From 177ef22673bca632005a534b590336eee7db444c Mon Sep 17 00:00:00 2001
From: Chinedu Francis Nwafili <frankie.nwafili@gmail.com>
Date: Mon, 14 Jan 2019 17:45:43 -0500
Subject: [PATCH 1/2] Add maybe adjust function

---
 crates/webidl/src/util.rs | 23 ++++++++++++++++++++++-
 1 file changed, 22 insertions(+), 1 deletion(-)

diff --git a/crates/webidl/src/util.rs b/crates/webidl/src/util.rs
index 412e45d7..629b94e8 100644
--- a/crates/webidl/src/util.rs
+++ b/crates/webidl/src/util.rs
@@ -433,7 +433,10 @@ impl<'src> FirstPassRecord<'src> {
                     );
                     signatures.push((signature, idl_args.clone()));
                 }
-                idl_args.push(arg.ty.to_idl_type(self));
+
+                let idl_type = arg.ty.to_idl_type(self);
+                let idl_type = maybe_adjust(idl_type, id);
+                idl_args.push(idl_type);
             }
             signatures.push((signature, idl_args));
         }
@@ -707,3 +710,21 @@ pub fn public() -> syn::Visibility {
         pub_token: Default::default(),
     })
 }
+
+/// When generating our web_sys APIs we default to setting slice references that
+/// get passed to JS as mutable in case they get mutated in JS.
+///
+/// In certain cases we know for sure that the slice will not get mutated - for
+/// example when working with the WebGlRenderingContext APIs.
+///
+/// Here we implement a whitelist for those cases. This whitelist is currently
+/// maintained by hand.
+fn maybe_adjust<'a> (idl_type: IdlType<'a>, id: &'a OperationId) -> IdlType<'a> {
+    match id {
+        OperationId::Operation(Some(op)) => {
+            // TODO: `match op` and return an adjusted idl_type if necessary
+            idl_type
+        }
+        _ => idl_type
+    }
+}

From 1d0c2fee9ef14ea484e83c524c365e90809dacdb Mon Sep 17 00:00:00 2001
From: Chinedu Francis Nwafili <frankie.nwafili@gmail.com>
Date: Thu, 17 Jan 2019 13:20:53 -0500
Subject: [PATCH 2/2] Add failing immutable slice test

---
 crates/web-sys/tests/wasm/element.js          |  5 ++++
 crates/web-sys/tests/wasm/immutable_slices.rs | 26 +++++++++++++++++++
 crates/web-sys/tests/wasm/main.rs             |  1 +
 crates/webidl-tests/lib.rs                    |  2 ++
 4 files changed, 34 insertions(+)
 create mode 100644 crates/web-sys/tests/wasm/immutable_slices.rs

diff --git a/crates/web-sys/tests/wasm/element.js b/crates/web-sys/tests/wasm/element.js
index ed51fbdb..3f34bc5f 100644
--- a/crates/web-sys/tests/wasm/element.js
+++ b/crates/web-sys/tests/wasm/element.js
@@ -151,6 +151,11 @@ export function new_title() {
   return document.createElement("title");
 }
 
+export function new_webgl_rendering_context() {
+  const canvas = document.createElement('canvas');
+  return canvas.getContext('webgl');
+}
+
 export function new_xpath_result() {
     let xmlDoc = new DOMParser().parseFromString("<root><value>tomato</value></root>", "application/xml");
     let xpathResult = xmlDoc.evaluate("/root//value", xmlDoc, null, XPathResult.ANY_TYPE, null);
diff --git a/crates/web-sys/tests/wasm/immutable_slices.rs b/crates/web-sys/tests/wasm/immutable_slices.rs
new file mode 100644
index 00000000..92b4cbd3
--- /dev/null
+++ b/crates/web-sys/tests/wasm/immutable_slices.rs
@@ -0,0 +1,26 @@
+//! When generating our web_sys APIs we default to setting slice references that
+//! get passed to JS as mutable in case they get mutated in JS.
+//!
+//! In certain cases we know for sure that the slice will not get mutated - for
+//! example when working with the WebGlRenderingContext APIs.
+//!
+//! These tests ensure that whitelisted methods do indeed accept mutable slices.
+//!
+//! @see https://github.com/rustwasm/wasm-bindgen/issues/1005
+
+use wasm_bindgen::prelude::*;
+use wasm_bindgen_test::*;
+use web_sys::WebGlRenderingContext;
+
+#[wasm_bindgen(module = "./tests/wasm/element.js")]
+extern "C" {
+    fn new_webgl_rendering_context() -> WebGlRenderingContext;
+}
+
+// Ensure that our whitelisted WebGlRenderingContext methods work
+#[wasm_bindgen_test]
+fn test_webgl_rendering_context_immutable_slices() {
+    let gl = new_webgl_rendering_context();
+
+    gl.vertex_attrib1fv_with_f32_array(0, &[5000.]);
+}
diff --git a/crates/web-sys/tests/wasm/main.rs b/crates/web-sys/tests/wasm/main.rs
index 8f2fd5ff..3b57604a 100644
--- a/crates/web-sys/tests/wasm/main.rs
+++ b/crates/web-sys/tests/wasm/main.rs
@@ -56,6 +56,7 @@ pub mod style_element;
 pub mod table_element;
 pub mod title_element;
 pub mod xpath_result;
+pub mod immutable_slices;
 
 #[wasm_bindgen_test]
 fn deref_works() {
diff --git a/crates/webidl-tests/lib.rs b/crates/webidl-tests/lib.rs
index 44425d9c..7d245db2 100644
--- a/crates/webidl-tests/lib.rs
+++ b/crates/webidl-tests/lib.rs
@@ -1 +1,3 @@
 // intentionally left blank
+
+// QUESTION FOR REVIEWER: WHY? (I'll include this context in the comment)