perf(transform/styled-components): optimize removal loop (#12284)

Follow-on after #12256. Optimize the `retain` loop for removing quasis and expressions by doing bounds check only once at start, rather than on each turn of the loop.

Author: overlookmotel

crates/oxc_transformer/Cargo.toml

@@ -25,7 +25,7 @@ oxc-browserslist = { workspace = true }
 oxc_allocator = { workspace = true }
 oxc_ast = { workspace = true }
 oxc_ast_visit = { workspace = true }
-oxc_data_structures = { workspace = true, features = ["inline_string", "rope", "stack"] }
+oxc_data_structures = { workspace = true, features = ["assert_unchecked", "inline_string", "rope", "stack"] }
 oxc_diagnostics = { workspace = true }
 oxc_ecmascript = { workspace = true }
 oxc_parser = { workspace = true }

crates/oxc_transformer/src/plugins/styled_components.rs

@@ -65,7 +65,7 @@ use serde::Deserialize;
 
 use oxc_allocator::{TakeIn, Vec as ArenaVec};
 use oxc_ast::{AstBuilder, NONE, ast::*};
-use oxc_data_structures::inline_string::InlineString;
+use oxc_data_structures::{assert_unchecked, inline_string::InlineString};
 use oxc_semantic::SymbolId;
 use oxc_span::SPAN;
 use oxc_traverse::{Ancestor, Traverse};
@@ -1093,11 +1093,26 @@ fn minify_template_literal<'a>(lit: &mut TemplateLiteral<'a>, ast: AstBuilder<'a
     let output_str = unsafe { std::str::from_utf8_unchecked(&output) };
     quasis.last_mut().unwrap().value.raw = ast.atom(output_str);
 
-    // Remove quasis that are marked for removal, and the expressions following them
+    // Remove quasis that are marked for removal, and the expressions following them.
+    // TODO: Remove scopes, symbols, and references for removed `Expression`s.
     if delete_some {
+        // We assert the lengths of `quasis` and `expressions` here so that we can safely remove
+        // the bounds check in the `retain` loop.
+        // It should always be true that `quasis.len() == expressions.len() + 1`
+        // but `quasis.len() >= expressions.len()` is all we need to ensure safety of `assert_unchecked!`
+        // below, and it's a cheaper check.
+        assert!(quasis.len() >= expressions.len());
+
         let mut quasis_iter = quasis.iter();
-        // TODO: Remove scopes, symbols, and references for removed `Expression`.
-        expressions.retain(|_| quasis_iter.next().unwrap().span != REMOVE_SENTINEL);
+        expressions.retain(|_| {
+            // This unchecked assertion removes bounds check in `unwrap`.
+            // SAFETY: We asserted above that there are at least as many quasis as expressions,
+            // so `quasis_iter` cannot be exhausted in this loop.
+            unsafe { assert_unchecked!(!quasis_iter.as_slice().is_empty()) };
+            let quasi = quasis_iter.next().unwrap();
+            quasi.span != REMOVE_SENTINEL
+        });
+
         quasis.retain(|quasi| quasi.span != REMOVE_SENTINEL);
     }
 }