now the method lookup actually works (and I understand why! 😄), reduces unnecessary loops, and has a few comments

tests/compile-fail/len_zero.rs

@@ -5,14 +5,14 @@ struct One;
 
 #[deny(len_without_is_empty)]
 impl One {
-	fn len(self: &Self) -> isize { //~ERROR Item 'One' has a '.len()' method
+	fn len(self: &Self) -> isize { //~ERROR Item 'One' has a '.len(_: &Self)'
 		1
 	}
 }
 
 #[deny(len_without_is_empty)]
 trait TraitsToo {
-	fn len(self: &Self) -> isize; //~ERROR Trait 'TraitsToo' has a '.len()' method,
+	fn len(self: &Self) -> isize; //~ERROR Trait 'TraitsToo' has a '.len(_:
 }
 
 impl TraitsToo for One {
@@ -21,17 +21,47 @@ impl TraitsToo for One {
 	}
 }
 
-#[allow(dead_code)]
 struct HasIsEmpty;
 
 #[deny(len_without_is_empty)]
-#[allow(dead_code)]
 impl HasIsEmpty {
 	fn len(self: &Self) -> isize {
 		1
 	}
+
+	fn is_empty(self: &Self) -> bool {
+		false
+	}
+}
+
+struct Wither;
+
+#[deny(len_without_is_empty)]
+trait WithIsEmpty {
+	fn len(self: &Self) -> isize;
+	fn is_empty(self: &Self) -> bool;
+}
+
+impl WithIsEmpty for Wither {
+	fn len(self: &Self) -> isize {
+		1
+	}
+
+	fn is_empty(self: &Self) -> bool {
+		false
+	}
+}
+
+struct HasWrongIsEmpty;
+
+#[deny(len_without_is_empty)]
+impl HasWrongIsEmpty {
+	fn len(self: &Self) -> isize { //~ERROR Item 'HasWrongIsEmpty' has a '.len(_: &Self)'
+		1
+	}
 	
-	fn is_empty() -> bool {
+	#[allow(dead_code, unused)]
+	fn is_empty(self: &Self, x : u32) -> bool {
 		false
 	}
 }
@@ -49,7 +79,24 @@ fn main() {
 	}
 	
 	let z : &TraitsToo = &y;
-	if z.len() > 0 { //~ERROR Consider replacing the len comparison
+	if z.len() > 0 { //no error, because TraitsToo has no .is_empty() method
 		println!("Nor should this!");
 	}
+	
+	let hie = HasIsEmpty;
+	if hie.len() == 0 { //~ERROR Consider replacing the len comparison
+		println!("Or this!");
+	}
+	assert!(!hie.is_empty());
+	
+	let wie : &WithIsEmpty = &Wither;
+	if wie.len() == 0 { //~ERROR Consider replacing the len comparison
+		println!("Or this!");
+	}
+	assert!(!wie.is_empty());
+	
+	let hwie = HasWrongIsEmpty;
+	if hwie.len() == 0 { //no error as HasWrongIsEmpty does not have .is_empty()
+		println!("Or this!");
+	}
 }