[librsvg] node.rs: Add a Node::with_impl() method, to access the NodeTrait impl out of band

[Federico Mena Quintero <federico src gnome org>]

commit 1112a20c355043cb962b0f3d4c89ee4c0e3978ae
Author: Federico Mena Quintero <federico gnome org>
Date:   Mon Feb 27 16:35:46 2017 -0600

    node.rs: Add a Node::with_impl() method, to access the NodeTrait impl out of band
    
    This is a bit sketchy, because we use the downcast-rs crate to actually
    be able to downcast a our "node_impl: Box<NodeTrait>" field into the
    concrete implementation type.  At first we'll use this in the marker.rs
    implementation, as markers are not rendered from the normal ::draw()
    method.  Maybe we'll find a way to make this cleaner at some point.

 rust/Cargo.lock  |    7 +++++++
 rust/Cargo.toml  |    4 ++++
 rust/src/lib.rs  |    7 +++++++
 rust/src/node.rs |   14 +++++++++++++-
 4 files changed, 31 insertions(+), 1 deletions(-)
---
diff --git a/rust/Cargo.lock b/rust/Cargo.lock
index 848b400..ef615c8 100644
--- a/rust/Cargo.lock
+++ b/rust/Cargo.lock
@@ -5,6 +5,7 @@ dependencies = [
  "bitflags 0.7.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "cairo-rs 0.1.1 (git+https://github.com/gtk-rs/cairo.git)",
  "cairo-sys-rs 0.3.2 (git+https://github.com/gtk-rs/cairo.git)",
+ "downcast-rs 1.0.0 (git+https://github.com/marcianx/downcast-rs)",
  "glib 0.1.1 (git+https://github.com/gtk-rs/glib)",
  "libc 0.2.20 (registry+https://github.com/rust-lang/crates.io-index)",
  "nom 2.1.0 (git+https://github.com/federicomenaquintero/nom.git)",
@@ -53,6 +54,11 @@ dependencies = [
 ]
 
 [[package]]
+name = "downcast-rs"
+version = "1.0.0"
+source = "git+https://github.com/marcianx/downcast-rs#5ace99daf704b91f2eeab366b67b78b368d51884";
+
+[[package]]
 name = "glib"
 version = "0.1.1"
 source = "git+https://github.com/gtk-rs/glib#91f2ceb47cdc2d3b209ce3b6c96e3de928223842";
@@ -111,6 +117,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index";
 "checksum c_vec 1.2.0 (registry+https://github.com/rust-lang/crates.io-index)" = 
"0059f5a658f62a4bd3937a7addc52ccfda144b75cce7a92b187e528629cdc507"
 "checksum cairo-rs 0.1.1 (git+https://github.com/gtk-rs/cairo.git)" = "<none>"
 "checksum cairo-sys-rs 0.3.2 (git+https://github.com/gtk-rs/cairo.git)" = "<none>"
+"checksum downcast-rs 1.0.0 (git+https://github.com/marcianx/downcast-rs)" = "<none>"
 "checksum glib 0.1.1 (git+https://github.com/gtk-rs/glib)" = "<none>"
 "checksum glib-sys 0.3.2 (git+https://github.com/gtk-rs/sys)" = "<none>"
 "checksum gobject-sys 0.3.2 (git+https://github.com/gtk-rs/sys)" = "<none>"
diff --git a/rust/Cargo.toml b/rust/Cargo.toml
index 9f9b6f6..25eb419 100644
--- a/rust/Cargo.toml
+++ b/rust/Cargo.toml
@@ -28,6 +28,10 @@ version = "0.1.1"
 git = "https://github.com/federicomenaquintero/nom.git";
 features = ["verbose-errors"]
 
+[dependencies.downcast-rs]
+git = "https://github.com/marcianx/downcast-rs";
+version = "1.0.0"
+
 [lib]
 name = "rsvg_internals"
 crate-type = ["staticlib"]
diff --git a/rust/src/lib.rs b/rust/src/lib.rs
index ae2e5b2..8b7fcd4 100644
--- a/rust/src/lib.rs
+++ b/rust/src/lib.rs
@@ -4,6 +4,9 @@ extern crate bitflags;
 #[macro_use]
 extern crate nom;
 
+#[macro_use]
+extern crate downcast_rs;
+
 pub use aspect_ratio::{
     FitMode,
     AlignMode,
@@ -43,6 +46,10 @@ pub use length::{
     rsvg_length_hand_normalize,
 };
 
+pub use marker::{
+    rsvg_node_marker_new,
+};
+
 pub use node::{
     rsvg_node_get_type,
     rsvg_node_get_parent,
diff --git a/rust/src/node.rs b/rust/src/node.rs
index 6cab60a..f845871 100644
--- a/rust/src/node.rs
+++ b/rust/src/node.rs
@@ -5,6 +5,8 @@ use std::rc::Weak;
 use std::cell::RefCell;
 use std::ptr;
 
+use downcast_rs::*;
+
 use drawing_ctx::RsvgDrawingCtx;
 use drawing_ctx;
 
@@ -25,12 +27,14 @@ pub type RsvgNode = Rc<Node>;
  */
 pub enum RsvgCNodeImpl {}
 
-pub trait NodeTrait {
+pub trait NodeTrait: Downcast {
     fn set_atts (&self, node: &RsvgNode, handle: *const RsvgHandle, pbag: *const RsvgPropertyBag);
     fn draw (&self, node: &RsvgNode, draw_ctx: *const RsvgDrawingCtx, dominate: i32);
     fn get_c_impl (&self) -> *const RsvgCNodeImpl;
 }
 
+impl_downcast! (NodeTrait);
+
 pub struct Node {
     node_type: NodeType,
     parent:    Option<Weak<Node>>,       // optional; weak ref to parent
@@ -133,6 +137,14 @@ impl Node {
     pub fn get_c_impl (&self) -> *const RsvgCNodeImpl {
         self.node_impl.get_c_impl ()
     }
+
+    pub fn with_impl<T: NodeTrait, F: FnOnce (&T)> (&self, f: F) {
+        if let Some (t) = (&self.node_impl).downcast_ref::<T> () {
+            f (t);
+        } else {
+            panic! ("could not downcast");
+        }
+    }
 }
 
 extern "C" {