg)|
+                ty::GenericParamDef {
+                    index: type_start + i as u32,
+                    name: Symbol::intern(arg).as_interned_str(),
+                    def_id,
+                    pure_wrt_drop: false,
+                    kind: ty::GenericParamDefKind::Type {
+                        has_default: false,
+                        object_lifetime_default: rl::Set1::Empty,
+                        synthetic: None,
+                    },
+                }
+            )
+        );
 
         tcx.with_freevars(node_id, |fv| {
             params.extend(fv.iter().zip((dummy_args.len() as u32)..).map(|(_, i)| {
@@ -1651,10 +1653,7 @@ fn explicit_predicates_of<'a, 'tcx>(
                                                                     &mut projections);
 
                             predicates.push(trait_ref.to_predicate());
-
-                            for projection in &projections {
-                                predicates.push(projection.to_predicate());
-                            }
+                            predicates.extend(projections.iter().map(|p| p.to_predicate()));
                         }
 
                         &hir::GenericBound::Outlives(ref lifetime) => {
diff --git a/src/librustdoc/core.rs b/src/librustdoc/core.rs
index 77375442d4c..769c9804a35 100644
--- a/src/librustdoc/core.rs
+++ b/src/librustdoc/core.rs
@@ -203,9 +203,7 @@ pub fn run_core(search_paths: SearchPaths,
                                      intra_link_resolution_failure_name.to_owned(),
                                      missing_docs.to_owned()];
 
-    for (lint, _) in &cmd_lints {
-        whitelisted_lints.push(lint.clone());
-    }
+    whitelisted_lints.extend(cmd_lints.iter().map(|(lint, _)| lint).cloned());
 
     let lints = lint::builtin::HardwiredLints.get_lints()
                     .into_iter()
diff --git a/src/librustdoc/lib.rs b/src/librustdoc/lib.rs
index 00cad5e376a..613ca01ff82 100644
--- a/src/librustdoc/lib.rs
+++ b/src/librustdoc/lib.rs
@@ -722,9 +722,7 @@ where R: 'static + Send,
                     },
                     _ => continue,
                 };
-                for p in value.as_str().split_whitespace() {
-                    sink.push(p.to_string());
-                }
+                sink.extend(value.as_str().split_whitespace().map(|p| p.to_string()));
             }
 
             if attr.is_word() && name == Some("document_private_items") {
diff --git a/src/libserialize/json.rs b/src/libserialize/json.rs
index 88cc9373113..b5986e88b66 100644
--- a/src/libserialize/json.rs
+++ b/src/libserialize/json.rs
@@ -1364,9 +1364,7 @@ impl Stack {
     // Used by Parser to insert StackElement::Key elements at the top of the stack.
     fn push_key(&mut self, key: string::String) {
         self.stack.push(InternalKey(self.str_buffer.len() as u16, key.len() as u16));
-        for c in key.as_bytes() {
-            self.str_buffer.push(*c);
-        }
+        self.str_buffer.extend(key.as_bytes());
     }
 
     // Used by Parser to insert StackElement::Index elements at the top of the stack.
@@ -2212,9 +2210,7 @@ impl ::Decoder for Decoder {
                 };
                 match o.remove(&"fields".to_string()) {
                     Some(Json::Array(l)) => {
-                        for field in l.into_iter().rev() {
-                            self.stack.push(field);
-                        }
+                        self.stack.extend(l.into_iter().rev());
                     },
                     Some(val) => {
                         return Err(ExpectedError("Array".to_owned(), format!("{}", val)))
@@ -2346,9 +2342,7 @@ impl ::Decoder for Decoder {
     {
         let array = expect!(self.pop(), Array)?;
         let len = array.len();
-        for v in array.into_iter().rev() {
-            self.stack.push(v);
-        }
+        self.stack.extend(array.into_iter().rev());
         f(self, len)
     }
 
diff --git a/src/libstd/sys/redox/process.rs b/src/libstd/sys/redox/process.rs
index 02bc467541e..2037616e6ac 100644
--- a/src/libstd/sys/redox/process.rs
+++ b/src/libstd/sys/redox/process.rs
@@ -13,6 +13,7 @@ use ffi::OsStr;
 use os::unix::ffi::OsStrExt;
 use fmt;
 use io::{self, Error, ErrorKind};
+use iter;
 use libc::{EXIT_SUCCESS, EXIT_FAILURE};
 use path::{Path, PathBuf};
 use sys::fd::FileDesc;
@@ -296,11 +297,11 @@ impl Command {
             t!(callback());
         }
 
-        let mut args: Vec<[usize; 2]> = Vec::new();
-        args.push([self.program.as_ptr() as usize, self.program.len()]);
-        for arg in self.args.iter() {
-            args.push([arg.as_ptr() as usize, arg.len()]);
-        }
+        let args: Vec<[usize; 2]> = iter::once(
+            [self.program.as_ptr() as usize, self.program.len()]
+        ).chain(
+            self.args.iter().map(|arg| [arg.as_ptr() as usize, arg.len()])
+        ).collect();
 
         self.env.apply();
 
diff --git a/src/libstd/sys/windows/process.rs b/src/libstd/sys/windows/process.rs
index be442f41374..4974a8de89c 100644
--- a/src/libstd/sys/windows/process.rs
+++ b/src/libstd/sys/windows/process.rs
@@ -487,9 +487,7 @@ fn make_command_line(prog: &OsStr, args: &[OsString]) -> io::Result<Vec<u16>> {
             } else {
                 if x == '"' as u16 {
                     // Add n+1 backslashes to total 2n+1 before internal '"'.
-                    for _ in 0..(backslashes+1) {
-                        cmd.push('\\' as u16);
-                    }
+                    cmd.extend((0..(backslashes + 1)).map(|_| '\\' as u16));
                 }
                 backslashes = 0;
             }
@@ -498,9 +496,7 @@ fn make_command_line(prog: &OsStr, args: &[OsString]) -> io::Result<Vec<u16>> {
 
         if quote {
             // Add n backslashes to total 2n before ending '"'.
-            for _ in 0..backslashes {
-                cmd.push('\\' as u16);
-            }
+            cmd.extend((0..backslashes).map(|_| '\\' as u16));
             cmd.push('"' as u16);
         }
         Ok(())
diff --git a/src/libsyntax/ast.rs b/src/libsyntax/ast.rs
index 1b6b47f5489..d59c9dd6b53 100644
--- a/src/libsyntax/ast.rs
+++ b/src/libsyntax/ast.rs
@@ -500,10 +500,7 @@ impl Pat {
             PatKind::Slice(pats, None, _) if pats.len() == 1 =>
                 pats[0].to_ty().map(TyKind::Slice)?,
             PatKind::Tuple(pats, None) => {
-                let mut tys = Vec::new();
-                for pat in pats {
-                    tys.push(pat.to_ty()?);
-                }
+                let tys = pats.iter().map(|pat| pat.to_ty()).collect::<Option<Vec<_>>>()?;
                 TyKind::Tup(tys)
             }
             _ => return None,
@@ -949,10 +946,7 @@ impl Expr {
             ExprKind::Array(exprs) if exprs.len() == 1 =>
                 exprs[0].to_ty().map(TyKind::Slice)?,
             ExprKind::Tup(exprs) => {
-                let mut tys = Vec::new();
-                for expr in exprs {
-                    tys.push(expr.to_ty()?);
-                }
+                let tys = exprs.iter().map(|expr| expr.to_ty()).collect::<Option<Vec<_>>>()?;
                 TyKind::Tup(tys)
             }
             ExprKind::Binary(binop, lhs, rhs) if binop.node == BinOpKind::Add =>
diff --git a/src/libsyntax/parse/lexer/mod.rs b/src/libsyntax/parse/lexer/mod.rs
index 4b077aa8dd4..f9b9e95ead1 100644
--- a/src/libsyntax/parse/lexer/mod.rs
+++ b/src/libsyntax/parse/lexer/mod.rs
@@ -265,9 +265,7 @@ impl<'a> StringReader<'a> {
                 m.push(c);
             }
             _ => {
-                for c in c.escape_default() {
-                    m.push(c);
-                }
+                m.extend(c.escape_default());
             }
         }
     }
diff --git a/src/libsyntax_ext/deriving/decodable.rs b/src/libsyntax_ext/deriving/decodable.rs
index 1e04d8fa22a..c2b92944999 100644
--- a/src/libsyntax_ext/deriving/decodable.rs
+++ b/src/libsyntax_ext/deriving/decodable.rs
@@ -131,8 +131,8 @@ fn decodable_substructure(cx: &mut ExtCtxt,
         StaticEnum(_, ref fields) => {
             let variant = cx.ident_of("i");
 
-            let mut arms = Vec::new();
-            let mut variants = Vec::new();
+            let mut arms = Vec::with_capacity(fields.len() + 1);
+            let mut variants = Vec::with_capacity(fields.len());
             let rvariant_arg = cx.ident_of("read_enum_variant_arg");
 
             for (i, &(ident, v_span, ref parts)) in fields.iter().enumerate() {
diff --git a/src/libsyntax_ext/deriving/generic/mod.rs b/src/libsyntax_ext/deriving/generic/mod.rs
index a9f60fd053c..e0f985c26c7 100644
--- a/src/libsyntax_ext/deriving/generic/mod.rs
+++ b/src/libsyntax_ext/deriving/generic/mod.rs
@@ -188,6 +188,7 @@ pub use self::StaticFields::*;
 pub use self::SubstructureFields::*;
 
 use std::cell::RefCell;
+use std::iter;
 use std::vec;
 
 use rustc_target::spec::abi::Abi;
@@ -558,15 +559,13 @@ impl<'a> TraitDef<'a> {
                     // type being derived upon
                     self.additional_bounds.iter().map(|p| {
                         cx.trait_bound(p.to_path(cx, self.span, type_ident, generics))
-                    }).collect();
-
-                // require the current trait
-                bounds.push(cx.trait_bound(trait_path.clone()));
-
-                // also add in any bounds from the declaration
-                for declared_bound in &param.bounds {
-                    bounds.push((*declared_bound).clone());
-                }
+                    }).chain(
+                        // require the current trait
+                        iter::once(cx.trait_bound(trait_path.clone()))
+                    ).chain(
+                        // also add in any bounds from the declaration
+                        param.bounds.iter().cloned()
+                    ).collect();
 
                 cx.typaram(self.span, param.ident, vec![], bounds, None)
             }
diff --git a/src/libsyntax_ext/deriving/hash.rs b/src/libsyntax_ext/deriving/hash.rs
index 7d22998487b..950e8c84f17 100644
--- a/src/libsyntax_ext/deriving/hash.rs
+++ b/src/libsyntax_ext/deriving/hash.rs
@@ -95,9 +95,8 @@ fn hash_substructure(cx: &mut ExtCtxt, trait_span: Span, substr: &Substructure)
         _ => cx.span_bug(trait_span, "impossible substructure in `derive(Hash)`"),
     };
 
-    for &FieldInfo { ref self_, span, .. } in fields {
-        stmts.push(call_hash(span, self_.clone()));
-    }
+    stmts.extend(fields.iter().map(|FieldInfo { ref self_, span, .. }|
+        call_hash(*span, self_.clone())));
 
     cx.expr_block(cx.block(trait_span, stmts))
 }
diff --git a/src/libsyntax_ext/format.rs b/src/libsyntax_ext/format.rs
index 98de3d80b1e..46c85497ee7 100644
--- a/src/libsyntax_ext/format.rs
+++ b/src/libsyntax_ext/format.rs
@@ -406,10 +406,7 @@ impl<'a, 'b> Context<'a, 'b> {
         // Map the arguments
         for i in 0..args_len {
             let ref arg_types = self.arg_types[i];
-            let mut arg_offsets = Vec::with_capacity(arg_types.len());
-            for offset in arg_types {
-                arg_offsets.push(sofar + *offset);
-            }
+            let arg_offsets = arg_types.iter().map(|offset| sofar + *offset).collect::<Vec<_>>();
             self.arg_index_map.push(arg_offsets);
             sofar += self.arg_unique_types[i].len();
         }
@@ -581,10 +578,12 @@ impl<'a, 'b> Context<'a, 'b> {
     /// Actually builds the expression which the format_args! block will be
     /// expanded to
     fn into_expr(self) -> P<ast::Expr> {
-        let mut locals = Vec::new();
-        let mut counts = Vec::new();
-        let mut pats = Vec::new();
-        let mut heads = Vec::new();
+        let mut locals = Vec::with_capacity(
+            (0..self.args.len()).map(|i| self.arg_unique_types[i].len()).sum()
+        );
+        let mut counts = Vec::with_capacity(self.count_args.len());
+        let mut pats = Vec::with_capacity(self.args.len());
+        let mut heads = Vec::with_capacity(self.args.len());
 
         let names_pos: Vec<_> = (0..self.args.len())
             .map(|i| self.ecx.ident_of(&format!("arg{}", i)).gensym())