Overhaul syntax::fold::Folder.

This commit changes `syntax::fold::Folder` from a functional style
(where most methods take a `T` and produce a new `T`) to a more
imperative style (where most methods take and modify a `&mut T`), and
renames it `syntax::mut_visit::MutVisitor`.

The first benefit is speed. The functional style does not require any
reallocations, due to the use of `P::map` and
`MoveMap::move_{,flat_}map`. However, every field in the AST must be
overwritten; even those fields that are unchanged are overwritten with
the same value. This causes a lot of unnecessary memory writes. The
imperative style reduces instruction counts by 1--3% across a wide range
of workloads, particularly incremental workloads.

The second benefit is conciseness; the imperative style is usually more
concise. E.g. compare the old functional style:
```
fn fold_abc(&mut self, abc: ABC) {
    ABC {
        a: fold_a(abc.a),
        b: fold_b(abc.b),
        c: abc.c,
    }
}
```
with the imperative style:
```
fn visit_abc(&mut self, ABC { a, b, c: _ }: &mut ABC) {
    visit_a(a);
    visit_b(b);
}
```
(The reductions get larger in more complex examples.)

Overall, the patch removes over 200 lines of code -- even though the new
code has more comments -- and a lot of the remaining lines have fewer
characters.

Some notes:

- The old style used methods called `fold_*`. The new style mostly uses
  methods called `visit_*`, but there are a few methods that map a `T`
  to something other than a `T`, which are called `flat_map_*` (`T` maps
  to multiple `T`s) or `filter_map_*` (`T` maps to 0 or 1 `T`s).

- `move_map.rs`/`MoveMap`/`move_map`/`move_flat_map` are renamed
  `map_in_place.rs`/`MapInPlace`/`map_in_place`/`flat_map_in_place` to
  reflect their slightly changed signatures.

- Although this commit renames the `fold` module as `mut_visit`, it
  keeps it in the `fold.rs` file, so as not to confuse git. The next
  commit will rename the file.

src/librustc_allocator/expand.rs

@@ -16,7 +16,7 @@ use syntax::{
         expand::ExpansionConfig,
         hygiene::{self, Mark, SyntaxContext},
     },
-    fold::{self, Folder},
+    mut_visit::{self, MutVisitor},
     parse::ParseSess,
     ptr::P,
     symbol::Symbol
@@ -28,18 +28,18 @@ use {AllocatorMethod, AllocatorTy, ALLOCATOR_METHODS};
 pub fn modify(
     sess: &ParseSess,
     resolver: &mut dyn Resolver,
-    krate: Crate,
+    krate: &mut Crate,
     crate_name: String,
     handler: &rustc_errors::Handler,
-) -> ast::Crate {
+) {
     ExpandAllocatorDirectives {
         handler,
         sess,
         resolver,
         found: false,
         crate_name: Some(crate_name),
         in_submod: -1, // -1 to account for the "root" module
-    }.fold_crate(krate)
+    }.visit_crate(krate);
 }
 
 struct ExpandAllocatorDirectives<'a> {
@@ -54,14 +54,14 @@ struct ExpandAllocatorDirectives<'a> {
     in_submod: isize,
 }
 
-impl<'a> Folder for ExpandAllocatorDirectives<'a> {
-    fn fold_item(&mut self, item: P<Item>) -> SmallVec<[P<Item>; 1]> {
+impl<'a> MutVisitor for ExpandAllocatorDirectives<'a> {
+    fn flat_map_item(&mut self, item: P<Item>) -> SmallVec<[P<Item>; 1]> {
         debug!("in submodule {}", self.in_submod);
 
         let name = if attr::contains_name(&item.attrs, "global_allocator") {
             "global_allocator"
         } else {
-            return fold::noop_fold_item(item, self);
+            return mut_visit::noop_flat_map_item(item, self);
         };
         match item.node {
             ItemKind::Static(..) => {}
@@ -139,25 +139,24 @@ impl<'a> Folder for ExpandAllocatorDirectives<'a> {
         let name = f.kind.fn_name("allocator_abi");
         let allocator_abi = Ident::with_empty_ctxt(Symbol::gensym(&name));
         let module = f.cx.item_mod(span, span, allocator_abi, Vec::new(), items);
-        let module = f.cx.monotonic_expander().fold_item(module).pop().unwrap();
+        let module = f.cx.monotonic_expander().flat_map_item(module).pop().unwrap();
 
         // Return the item and new submodule
         smallvec![item, module]
     }
 
     // If we enter a submodule, take note.
-    fn fold_mod(&mut self, m: Mod) -> Mod {
+    fn visit_mod(&mut self, m: &mut Mod) {
         debug!("enter submodule");
         self.in_submod += 1;
-        let ret = fold::noop_fold_mod(m, self);
+        mut_visit::noop_visit_mod(m, self);
         self.in_submod -= 1;
         debug!("exit submodule");
-        ret
     }
 
-    // `fold_mac` is disabled by default. Enable it here.
-    fn fold_mac(&mut self, mac: Mac) -> Mac {
-        fold::noop_fold_mac(mac, self)
+    // `visit_mac` is disabled by default. Enable it here.
+    fn visit_mac(&mut self, mac: &mut Mac) {
+        mut_visit::noop_visit_mac(mac, self)
     }
 }
 

src/librustc_data_structures/thin_vec.rs

@@ -39,6 +39,15 @@ impl<T> ::std::ops::Deref for ThinVec<T> {
     }
 }
 
+impl<T> ::std::ops::DerefMut for ThinVec<T> {
+    fn deref_mut(&mut self) -> &mut [T] {
+        match *self {
+            ThinVec(None) => &mut [],
+            ThinVec(Some(ref mut vec)) => vec,
+        }
+    }
+}
+
 impl<T> Extend<T> for ThinVec<T> {
     fn extend<I: IntoIterator<Item = T>>(&mut self, iter: I) {
         match *self {

src/librustc_driver/driver.rs

@@ -32,7 +32,7 @@ use rustc_typeck as typeck;
 use syntax::{self, ast, attr, diagnostics, visit};
 use syntax::early_buffered_lints::BufferedEarlyLint;
 use syntax::ext::base::ExtCtxt;
-use syntax::fold::Folder;
+use syntax::mut_visit::MutVisitor;
 use syntax::parse::{self, PResult};
 use syntax::util::node_count::NodeCounter;
 use syntax::util::lev_distance::find_best_match_for_name;
@@ -1000,12 +1000,12 @@ where
     });
     sess.profiler(|p| p.end_activity(ProfileCategory::Expansion));
 
-    krate = time(sess, "maybe building test harness", || {
+    time(sess, "maybe building test harness", || {
         syntax::test::modify_for_testing(
             &sess.parse_sess,
             &mut resolver,
             sess.opts.test,
-            krate,
+            &mut krate,
             sess.diagnostic(),
             &sess.features_untracked(),
         )
@@ -1014,7 +1014,7 @@ where
     // If we're actually rustdoc then there's no need to actually compile
     // anything, so switch everything to just looping
     if sess.opts.actually_rustdoc {
-        krate = ReplaceBodyWithLoop::new(sess).fold_crate(krate);
+        ReplaceBodyWithLoop::new(sess).visit_crate(&mut krate);
     }
 
     let (has_proc_macro_decls, has_global_allocator) = time(sess, "AST validation", || {
@@ -1045,11 +1045,11 @@ where
 
     if has_global_allocator {
         // Expand global allocators, which are treated as an in-tree proc macro
-        krate = time(sess, "creating allocators", || {
+        time(sess, "creating allocators", || {
             allocator::expand::modify(
                 &sess.parse_sess,
                 &mut resolver,
-                krate,
+                &mut krate,
                 crate_name.to_string(),
                 sess.diagnostic(),
             )

src/librustc_driver/lib.rs

@@ -870,9 +870,9 @@ impl<'a> CompilerCalls<'a> for RustcDefaultCalls {
                 control.after_hir_lowering.stop = Compilation::Stop;
 
                 control.after_parse.callback = box move |state| {
-                    state.krate = Some(pretty::fold_crate(state.session,
-                                                          state.krate.take().unwrap(),
-                                                          ppm));
+                    let mut krate = state.krate.take().unwrap();
+                    pretty::visit_crate(state.session, &mut krate, ppm);
+                    state.krate = Some(krate);
                 };
                 control.after_hir_lowering.callback = box move |state| {
                     pretty::print_after_hir_lowering(state.session,
@@ -891,7 +891,8 @@ impl<'a> CompilerCalls<'a> for RustcDefaultCalls {
                 control.after_parse.stop = Compilation::Stop;
 
                 control.after_parse.callback = box move |state| {
-                    let krate = pretty::fold_crate(state.session, state.krate.take().unwrap(), ppm);
+                    let mut krate = state.krate.take().unwrap();
+                    pretty::visit_crate(state.session, &mut krate, ppm);
                     pretty::print_after_parsing(state.session,
                                                 state.input,
                                                 &krate,

src/librustc_driver/pretty.rs

@@ -16,7 +16,7 @@ use rustc_metadata::cstore::CStore;
 use rustc_mir::util::{write_mir_pretty, write_mir_graphviz};
 
 use syntax::ast::{self, BlockCheckMode};
-use syntax::fold::{self, Folder};
+use syntax::mut_visit::{*, MutVisitor, visit_clobber};
 use syntax::print::{pprust};
 use syntax::print::pprust::PrintState;
 use syntax::ptr::P;
@@ -28,6 +28,7 @@ use smallvec::SmallVec;
 use std::cell::Cell;
 use std::fs::File;
 use std::io::{self, Write};
+use std::ops::DerefMut;
 use std::option;
 use std::path::Path;
 use std::str::FromStr;
@@ -703,42 +704,42 @@ impl<'a> ReplaceBodyWithLoop<'a> {
     }
 }
 
-impl<'a> fold::Folder for ReplaceBodyWithLoop<'a> {
-    fn fold_item_kind(&mut self, i: ast::ItemKind) -> ast::ItemKind {
+impl<'a> MutVisitor for ReplaceBodyWithLoop<'a> {
+    fn visit_item_kind(&mut self, i: &mut ast::ItemKind) {
         let is_const = match i {
             ast::ItemKind::Static(..) | ast::ItemKind::Const(..) => true,
             ast::ItemKind::Fn(ref decl, ref header, _, _) =>
                 header.constness.node == ast::Constness::Const || Self::should_ignore_fn(decl),
             _ => false,
         };
-        self.run(is_const, |s| fold::noop_fold_item_kind(i, s))
+        self.run(is_const, |s| noop_visit_item_kind(i, s))
     }
 
-    fn fold_trait_item(&mut self, i: ast::TraitItem) -> SmallVec<[ast::TraitItem; 1]> {
+    fn flat_map_trait_item(&mut self, i: ast::TraitItem) -> SmallVec<[ast::TraitItem; 1]> {
         let is_const = match i.node {
             ast::TraitItemKind::Const(..) => true,
             ast::TraitItemKind::Method(ast::MethodSig { ref decl, ref header, .. }, _) =>
                 header.constness.node == ast::Constness::Const || Self::should_ignore_fn(decl),
             _ => false,
         };
-        self.run(is_const, |s| fold::noop_fold_trait_item(i, s))
+        self.run(is_const, |s| noop_flat_map_trait_item(i, s))
     }
 
-    fn fold_impl_item(&mut self, i: ast::ImplItem) -> SmallVec<[ast::ImplItem; 1]> {
+    fn flat_map_impl_item(&mut self, i: ast::ImplItem) -> SmallVec<[ast::ImplItem; 1]> {
         let is_const = match i.node {
             ast::ImplItemKind::Const(..) => true,
             ast::ImplItemKind::Method(ast::MethodSig { ref decl, ref header, .. }, _) =>
                 header.constness.node == ast::Constness::Const || Self::should_ignore_fn(decl),
             _ => false,
         };
-        self.run(is_const, |s| fold::noop_fold_impl_item(i, s))
+        self.run(is_const, |s| noop_flat_map_impl_item(i, s))
     }
 
-    fn fold_anon_const(&mut self, c: ast::AnonConst) -> ast::AnonConst {
-        self.run(true, |s| fold::noop_fold_anon_const(c, s))
+    fn visit_anon_const(&mut self, c: &mut ast::AnonConst) {
+        self.run(true, |s| noop_visit_anon_const(c, s))
     }
 
-    fn fold_block(&mut self, b: P<ast::Block>) -> P<ast::Block> {
+    fn visit_block(&mut self, b: &mut P<ast::Block>) {
         fn stmt_to_block(rules: ast::BlockCheckMode,
                          s: Option<ast::Stmt>,
                          sess: &Session) -> ast::Block {
@@ -780,14 +781,14 @@ impl<'a> fold::Folder for ReplaceBodyWithLoop<'a> {
         };
 
         if self.within_static_or_const {
-            fold::noop_fold_block(b, self)
+            noop_visit_block(b, self)
         } else {
-            b.map(|b| {
+            visit_clobber(b.deref_mut(), |b| {
                 let mut stmts = vec![];
                 for s in b.stmts {
                     let old_blocks = self.nested_blocks.replace(vec![]);
 
-                    stmts.extend(self.fold_stmt(s).into_iter().filter(|s| s.is_item()));
+                    stmts.extend(self.flat_map_stmt(s).into_iter().filter(|s| s.is_item()));
 
                     // we put a Some in there earlier with that replace(), so this is valid
                     let new_blocks = self.nested_blocks.take().unwrap();
@@ -818,9 +819,9 @@ impl<'a> fold::Folder for ReplaceBodyWithLoop<'a> {
     }
 
     // in general the pretty printer processes unexpanded code, so
-    // we override the default `fold_mac` method which panics.
-    fn fold_mac(&mut self, mac: ast::Mac) -> ast::Mac {
-        fold::noop_fold_mac(mac, self)
+    // we override the default `visit_mac` method which panics.
+    fn visit_mac(&mut self, mac: &mut ast::Mac) {
+        noop_visit_mac(mac, self)
     }
 }
 
@@ -889,12 +890,9 @@ fn print_flowgraph<'a, 'tcx, W: Write>(variants: Vec<borrowck_dot::Variant>,
     }
 }
 
-pub fn fold_crate(sess: &Session, krate: ast::Crate, ppm: PpMode) -> ast::Crate {
+pub fn visit_crate(sess: &Session, krate: &mut ast::Crate, ppm: PpMode) {
     if let PpmSource(PpmEveryBodyLoops) = ppm {
-        let mut fold = ReplaceBodyWithLoop::new(sess);
-        fold.fold_crate(krate)
-    } else {
-        krate
+        ReplaceBodyWithLoop::new(sess).visit_crate(krate);
     }
 }