eliben/code-for-blog:optimize-2022-rust-tablegen-lexer

a.patch

commit 3fdd4fa036d925e1d2bba15f23db0790512729cf
Author: Utkarsh Kukreti <utkarshkukreti@gmail.com>
Date:   Tue May 31 01:01:16 2022 +0530

    optimize lexer/slice_all_push: ~4.2ms -> ~3.5ms (-18%)
    
    `CharIndices` implements a cheap `Clone` operation which is much faster
    than wrapping the whole iterator in `Peekable`, especially when the
    `peek()` operation is rarely used.

diff --git a/2022/rust-tablegen-lexer/src/slice.rs b/2022/rust-tablegen-lexer/src/slice.rs
index f87e3c4..6069b2d 100644
--- a/2022/rust-tablegen-lexer/src/slice.rs
+++ b/2022/rust-tablegen-lexer/src/slice.rs
@@ -55,7 +55,7 @@ pub struct Token<'source> {
 // the Iterator trait for a higher-level iterator API.
 pub struct Lexer<'source> {
     input: &'source str,
-    iter: Peekable<CharIndices<'source>>,
+    iter: CharIndices<'source>,
 
     // c is the last char taken from iter, and ci is its offset in the input.
     c: char,
@@ -66,13 +66,12 @@ pub struct Lexer<'source> {
 }
 
 use core::str::CharIndices;
-use std::iter::Peekable;
 
 impl<'source> Lexer<'source> {
     pub fn new(input: &'source str) -> Self {
         let mut lex = Self {
             input,
-            iter: input.char_indices().peekable(),
+            iter: input.char_indices(),
             c: '\x00',
             ci: 0,
             error: false,
@@ -104,8 +103,8 @@ impl<'source> Lexer<'source> {
             '-' => TokenValue::Minus,
             '*' => TokenValue::Multiply,
             '/' => {
-                if let Some((_, chr)) = self.iter.peek() {
-                    if *chr == '/' {
+                if let Some((_, chr)) = self.iter.clone().next() {
+                    if chr == '/' {
                         return self.scan_comment();
                     }
                 }