compiler/rust: Add a DepthFirstSearch trait

There are four depth first searches in cfg.rs.  This adds a DFS
abstraction which we can later optimize.

Reviewed-by: Mel Henning <mhenning@darkrefraction.com>
Reviewed-by: Lorenzo Rossi <git@rossilorenzo.dev>
Reviewed-by: Mary Guillemard <mary@mary.zone>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/37536>

src/compiler/rust/depth_first_search.rs

@@ -0,0 +1,54 @@
+// Copyright © 2025 Collabora, Ltd.
+// SPDX-License-Identifier: MIT
+
+use crate::bitset::BitSet;
+
+/// A trait implementing a depth-first search over a graph
+pub trait DepthFirstSearch {
+    type ChildIter;
+
+    /// Pre-visit a node.  This is called before any children or edges are
+    /// visited.  Returns an iterator to this node's children in the graph.
+    fn pre(&mut self, id: usize) -> Self::ChildIter;
+
+    /// Visit an edge.  An edge is visited before the child at the end of that
+    /// edge is pre-visited.  Every edge is visited, even if if the node this
+    /// edge points to has already been visited.
+    fn edge(&mut self, _parent: usize, _child: usize) {
+        // Does nothing by default
+    }
+
+    /// Post-visit a node.  This is called after all the children have been
+    /// visited.
+    fn post(&mut self, _id: usize) {
+        // Does nothing by default
+    }
+}
+
+fn dfs_impl<I, D>(dfs: &mut D, seen: &mut BitSet, id: usize)
+where
+    I: Iterator<Item = usize>,
+    D: DepthFirstSearch<ChildIter = I>,
+{
+    if seen.contains(id) {
+        return;
+    }
+
+    seen.insert(id);
+
+    let children = dfs.pre(id);
+    for child in children {
+        dfs.edge(id, child);
+        dfs_impl(dfs, seen, child);
+    }
+    dfs.post(id);
+}
+
+pub fn dfs<I, D>(dfs: &mut D, start: usize)
+where
+    I: Iterator<Item = usize>,
+    D: DepthFirstSearch<ChildIter = I>,
+{
+    let mut seen = BitSet::new();
+    dfs_impl(dfs, &mut seen, start);
+}

src/compiler/rust/lib.rs

@@ -6,6 +6,7 @@ pub mod bindings;
 pub mod bitset;
 pub mod cfg;
 pub mod dataflow;
+pub mod depth_first_search;
 pub mod memstream;
 pub mod nir;
 pub mod nir_instr_printer;

src/compiler/rust/meson.build

@@ -6,6 +6,7 @@ _compiler_rs_sources = [
   'bitset.rs',
   'cfg.rs',
   'dataflow.rs',
+  'depth_first_search.rs',
   'memstream.rs',
   'nir_instr_printer.rs',
   'nir.rs',