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Rollup merge of #55383 - nnethercote:coerce_unsized-SmallVec, r=davidtwco

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Use `SmallVec` for the queue in `coerce_unsized`.

This reduces the number of allocations done for the `tuple-stress`
benchmark by 4%.
This commit is contained in:
kennytm 2018-10-26 23:10:06 +08:00
commit 5a16579d08
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1 changed files with 20 additions and 16 deletions
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36
src/librustc_typeck/check/coercion.rs
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@ -61,7 +61,7 @@
//! we may want to adjust precisely when coercions occur.
use check::{FnCtxt, Needs};
use errors::DiagnosticBuilder;
use rustc::hir;
use rustc::hir::def_id::DefId;
use rustc::infer::{Coercion, InferResult, InferOk};
@ -72,14 +72,12 @@ use rustc::ty::{self, TypeAndMut, Ty, ClosureSubsts};
use rustc::ty::fold::TypeFoldable;
use rustc::ty::error::TypeError;
use rustc::ty::relate::RelateResult;
use errors::DiagnosticBuilder;
use smallvec::{smallvec, SmallVec};
use std::ops::Deref;
use syntax::feature_gate;
use syntax::ptr::P;
use syntax_pos;
use std::collections::VecDeque;
use std::ops::Deref;
struct Coerce<'a, 'gcx: 'a + 'tcx, 'tcx: 'a> {
fcx: &'a FnCtxt<'a, 'gcx, 'tcx>,
cause: ObligationCause<'tcx>,
@ -536,18 +534,23 @@ impl<'f, 'gcx, 'tcx> Coerce<'f, 'gcx, 'tcx> {
let mut selcx = traits::SelectionContext::new(self);
// Use a FIFO queue for this custom fulfillment procedure. (The maximum
// length is almost always 1.)
let mut queue = VecDeque::with_capacity(1);
// Create an obligation for `Source: CoerceUnsized<Target>`.
let cause = ObligationCause::misc(self.cause.span, self.body_id);
queue.push_back(self.tcx.predicate_for_trait_def(self.fcx.param_env,
cause,
coerce_unsized_did,
0,
coerce_source,
&[coerce_target.into()]));
// Use a FIFO queue for this custom fulfillment procedure.
//
// A Vec (or SmallVec) is not a natural choice for a queue. However,
// this code path is hot, and this queue usually has a max length of 1
// and almost never more than 3. By using a SmallVec we avoid an
// allocation, at the (very small) cost of (occasionally) having to
// shift subsequent elements down when removing the front element.
let mut queue: SmallVec<[_; 4]> =
smallvec![self.tcx.predicate_for_trait_def(self.fcx.param_env,
cause,
coerce_unsized_did,
0,
coerce_source,
&[coerce_target.into()])];
let mut has_unsized_tuple_coercion = false;
@ -555,7 +558,8 @@ impl<'f, 'gcx, 'tcx> Coerce<'f, 'gcx, 'tcx> {
// emitting a coercion in cases like `Foo<$1>` -> `Foo<$2>`, where
// inference might unify those two inner type variables later.
let traits = [coerce_unsized_did, unsize_did];
while let Some(obligation) = queue.pop_front() {
while !queue.is_empty() {
let obligation = queue.remove(0);
debug!("coerce_unsized resolve step: {:?}", obligation);
let trait_ref = match obligation.predicate {
ty::Predicate::Trait(ref tr) if traits.contains(&tr.def_id()) => {