Use mem::take instead of mem::replace with default

src/liballoc/collections/btree/map.rs

@@ -770,8 +770,8 @@ impl<K: Ord, V> BTreeMap<K, V> {
         }
 
         // First, we merge `self` and `other` into a sorted sequence in linear time.
-        let self_iter = mem::replace(self, BTreeMap::new()).into_iter();
-        let other_iter = mem::replace(other, BTreeMap::new()).into_iter();
+        let self_iter = mem::take(self).into_iter();
+        let other_iter = mem::take(other).into_iter();
         let iter = MergeIter {
             left: self_iter.peekable(),
             right: other_iter.peekable(),

src/liballoc/collections/linked_list.rs

@@ -708,7 +708,7 @@ impl<T> LinkedList<T> {
         let len = self.len();
         assert!(at <= len, "Cannot split off at a nonexistent index");
         if at == 0 {
-            return mem::replace(self, Self::new());
+            return mem::take(self);
         } else if at == len {
             return Self::new();
         }

src/liballoc/lib.rs

@@ -112,6 +112,7 @@
 #![feature(maybe_uninit_extra, maybe_uninit_slice, maybe_uninit_array)]
 #![feature(alloc_layout_extra)]
 #![feature(try_trait)]
+#![feature(mem_take)]
 
 // Allow testing this library
 

src/liballoc/str.rs

@@ -203,7 +203,7 @@ impl ToOwned for str {
     }
 
     fn clone_into(&self, target: &mut String) {
-        let mut b = mem::replace(target, String::new()).into_bytes();
+        let mut b = mem::take(target).into_bytes();
         self.as_bytes().clone_into(&mut b);
         *target = unsafe { String::from_utf8_unchecked(b) }
     }

src/libcore/lib.rs

@@ -126,6 +126,7 @@
 #![feature(adx_target_feature)]
 #![feature(maybe_uninit_slice, maybe_uninit_array)]
 #![feature(external_doc)]
+#![feature(mem_take)]
 
 #[prelude_import]
 #[allow(unused)]

src/libcore/option.rs

@@ -845,7 +845,7 @@ impl<T> Option<T> {
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn take(&mut self) -> Option<T> {
-        mem::replace(self, None)
+        mem::take(self)
     }
 
     /// Replaces the actual value in the option by the value given in parameter,

src/libproc_macro/bridge/buffer.rs

@@ -78,7 +78,7 @@ impl<T: Copy> Buffer<T> {
     }
 
     pub(super) fn take(&mut self) -> Self {
-        mem::replace(self, Self::default())
+        mem::take(self)
     }
 
     pub(super) fn extend_from_slice(&mut self, xs: &[T]) {

src/libproc_macro/lib.rs

@@ -25,6 +25,7 @@
 #![feature(extern_types)]
 #![feature(in_band_lifetimes)]
 #![feature(optin_builtin_traits)]
+#![feature(mem_take)]
 #![feature(non_exhaustive)]
 #![feature(specialization)]
 

src/librustc/hir/lowering.rs

@@ -1278,8 +1278,8 @@ impl<'a> LoweringContext<'a> {
         let was_in_loop_condition = self.is_in_loop_condition;
         self.is_in_loop_condition = false;
 
-        let catch_scopes = mem::replace(&mut self.catch_scopes, Vec::new());
-        let loop_scopes = mem::replace(&mut self.loop_scopes, Vec::new());
+        let catch_scopes = mem::take(&mut self.catch_scopes);
+        let loop_scopes = mem::take(&mut self.loop_scopes);
         let ret = f(self);
         self.catch_scopes = catch_scopes;
         self.loop_scopes = loop_scopes;

src/librustc/infer/nll_relate/mod.rs

@@ -364,7 +364,7 @@ where
         // been fully instantiated and hence the set of scopes we have
         // doesn't matter -- just to be sure, put an empty vector
         // in there.
-        let old_a_scopes = ::std::mem::replace(pair.vid_scopes(self), vec![]);
+        let old_a_scopes = ::std::mem::take(pair.vid_scopes(self));
 
         // Relate the generalized kind to the original one.
         let result = pair.relate_generalized_ty(self, generalized_ty);

src/librustc/infer/outlives/obligations.rs

@@ -112,7 +112,7 @@ impl<'cx, 'tcx> InferCtxt<'cx, 'tcx> {
 
     /// Trait queries just want to pass back type obligations "as is"
     pub fn take_registered_region_obligations(&self) -> Vec<(hir::HirId, RegionObligation<'tcx>)> {
-        ::std::mem::replace(&mut *self.region_obligations.borrow_mut(), vec![])
+        ::std::mem::take(&mut *self.region_obligations.borrow_mut())
     }
 
     /// Process the region obligations that must be proven (during

src/librustc/infer/region_constraints/mod.rs

@@ -410,7 +410,7 @@ impl<'tcx> RegionConstraintCollector<'tcx> {
             *any_unifications = false;
         }
 
-        mem::replace(data, RegionConstraintData::default())
+        mem::take(data)
     }
 
     pub fn data(&self) -> &RegionConstraintData<'tcx> {

src/librustc/lib.rs

@@ -65,6 +65,7 @@
 #![feature(crate_visibility_modifier)]
 #![feature(proc_macro_hygiene)]
 #![feature(log_syntax)]
+#![feature(mem_take)]
 
 #![recursion_limit="512"]
 

src/librustc/middle/region.rs

@@ -1375,7 +1375,7 @@ impl<'tcx> Visitor<'tcx> for RegionResolutionVisitor<'tcx> {
 
         let outer_ec = mem::replace(&mut self.expr_and_pat_count, 0);
         let outer_cx = self.cx;
-        let outer_ts = mem::replace(&mut self.terminating_scopes, FxHashSet::default());
+        let outer_ts = mem::take(&mut self.terminating_scopes);
         self.terminating_scopes.insert(body.value.hir_id.local_id);
 
         if let Some(root_id) = self.cx.root_id {

src/librustc/middle/resolve_lifetime.rs

@@ -18,7 +18,7 @@ use errors::{Applicability, DiagnosticBuilder};
 use rustc_macros::HashStable;
 use std::borrow::Cow;
 use std::cell::Cell;
-use std::mem::replace;
+use std::mem::{replace, take};
 use syntax::ast;
 use syntax::attr;
 use syntax::ptr::P;
@@ -441,7 +441,7 @@ impl<'a, 'tcx> Visitor<'tcx> for LifetimeContext<'a, 'tcx> {
 
     fn visit_nested_body(&mut self, body: hir::BodyId) {
         // Each body has their own set of labels, save labels.
-        let saved = replace(&mut self.labels_in_fn, vec![]);
+        let saved = take(&mut self.labels_in_fn);
         let body = self.tcx.hir().body(body);
         extract_labels(self, body);
         self.with(
@@ -1405,9 +1405,8 @@ impl<'a, 'tcx> LifetimeContext<'a, 'tcx> {
             lifetime_uses,
             ..
         } = self;
-        let labels_in_fn = replace(&mut self.labels_in_fn, vec![]);
-        let xcrate_object_lifetime_defaults =
-            replace(&mut self.xcrate_object_lifetime_defaults, DefIdMap::default());
+        let labels_in_fn = take(&mut self.labels_in_fn);
+        let xcrate_object_lifetime_defaults = take(&mut self.xcrate_object_lifetime_defaults);
         let mut this = LifetimeContext {
             tcx: *tcx,
             map: map,

src/librustc_codegen_llvm/back/archive.rs

@@ -205,8 +205,8 @@ impl<'a> LlvmArchiveBuilder<'a> {
     }
 
     fn build_with_llvm(&mut self, kind: ArchiveKind) -> io::Result<()> {
-        let removals = mem::replace(&mut self.removals, Vec::new());
-        let mut additions = mem::replace(&mut self.additions, Vec::new());
+        let removals = mem::take(&mut self.removals);
+        let mut additions = mem::take(&mut self.additions);
         let mut strings = Vec::new();
         let mut members = Vec::new();
 

src/librustc_codegen_llvm/lib.rs

@@ -21,6 +21,7 @@
 #![feature(link_args)]
 #![feature(static_nobundle)]
 #![feature(trusted_len)]
+#![feature(mem_take)]
 #![deny(rust_2018_idioms)]
 #![deny(internal)]
 #![deny(unused_lifetimes)]

src/librustc_codegen_ssa/back/command.rs

@@ -110,7 +110,7 @@ impl Command {
     }
 
     pub fn take_args(&mut self) -> Vec<OsString> {
-        mem::replace(&mut self.args, Vec::new())
+        mem::take(&mut self.args)
     }
 
     /// Returns a `true` if we're pretty sure that this'll blow OS spawn limits,

src/librustc_codegen_ssa/back/write.rs

@@ -1345,12 +1345,9 @@ fn start_executing_work<B: ExtraBackendMethods>(
                     assert!(!started_lto);
                     started_lto = true;
 
-                    let needs_fat_lto =
-                        mem::replace(&mut needs_fat_lto, Vec::new());
-                    let needs_thin_lto =
-                        mem::replace(&mut needs_thin_lto, Vec::new());
-                    let import_only_modules =
-                        mem::replace(&mut lto_import_only_modules, Vec::new());
+                    let needs_fat_lto = mem::take(&mut needs_fat_lto);
+                    let needs_thin_lto = mem::take(&mut needs_thin_lto);
+                    let import_only_modules = mem::take(&mut lto_import_only_modules);
 
                     for (work, cost) in generate_lto_work(&cgcx, needs_fat_lto,
                                                           needs_thin_lto, import_only_modules) {

src/librustc_codegen_ssa/lib.rs

@@ -10,6 +10,7 @@
 #![feature(in_band_lifetimes)]
 #![feature(nll)]
 #![feature(trusted_len)]
+#![feature(mem_take)]
 #![allow(unused_attributes)]
 #![allow(dead_code)]
 #![deny(rust_2018_idioms)]

src/librustc_mir/borrow_check/mod.rs

@@ -275,7 +275,7 @@ fn do_mir_borrowck<'a, 'tcx>(
     mbcx.analyze_results(&mut state); // entry point for DataflowResultsConsumer
 
     // Convert any reservation warnings into lints.
-    let reservation_warnings = mem::replace(&mut mbcx.reservation_warnings, Default::default());
+    let reservation_warnings = mem::take(&mut mbcx.reservation_warnings);
     for (_, (place, span, location, bk, borrow)) in reservation_warnings {
         let mut initial_diag =
             mbcx.report_conflicting_borrow(location, (&place, span), bk, &borrow);

src/librustc_mir/build/matches/simplify.rs

@@ -28,7 +28,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                                     candidate: &mut Candidate<'pat, 'tcx>) {
         // repeatedly simplify match pairs until fixed point is reached
         loop {
-            let match_pairs = mem::replace(&mut candidate.match_pairs, vec![]);
+            let match_pairs = mem::take(&mut candidate.match_pairs);
             let mut changed = false;
             for match_pair in match_pairs {
                 match self.simplify_match_pair(match_pair, candidate) {

src/librustc_mir/lib.rs

@@ -24,6 +24,7 @@ Rust MIR: a lowered representation of Rust. Also: an experiment!
 #![feature(slice_concat_ext)]
 #![feature(trusted_len)]
 #![feature(try_blocks)]
+#![feature(mem_take)]
 
 #![recursion_limit="256"]
 

src/librustc_mir/util/def_use.rs

@@ -31,7 +31,7 @@ impl DefUseAnalysis {
         self.clear();
 
         let mut finder = DefUseFinder {
-            info: mem::replace(&mut self.info, IndexVec::new()),
+            info: mem::take(&mut self.info),
         };
         finder.visit_body(body);
         self.info = finder.info

src/librustc_resolve/lib.rs

@@ -4,6 +4,7 @@
 
 #![feature(crate_visibility_modifier)]
 #![feature(label_break_value)]
+#![feature(mem_take)]
 #![feature(nll)]
 #![feature(rustc_diagnostic_macros)]
 #![cfg_attr(bootstrap, feature(type_alias_enum_variants))]

src/librustc_resolve/macros.rs

@@ -946,7 +946,7 @@ impl<'a> Resolver<'a> {
         };
 
         let macro_resolutions =
-            mem::replace(&mut *module.multi_segment_macro_resolutions.borrow_mut(), Vec::new());
+            mem::take(&mut *module.multi_segment_macro_resolutions.borrow_mut());
         for (mut path, path_span, kind, parent_scope, initial_res) in macro_resolutions {
             // FIXME: Path resolution will ICE if segment IDs present.
             for seg in &mut path { seg.id = None; }
@@ -973,7 +973,7 @@ impl<'a> Resolver<'a> {
         }
 
         let macro_resolutions =
-            mem::replace(&mut *module.single_segment_macro_resolutions.borrow_mut(), Vec::new());
+            mem::take(&mut *module.single_segment_macro_resolutions.borrow_mut());
         for (ident, kind, parent_scope, initial_binding) in macro_resolutions {
             match self.early_resolve_ident_in_lexical_scope(ident, ScopeSet::Macro(kind),
                                                             &parent_scope, true, true, ident.span) {
@@ -998,7 +998,7 @@ impl<'a> Resolver<'a> {
             }
         }
 
-        let builtin_attrs = mem::replace(&mut *module.builtin_attrs.borrow_mut(), Vec::new());
+        let builtin_attrs = mem::take(&mut *module.builtin_attrs.borrow_mut());
         for (ident, parent_scope) in builtin_attrs {
             let _ = self.early_resolve_ident_in_lexical_scope(
                 ident, ScopeSet::Macro(MacroKind::Attr), &parent_scope, true, true, ident.span

src/librustc_resolve/resolve_imports.rs

@@ -682,7 +682,7 @@ impl<'a, 'b> ImportResolver<'a, 'b> {
         let mut prev_num_indeterminates = self.indeterminate_imports.len() + 1;
         while self.indeterminate_imports.len() < prev_num_indeterminates {
             prev_num_indeterminates = self.indeterminate_imports.len();
-            for import in mem::replace(&mut self.indeterminate_imports, Vec::new()) {
+            for import in mem::take(&mut self.indeterminate_imports) {
                 match self.resolve_import(&import) {
                     true => self.determined_imports.push(import),
                     false => self.indeterminate_imports.push(import),

src/librustc_typeck/check/method/probe.rs

@@ -970,9 +970,9 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
 
         debug!("pick: actual search failed, assemble diagnotics");
 
-        let static_candidates = mem::replace(&mut self.static_candidates, vec![]);
+        let static_candidates = mem::take(&mut self.static_candidates);
         let private_candidate = self.private_candidate.take();
-        let unsatisfied_predicates = mem::replace(&mut self.unsatisfied_predicates, vec![]);
+        let unsatisfied_predicates = mem::take(&mut self.unsatisfied_predicates);
 
         // things failed, so lets look at all traits, for diagnostic purposes now:
         self.reset();

src/librustc_typeck/lib.rs

@@ -69,6 +69,7 @@ This API is completely unstable and subject to change.
 #![feature(slice_patterns)]
 #![feature(never_type)]
 #![feature(inner_deref)]
+#![feature(mem_take)]
 
 #![recursion_limit="256"]
 

src/librustdoc/clean/mod.rs

@@ -4408,7 +4408,7 @@ pub fn enter_impl_trait<F, R>(cx: &DocContext<'_>, f: F) -> R
 where
     F: FnOnce() -> R,
 {
-    let old_bounds = mem::replace(&mut *cx.impl_trait_bounds.borrow_mut(), Default::default());
+    let old_bounds = mem::take(&mut *cx.impl_trait_bounds.borrow_mut());
     let r = f();
     assert!(cx.impl_trait_bounds.borrow().is_empty());
     *cx.impl_trait_bounds.borrow_mut() = old_bounds;

src/librustdoc/clean/simplify.rs

@@ -131,7 +131,7 @@ pub fn ty_params(mut params: Vec<clean::GenericParamDef>) -> Vec<clean::GenericP
     for param in &mut params {
         match param.kind {
             clean::GenericParamDefKind::Type { ref mut bounds, .. } => {
-                *bounds = ty_bounds(mem::replace(bounds, Vec::new()));
+                *bounds = ty_bounds(mem::take(bounds));
             }
             _ => panic!("expected only type parameters"),
         }

src/librustdoc/html/render.rs

@@ -660,7 +660,7 @@ pub fn run(mut krate: clean::Crate,
         deref_trait_did,
         deref_mut_trait_did,
         owned_box_did,
-        masked_crates: mem::replace(&mut krate.masked_crates, Default::default()),
+        masked_crates: mem::take(&mut krate.masked_crates),
         param_names: external_param_names,
         aliases: Default::default(),
     };

src/librustdoc/lib.rs

@@ -21,6 +21,7 @@
 #![feature(drain_filter)]
 #![feature(inner_deref)]
 #![feature(never_type)]
+#![feature(mem_take)]
 
 #![recursion_limit="256"]
 

src/librustdoc/passes/collapse_docs.rs

@@ -4,7 +4,7 @@ use crate::fold;
 use crate::fold::{DocFolder};
 use crate::passes::Pass;
 
-use std::mem::replace;
+use std::mem::take;
 
 pub const COLLAPSE_DOCS: Pass = Pass {
     name: "collapse-docs",
@@ -46,7 +46,7 @@ fn collapse(doc_strings: &mut Vec<DocFragment>) {
     let mut docs = vec![];
     let mut last_frag: Option<DocFragment> = None;
 
-    for frag in replace(doc_strings, vec![]) {
+    for frag in take(doc_strings) {
         if let Some(mut curr_frag) = last_frag.take() {
             let curr_kind = curr_frag.kind();
             let new_kind = frag.kind();

src/libstd/lib.rs

@@ -272,6 +272,7 @@
 #![feature(libc)]
 #![feature(link_args)]
 #![feature(linkage)]
+#![feature(mem_take)]
 #![feature(needs_panic_runtime)]
 #![feature(never_type)]
 #![feature(nll)]

src/libstd/panicking.rs

@@ -364,7 +364,7 @@ fn continue_panic_fmt(info: &PanicInfo<'_>) -> ! {
 
     unsafe impl<'a> BoxMeUp for PanicPayload<'a> {
         fn box_me_up(&mut self) -> *mut (dyn Any + Send) {
-            let contents = mem::replace(self.fill(), String::new());
+            let contents = mem::take(self.fill());
             Box::into_raw(Box::new(contents))
         }
 

src/libstd/sync/mpsc/sync.rs

@@ -383,7 +383,7 @@ impl<T> Packet<T> {
         // needs to be careful to destroy the data *outside* of the lock to
         // prevent deadlock.
         let _data = if guard.cap != 0 {
-            mem::replace(&mut guard.buf.buf, Vec::new())
+            mem::take(&mut guard.buf.buf)
         } else {
             Vec::new()
         };

src/libstd/sys/windows/pipe.rs

@@ -342,7 +342,7 @@ impl<'a> Drop for AsyncPipe<'a> {
         // If anything here fails, there's not really much we can do, so we leak
         // the buffer/OVERLAPPED pointers to ensure we're at least memory safe.
         if self.pipe.cancel_io().is_err() || self.result().is_err() {
-            let buf = mem::replace(self.dst, Vec::new());
+            let buf = mem::take(self.dst);
             let overlapped = Box::new(unsafe { mem::zeroed() });
             let overlapped = mem::replace(&mut self.overlapped, overlapped);
             mem::forget((buf, overlapped));