New lint: mut_mut (closes issue #9)

README.md

@@ -22,6 +22,7 @@ Lints included in this crate:
  - `precedence`: Warns on expressions where precedence may trip up the unwary reader of the source and suggests adding parenthesis, e.g. `x << 2 + y` will be parsed as `x << (2 + y)`
  - `redundant_closure`: Warns on usage of eta-reducible closures like `|a| foo(a)` (which can be written as just `foo`)
  - `identity_op`: Warns on identity operations like `x + 0` or `y / 1` (which can be reduced to `x` and `y`, respectively)
+ - `mut_mut`: Warns on `&mut &mut` which is either a copy'n'paste error, or shows a fundamental misunderstanding of references
 
 To use, add the following lines to your Cargo.toml:
 

src/lib.rs

@@ -23,6 +23,7 @@ pub mod needless_bool;
 pub mod approx_const;
 pub mod eta_reduction;
 pub mod identity_op;
+pub mod mut_mut;
 
 #[plugin_registrar]
 pub fn plugin_registrar(reg: &mut Registry) {
@@ -40,6 +41,7 @@ pub fn plugin_registrar(reg: &mut Registry) {
     reg.register_lint_pass(box misc::Precedence as LintPassObject);
     reg.register_lint_pass(box eta_reduction::EtaPass as LintPassObject);
     reg.register_lint_pass(box identity_op::IdentityOp as LintPassObject);
+    reg.register_lint_pass(box mut_mut::MutMut as LintPassObject);
     
     reg.register_lint_group("clippy", vec![types::BOX_VEC, types::LINKEDLIST,
                                            misc::SINGLE_MATCH, misc::STR_TO_STRING,
@@ -53,5 +55,6 @@ pub fn plugin_registrar(reg: &mut Registry) {
                                            misc::PRECEDENCE,
                                            eta_reduction::REDUNDANT_CLOSURE,
                                            identity_op::IDENTITY_OP,
+                                           mut_mut::MUT_MUT,
                                            ]);
 }

src/mut_mut.rs

@@ -0,0 +1,51 @@
+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};
+
+declare_lint!(pub MUT_MUT, Warn,
+              "Warn on usage of double-mut refs, e.g. '&mut &mut ...'");
+
+#[derive(Copy,Clone)]
+pub struct MutMut;
+
+impl LintPass for MutMut {
+	fn get_lints(&self) -> LintArray {
+        lint_array!(MUT_MUT)
+	}
+	
+	fn check_expr(&mut self, cx: &Context, expr: &Expr) {
+		
+		fn unwrap_addr(expr : &Expr) -> Option<&Expr> {
+			match expr.node {
+				ExprAddrOf(MutMutable, ref e) => Option::Some(e),
+				_ => Option::None
+			}
+		}
+		
+		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.")
+		}
+	}
+	
+	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.")
+		}
+	}
+}

tests/compile-fail/mut_mut.rs

@@ -0,0 +1,20 @@
+#![feature(plugin)]
+#![plugin(clippy)]
+
+#[deny(mut_mut)]
+fn fun(x : &mut &mut u32) -> bool { //~ERROR
+	**x > 0
+}
+
+#[deny(mut_mut)]
+fn main() {
+	let mut x = &mut &mut 1u32; //~ERROR
+	if fun(x) {
+		let y : &mut &mut &mut u32 = &mut &mut &mut 2; 
+			 //~^ ERROR
+                  //~^^ ERROR
+                                  //~^^^ ERROR
+									   //~^^^^ ERROR
+		***y + **x;
+	}
+}