Implement a lint to check for args like fn foo(ref x: u8), as the ref is effectively ignored by rustc.

src/lib.rs

@@ -2,7 +2,7 @@
 
 #![allow(unused_imports)]
 
-#[phase(plugin,link)]
+#[phase(plugin, link)]
 extern crate syntax;
 #[phase(plugin, link)]
 extern crate rustc;
@@ -21,4 +21,5 @@ pub fn plugin_registrar(reg: &mut Registry) {
     reg.register_lint_pass(box types::TypePass as LintPassObject);
     reg.register_lint_pass(box misc::MiscPass as LintPassObject);
     reg.register_lint_pass(box misc::StrToStringPass as LintPassObject);
+    reg.register_lint_pass(box misc::TopLevelRefPass as LintPassObject);
 }

src/misc.rs

@@ -1,6 +1,7 @@
 use syntax::ptr::P;
 use syntax::ast;
 use syntax::ast::*;
+use syntax::visit::{FnKind};
 use rustc::lint::{Context, LintPass, LintArray, Lint, Level};
 use rustc::middle::ty::{mod, expr_ty, ty_str, ty_ptr, ty_rptr};
 use syntax::codemap::Span;
@@ -81,3 +82,26 @@ impl LintPass for StrToStringPass {
         }
     }
 }
+
+
+declare_lint!(CLIPPY_TOPLEVEL_REF_ARG, Warn, "Warn about pattern matches with top-level `ref` bindings");
+
+pub struct TopLevelRefPass;
+
+impl LintPass for TopLevelRefPass {
+    fn get_lints(&self) -> LintArray {
+        lint_array!(CLIPPY_TOPLEVEL_REF_ARG)
+    }
+
+    fn check_fn(&mut self, cx: &Context, _: FnKind, decl: &FnDecl, _: &Block, _: Span, _: NodeId) {
+        for ref arg in decl.inputs.iter() {
+            if let PatIdent(BindByRef(_), _, _) = arg.pat.node {
+                cx.span_lint(
+                    CLIPPY_TOPLEVEL_REF_ARG,
+                    arg.pat.span,
+                    "`ref` directly on a function argument is ignored. Have you considered using a reference type instead?"
+                );
+            }
+        }
+    }
+}