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Remove TokenStreamBuilder.

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`TokenStreamBuilder` exists to concatenate multiple `TokenStream`s
together. This commit removes it, and moves the concatenation
functionality directly into `TokenStream`, via two new methods
`push_tree` and `push_stream`. This makes things both simpler and
faster.

`push_tree` is particularly important. `TokenStreamBuilder` only had a
single `push` method, which pushed a stream. But in practice most of the
time we push a single token tree rather than a stream, and `push_tree`
avoids the need to build a token stream with a single entry (which
requires two allocations, one for the `Lrc` and one for the `Vec`).

The main `push_tree` use arises from a change to one of the `ToInternal`
impls in `proc_macro_server.rs`. It now returns a `SmallVec` instead of
a `TokenStream`. This return value is then iterated over by
`concat_trees`, which does `push_tree` on each element. Furthermore, the
use of `SmallVec` avoids more allocations, because there is always only
one or two token trees.

Note: the removed `TokenStreamBuilder::push` method had some code to
deal with a quadratic blowup case from #57735. This commit removes the
code. I tried and failed to reproduce the blowup from that PR, before
and after this change. Various other changes have happened to
`TokenStreamBuilder` in the meantime, so I suspect the original problem
is no longer relevant, though I don't have proof of this. Generally
speaking, repeatedly extending a `Vec` without pre-determining its
capacity is *not* quadratic. It's also incredibly common, within rustc
and many other Rust programs, so if there were performance problems
there you'd think it would show up in other places, too.
This commit is contained in:
Nicholas Nethercote
2022-10-05 10:38:15 +11:00
parent 1e8dc45fb5
commit 1e848a564b
3 changed files with 82 additions and 110 deletions
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18
compiler/rustc_expand/src/tokenstream/tests.rs
View File

@@ -1,7 +1,7 @@
use crate::tests::string_to_stream;
use rustc_ast::token;
use rustc_ast::tokenstream::{TokenStream, TokenStreamBuilder};
use rustc_ast::tokenstream::{TokenStream, TokenTree};
use rustc_span::create_default_session_globals_then;
use rustc_span::{BytePos, Span, Symbol};
@@ -19,10 +19,9 @@ fn test_concat() {
let test_res = string_to_ts("foo::bar::baz");
let test_fst = string_to_ts("foo::bar");
let test_snd = string_to_ts("::baz");
let mut builder = TokenStreamBuilder::new();
builder.push(test_fst);
builder.push(test_snd);
let eq_res = builder.build();
let mut eq_res = TokenStream::default();
eq_res.push_stream(test_fst);
eq_res.push_stream(test_snd);
assert_eq!(test_res.trees().count(), 5);
assert_eq!(eq_res.trees().count(), 5);
assert_eq!(test_res.eq_unspanned(&eq_res), true);
@@ -99,11 +98,10 @@ fn test_is_empty() {
#[test]
fn test_dotdotdot() {
create_default_session_globals_then(|| {
let mut builder = TokenStreamBuilder::new();
builder.push(TokenStream::token_joint(token::Dot, sp(0, 1)));
builder.push(TokenStream::token_joint(token::Dot, sp(1, 2)));
builder.push(TokenStream::token_alone(token::Dot, sp(2, 3)));
let stream = builder.build();
let mut stream = TokenStream::default();
stream.push_tree(TokenTree::token_joint(token::Dot, sp(0, 1)));
stream.push_tree(TokenTree::token_joint(token::Dot, sp(1, 2)));
stream.push_tree(TokenTree::token_alone(token::Dot, sp(2, 3)));
assert!(stream.eq_unspanned(&string_to_ts("...")));
assert_eq!(stream.trees().count(), 1);
})