Warns if variable name is composed only of underscores and digits.

clippy_lints/src/non_expressive_names.rs

@@ -42,13 +42,33 @@ declare_lint! {
     "too many single character bindings"
 }
 
+/// **What it does:** Checks if you have variables whose name consists of just
+/// underscores and digits.
+///
+/// **Why is this bad?** It's hard to memorize what a variable means without a
+/// descriptive name.
+///
+/// **Known problems:** None?
+///
+/// **Example:**
+/// ```rust
+/// let _1 = 1;
+/// let ___1 = 1;
+/// let __1___2 = 11;
+/// ```
+declare_lint! {
+    pub JUST_UNDERSCORES_AND_DIGITS,
+    Warn,
+    "unclear name"
+}
+
 pub struct NonExpressiveNames {
     pub single_char_binding_names_threshold: u64,
 }
 
 impl LintPass for NonExpressiveNames {
     fn get_lints(&self) -> LintArray {
-        lint_array!(SIMILAR_NAMES, MANY_SINGLE_CHAR_NAMES)
+        lint_array!(SIMILAR_NAMES, MANY_SINGLE_CHAR_NAMES, JUST_UNDERSCORES_AND_DIGITS)
     }
 }
 
@@ -133,6 +153,15 @@ impl<'a, 'tcx, 'b> SimilarNamesNameVisitor<'a, 'tcx, 'b> {
         if interned_name.chars().any(char::is_uppercase) {
             return;
         }
+        if interned_name.chars().all(|c| c.is_digit(10) || c == '_') {
+            span_lint(
+                self.0.cx,
+                JUST_UNDERSCORES_AND_NUMBERS,
+                span,
+                "binding whose name is just underscores and digits",
+            );
+            return;
+        }
         let count = interned_name.chars().count();
         if count < 3 {
             if count == 1 {

tests/ui/non_expressive_names.rs

@@ -134,3 +134,10 @@ fn bla() {
         }
     }
 }
+
+fn underscores_and_numbers() {
+    let _1 = 1; //~ERROR Consider a more descriptive name
+    let ____1 = 1; //~ERROR Consider a more descriptive name
+    let __1___2 = 12; //~ERROR Consider a more descriptive name
+    let _1_ok= 1;
+}

tests/ui/non_expressive_names.stderr

@@ -129,3 +129,23 @@ error: 5th binding whose name is just one char
 129 |             e => panic!(),
     |             ^
 
+error: binding whose name is just underscores and digits
+   --> $DIR/non_expressive_names.rs:139:9
+    |
+139 |     let _1 = 1; //~ERROR Consider a more descriptive name
+    |         ^^
+    |
+    = note: `-D just-underscores-and-numbers` implied by `-D warnings`
+
+error: binding whose name is just underscores and digits
+   --> $DIR/non_expressive_names.rs:140:9
+    |
+140 |     let ____1 = 1; //~ERROR Consider a more descriptive name
+    |         ^^^^^
+
+error: binding whose name is just underscores and digits
+   --> $DIR/non_expressive_names.rs:141:9
+    |
+141 |     let __1___2 = 12; //~ERROR Consider a more descriptive name
+    |         ^^^^^^^
+