mk_graph: add new DOT backend behind env gate with GraphBuilder stubs

Introduce a new DOT implementation wired through the GraphBuilder abstraction and gate it behind `SMIR_DOT_NEW`.

Add `to_dot_file_new` entry point and a `DOTBuilder` with stubbed trait method implementations to establish the integration path.

This enables incremental migration to the new traversal-based architecture while preserving the existing DOT backend as default.

Author: 0xh4ty

src/mk_graph/mod.rs

@@ -28,7 +28,13 @@ pub use util::GraphLabelString;
 
 /// Entry point to write the DOT file
 pub fn emit_dotfile(tcx: TyCtxt<'_>) {
-    let smir_dot = collect_smir(tcx).to_dot_file();
+    let smir = collect_smir(tcx);
+
+    let smir_dot = if std::env::var("SMIR_DOT_NEW").is_ok() {
+        smir.to_dot_file_new()
+    } else {
+        smir.to_dot_file()
+    };
 
     match mir_output_path(tcx, "smir.dot") {
         OutputDest::Stdout => {

src/mk_graph/output/dot.rs

@@ -13,6 +13,9 @@ use crate::MonoItemKind;
 use crate::mk_graph::context::GraphContext;
 use crate::mk_graph::util::{block_name, is_unqualified, name_lines, short_name, GraphLabelString};
 
+use crate::mk_graph::traverse::render_graph;
+use crate::mk_graph::traverse::{GraphBuilder, RenderedFunction};
+
 impl SmirJson {
     /// Convert the MIR to DOT (Graphviz) format
     pub fn to_dot_file(self) -> String {
@@ -306,3 +309,56 @@ impl SmirJson {
         String::from_utf8(bytes).expect("Error converting dot file")
     }
 }
+
+// =============================================================================
+// DOT Builder
+// =============================================================================
+
+pub struct DOTBuilder {
+    buf: String,
+}
+
+impl DOTBuilder {
+    pub fn new() -> Self {
+        Self { buf: String::new() }
+    }
+}
+
+impl Default for DOTBuilder {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+impl GraphBuilder for DOTBuilder {
+    type Output = String;
+
+    fn begin_graph(&mut self, _name: &str) {}
+
+    fn alloc_legend(&mut self, _lines: &[String]) {}
+
+    fn type_legend(&mut self, _lines: &[String]) {}
+
+    fn external_function(&mut self, _id: &str, _name: &str) {}
+
+    fn render_function(&mut self, _func: &RenderedFunction) {}
+
+    fn static_item(&mut self, _id: &str, _name: &str) {}
+
+    fn asm_item(&mut self, _id: &str, _content: &str) {}
+
+    fn finish(self) -> Self::Output {
+        self.buf
+    }
+}
+
+// =============================================================================
+// Public entry point (new)
+// =============================================================================
+
+impl SmirJson {
+    /// Convert the MIR to DOT using GraphBuilder traversal
+    pub fn to_dot_file_new(&self) -> String {
+        render_graph(self, DOTBuilder::new())
+    }
+}