Implement fuzzing for the GLSL parser (gfx-rs#1301)

* Implement fuzzing for the GLSL parser

* Remove arbitrary dependency from naga

Derive `Arbitrary` for proxy objects in `fuzz/fuzz_targets/glsl_parser.rs`
instead.

fuzz/Cargo.toml

@@ -9,11 +9,12 @@ edition = "2018"
 cargo-fuzz = true
 
 [dependencies]
+arbitrary = { version = "1.0.2", features = ["derive"] }
 libfuzzer-sys = "0.4"
 
 [dependencies.naga]
 path = ".."
-features = ["spv-in", "wgsl-in"]
+features = ["spv-in", "wgsl-in", "glsl-in"]
 
 # Prevent this from interfering with workspaces
 [workspace]
@@ -30,3 +31,9 @@ name = "wgsl_parser"
 path = "fuzz_targets/wgsl_parser.rs"
 test = false
 doc = false
+
+[[bin]]
+name = "glsl_parser"
+path = "fuzz_targets/glsl_parser.rs"
+test = false
+doc = false

fuzz/fuzz_targets/glsl_parser.rs

@@ -0,0 +1,46 @@
+#![no_main]
+use arbitrary::Arbitrary;
+use libfuzzer_sys::fuzz_target;
+use naga::{
+    front::glsl::{Options, Parser},
+    FastHashMap, ShaderStage,
+};
+
+#[derive(Debug, Arbitrary)]
+enum ShaderStageProxy {
+    Vertex,
+    Fragment,
+    Compute,
+}
+
+impl From<ShaderStageProxy> for ShaderStage {
+    fn from(proxy: ShaderStageProxy) -> Self {
+        match proxy {
+            ShaderStageProxy::Vertex => ShaderStage::Vertex,
+            ShaderStageProxy::Fragment => ShaderStage::Fragment,
+            ShaderStageProxy::Compute => ShaderStage::Compute,
+        }
+    }
+}
+
+#[derive(Debug, Arbitrary)]
+struct OptionsProxy {
+    pub stage: ShaderStageProxy,
+    pub defines: FastHashMap<String, String>,
+}
+
+impl From<OptionsProxy> for Options {
+    fn from(proxy: OptionsProxy) -> Self {
+        Options {
+            stage: proxy.stage.into(),
+            defines: proxy.defines,
+        }
+    }
+}
+
+fuzz_target!(|data: (OptionsProxy, String)| {
+    let (options, source) = data;
+    // Ensure the parser can handle potentially malformed strings without crashing.
+    let mut parser = Parser::default();
+    let _result = parser.parse(&options.into(), &source);
+});