feat(minifier): add skeleton for RemoveDeadCode ast pass (#3802)

crates/oxc_minifier/src/ast_passes/mod.rs

@@ -1,3 +1,7 @@
+#![allow(clippy::wildcard_imports)]
+
+mod remove_dead_code;
 mod remove_parens;
 
+pub use remove_dead_code::RemoveDeadCode;
 pub use remove_parens::RemoveParens;

crates/oxc_minifier/src/ast_passes/remove_dead_code.rs

@@ -0,0 +1,45 @@
+use oxc_allocator::Allocator;
+use oxc_ast::{ast::*, visit::walk_mut, AstBuilder, VisitMut};
+use oxc_span::SPAN;
+
+/// Remove Dead Code from the AST.
+///
+/// Terser option: `dead_code: true`.
+#[derive(Clone, Copy)]
+pub struct RemoveDeadCode<'a> {
+    ast: AstBuilder<'a>,
+}
+
+impl<'a> RemoveDeadCode<'a> {
+    pub fn new(allocator: &'a Allocator) -> Self {
+        Self { ast: AstBuilder::new(allocator) }
+    }
+
+    pub fn build(&mut self, program: &mut Program<'a>) {
+        self.visit_program(program);
+    }
+
+    pub fn remove_if(&mut self, stmt: &mut Statement<'a>) {
+        let Statement::IfStatement(if_stmt) = stmt else { return };
+        match if_stmt.test.get_boolean_value() {
+            Some(true) => {
+                *stmt = self.ast.move_statement(&mut if_stmt.consequent);
+            }
+            Some(false) => {
+                *stmt = if let Some(alternate) = &mut if_stmt.alternate {
+                    self.ast.move_statement(alternate)
+                } else {
+                    self.ast.empty_statement(SPAN)
+                };
+            }
+            _ => {}
+        }
+    }
+}
+
+impl<'a> VisitMut<'a> for RemoveDeadCode<'a> {
+    fn visit_statement(&mut self, stmt: &mut Statement<'a>) {
+        self.remove_if(stmt);
+        walk_mut::walk_statement_mut(self, stmt);
+    }
+}

crates/oxc_minifier/src/ast_passes/remove_parens.rs

@@ -1,7 +1,9 @@
 use oxc_allocator::{Allocator, Vec};
-use oxc_ast::visit::walk_mut::{walk_expression_mut, walk_statements_mut};
-#[allow(clippy::wildcard_imports)]
-use oxc_ast::{ast::*, AstBuilder, VisitMut};
+use oxc_ast::{
+    ast::*,
+    visit::walk_mut::{walk_expression_mut, walk_statements_mut},
+    AstBuilder, VisitMut,
+};
 
 /// Remove Parenthesized Expression from the AST.
 #[derive(Clone, Copy)]

crates/oxc_minifier/src/lib.rs

@@ -8,7 +8,7 @@ use oxc_allocator::Allocator;
 use oxc_ast::ast::Program;
 
 pub use crate::{
-    ast_passes::RemoveParens,
+    ast_passes::{RemoveDeadCode, RemoveParens},
     compressor::{CompressOptions, Compressor},
     mangler::ManglerBuilder,
 };

crates/oxc_minifier/tests/oxc/mod.rs

@@ -1,3 +1,4 @@
 mod code_removal;
 mod folding;
 mod precedence;
+mod remove_dead_code;

crates/oxc_minifier/tests/oxc/remove_dead_code.rs

@@ -0,0 +1,26 @@
+use oxc_allocator::Allocator;
+use oxc_codegen::WhitespaceRemover;
+use oxc_minifier::RemoveDeadCode;
+use oxc_parser::Parser;
+use oxc_span::SourceType;
+
+fn minify(source_text: &str) -> String {
+    let source_type = SourceType::default();
+    let allocator = Allocator::default();
+    let ret = Parser::new(&allocator, source_text, source_type).parse();
+    let program = allocator.alloc(ret.program);
+    RemoveDeadCode::new(&allocator).build(program);
+    WhitespaceRemover::new().build(program).source_text
+}
+
+pub(crate) fn test(source_text: &str, expected: &str) {
+    let minified = minify(source_text);
+    assert_eq!(minified, expected, "for source {source_text}");
+}
+
+#[test]
+fn remove_dead_code() {
+    test("if (true) { foo }", "{foo}");
+    test("if (true) { foo } else { bar }", "{foo}");
+    test("if (false) { foo } else { bar }", "{bar}");
+}