mut_mut now more robust (thanks to Manishearth, see issue #9)

src/mut_mut.rs

@@ -1,11 +1,7 @@
 use syntax::ptr::P;
-use syntax::ast;
 use syntax::ast::*;
-use syntax::ast_util::{is_comparison_binop, binop_to_string};
-use syntax::visit::{FnKind};
-use rustc::lint::{Context, LintPass, LintArray, Lint, Level};
-use rustc::middle::ty::{self, expr_ty, ty_str, ty_ptr, ty_rptr, ty_float};
-use syntax::codemap::{Span, Spanned};
+use rustc::lint::{Context, LintPass, LintArray, Lint};
+use rustc::middle::ty::{expr_ty, sty, ty_ptr, ty_rptr, mt};
 
 declare_lint!(pub MUT_MUT, Warn,
               "Warn on usage of double-mut refs, e.g. '&mut &mut ...'");
@@ -27,25 +23,34 @@ impl LintPass for MutMut {
 			}
 		}
 		
-		if unwrap_addr(expr).and_then(unwrap_addr).is_some() {
-			cx.span_lint(MUT_MUT, expr.span, 
-				"We're not sure what this means, so if you know, please tell us.")
-		}
+		unwrap_addr(expr).map(|e| {
+			if unwrap_addr(e).is_some() {
+				cx.span_lint(MUT_MUT, expr.span, 
+					"We're not sure what this means, so if you know, please tell us.")
+			} else {
+				match expr_ty(cx.tcx, e).sty {
+					ty_ptr(mt{ty: _, mutbl: MutMutable}) |
+					ty_rptr(_, mt{ty: _, mutbl: MutMutable}) => 
+						cx.span_lint(MUT_MUT, expr.span,
+							"This expression mutably borrows a mutable reference. Consider direct reborrowing"),
+					_ => ()
+				}
+			}
+		});
 	}
 	
 	fn check_ty(&mut self, cx: &Context, ty: &Ty) {
-		
-		fn unwrap_mut(ty : &Ty) -> Option<&Ty> {
-			match ty.node {
-				TyPtr(MutTy{ ty: ref pty, mutbl: MutMutable }) => Option::Some(pty),
-				TyRptr(_, MutTy{ ty: ref pty, mutbl: MutMutable }) => Option::Some(pty),
-				_ => Option::None
-			}
-		}
-		
 		if unwrap_mut(ty).and_then(unwrap_mut).is_some() {
 			cx.span_lint(MUT_MUT, ty.span, 
 				"We're not sure what this means, so if you know, please tell us.")
 		}
 	}
 }
+
+fn unwrap_mut(ty : &Ty) -> Option<&Ty> {
+	match ty.node {
+		TyPtr(MutTy{ ty: ref pty, mutbl: MutMutable }) => Option::Some(pty),
+		TyRptr(_, MutTy{ ty: ref pty, mutbl: MutMutable }) => Option::Some(pty),
+		_ => Option::None
+	}
+}

tests/compile-fail/mut_mut.rs

@@ -7,8 +7,13 @@ fn fun(x : &mut &mut u32) -> bool { //~ERROR
 }
 
 #[deny(mut_mut)]
+#[allow(unused_mut, unused_variables)]
 fn main() {
 	let mut x = &mut &mut 1u32; //~ERROR
+	{
+		let mut y = &mut x; //~ERROR
+	}
+	
 	if fun(x) {
 		let y : &mut &mut &mut u32 = &mut &mut &mut 2; 
 			 //~^ ERROR