Implement some minor performance optimizations

Author: charliermarsh

parser/Cargo.toml

@@ -32,6 +32,7 @@ unic-emoji-char = "0.9.0"
 unic-ucd-ident  = "0.9.0"
 unicode_names2 = "0.5.0"
 thiserror = "1.0"
+fnv = "1.0.7"
 
 [dev-dependencies]
 insta = "1.14.0"

parser/src/function.rs

@@ -1,7 +1,6 @@
 use crate::ast;
 use crate::error::{LexicalError, LexicalErrorType};
-use ahash::RandomState;
-use std::collections::HashSet;
+use fnv::FnvHashSet;
 
 pub struct ArgumentList {
     pub args: Vec<ast::Expr>,
@@ -54,7 +53,8 @@ pub fn parse_args(func_args: Vec<FunctionArgument>) -> Result<ArgumentList, Lexi
     let mut args = vec![];
     let mut keywords = vec![];
 
-    let mut keyword_names = HashSet::with_capacity_and_hasher(func_args.len(), RandomState::new());
+    let mut keyword_names =
+        FnvHashSet::with_capacity_and_hasher(func_args.len(), Default::default());
     for (name, value) in func_args {
         match name {
             Some((start, end, name)) => {

parser/src/string.rs

@@ -14,6 +14,19 @@ pub fn parse_strings(
     let initial_end = values[0].2;
     let initial_kind = (values[0].1 .1 == StringKind::U).then(|| "u".to_owned());
 
+    // Optimization: fast-track the common case of a single string.
+    if values.len() == 1 && matches!(&values[0].1 .1, StringKind::Normal | StringKind::U) {
+        let value = values.into_iter().last().unwrap().1 .0;
+        return Ok(Expr::new(
+            initial_start,
+            initial_end,
+            ExprKind::Constant {
+                value: Constant::Str(value),
+                kind: initial_kind,
+            },
+        ));
+    }
+
     // Determine whether the list of values contains any f-strings. (If not, we can return a
     // single Constant at the end, rather than a JoinedStr.)
     let mut has_fstring = false;