feat(transformer): introduce StatementInjector helper (#6653)

close: #6641

crates/oxc_transformer/src/common/statement_injector.rs

@@ -137,3 +137,142 @@ impl<'a> StatementInjectorStore<'a> {
         *statements = new_statements;
     }
 }
+//! Utility transform to add new statements before or after the specified statement.
+//!
+//! `StatementInjectorStore` contains a `FxHashMap<Address, Vec<AdjacentStatement>>`. It is stored on `TransformCtx`.
+//!
+//! `StatementInjector` transform inserts new statements before or after a statement which is determined by the address of the statement.
+//!
+//! Other transforms can add statements to the store with following methods:
+//!
+//! ```rs
+//! self.ctx.statement_injector.insert_before(address, statement);
+//! self.ctx.statement_injector.insert_after(address, statement);
+//! self.ctx.statement_injector.insert_many_after(address, statements);
+//! ```
+
+use std::cell::RefCell;
+
+use oxc_allocator::{Address, Vec as OxcVec};
+
+use oxc_ast::{address::GetAddress, ast::*};
+use oxc_traverse::{Traverse, TraverseCtx};
+use rustc_hash::FxHashMap;
+
+use crate::TransformCtx;
+
+/// Transform that inserts any statements which have been requested insertion via `StatementInjectorStore`
+pub struct StatementInjector<'a, 'ctx> {
+    ctx: &'ctx TransformCtx<'a>,
+}
+
+impl<'a, 'ctx> StatementInjector<'a, 'ctx> {
+    pub fn new(ctx: &'ctx TransformCtx<'a>) -> Self {
+        Self { ctx }
+    }
+}
+
+impl<'a, 'ctx> Traverse<'a> for StatementInjector<'a, 'ctx> {
+    fn exit_statements(
+        &mut self,
+        statements: &mut OxcVec<'a, Statement<'a>>,
+        ctx: &mut TraverseCtx<'a>,
+    ) {
+        self.ctx.statement_injector.insert_into_statements(statements, ctx);
+    }
+}
+
+enum Direction {
+    Before,
+    After,
+}
+
+struct AdjacentStatement<'a> {
+    stmt: Statement<'a>,
+    direction: Direction,
+}
+
+/// Store for statements to be added to the statements.
+pub struct StatementInjectorStore<'a> {
+    insertions: RefCell<FxHashMap<Address, Vec<AdjacentStatement<'a>>>>,
+}
+
+// Public methods
+impl<'a> StatementInjectorStore<'a> {
+    /// Create new `StatementInjectorStore`.
+    pub fn new() -> Self {
+        Self { insertions: RefCell::new(FxHashMap::default()) }
+    }
+
+    /// Add a statement to be inserted immediately before the target statement.
+    #[expect(dead_code)]
+    pub fn insert_before(&self, target: Address, stmt: Statement<'a>) {
+        let mut insertions = self.insertions.borrow_mut();
+        let adjacent_stmts = insertions.entry(target).or_default();
+        let index = adjacent_stmts
+            .iter()
+            .position(|s| matches!(s.direction, Direction::After))
+            .unwrap_or(adjacent_stmts.len());
+        adjacent_stmts.insert(index, AdjacentStatement { stmt, direction: Direction::Before });
+    }
+
+    /// Add a statement to be inserted immediately after the target statement.
+    #[expect(dead_code)]
+    pub fn insert_after(&self, target: Address, stmt: Statement<'a>) {
+        let mut insertions = self.insertions.borrow_mut();
+        let adjacent_stmts = insertions.entry(target).or_default();
+        adjacent_stmts.push(AdjacentStatement { stmt, direction: Direction::After });
+    }
+
+    /// Add multiple statements to be inserted immediately after the target statement.
+    #[expect(dead_code)]
+    pub fn insert_many_after(&self, target: Address, stmts: Vec<Statement<'a>>) {
+        let mut insertions = self.insertions.borrow_mut();
+        let adjacent_stmts = insertions.entry(target).or_default();
+        adjacent_stmts.extend(
+            stmts.into_iter().map(|stmt| AdjacentStatement { stmt, direction: Direction::After }),
+        );
+    }
+
+    /// Insert statements immediately before / after the target statement.
+    pub(self) fn insert_into_statements(
+        &self,
+        statements: &mut OxcVec<'a, Statement<'a>>,
+        ctx: &mut TraverseCtx<'a>,
+    ) {
+        let mut insertions = self.insertions.borrow_mut();
+        if insertions.is_empty() {
+            return;
+        }
+
+        let new_statement_count = statements
+            .iter()
+            .filter_map(|s| insertions.get(&s.address()).map(Vec::len))
+            .sum::<usize>();
+        if new_statement_count == 0 {
+            return;
+        }
+
+        let mut new_statements = ctx.ast.vec_with_capacity(statements.len() + new_statement_count);
+
+        for stmt in statements.drain(..) {
+            if let Some(mut adjacent_stmts) = insertions.remove(&stmt.address()) {
+                let first_after_stmt_index = adjacent_stmts
+                    .iter()
+                    .position(|s| matches!(s.direction, Direction::After))
+                    .unwrap_or(adjacent_stmts.len());
+                if first_after_stmt_index != 0 {
+                    let right = adjacent_stmts.split_off(first_after_stmt_index);
+                    new_statements.extend(adjacent_stmts.into_iter().map(|s| s.stmt));
+                    new_statements.push(stmt);
+                    new_statements.extend(right.into_iter().map(|s| s.stmt));
+                } else {
+                    new_statements.push(stmt);
+                    new_statements.extend(adjacent_stmts.into_iter().map(|s| s.stmt));
+                }
+            }
+        }
+
+        *statements = new_statements;
+    }
+}