Add small underscore scope lint

CHANGELOG.md

@@ -1056,6 +1056,7 @@ All notable changes to this project will be documented in this file.
 [`single_match`]: https://rust-lang.github.io/rust-clippy/master/index.html#single_match
 [`single_match_else`]: https://rust-lang.github.io/rust-clippy/master/index.html#single_match_else
 [`slow_vector_initialization`]: https://rust-lang.github.io/rust-clippy/master/index.html#slow_vector_initialization
+[`small_underscore_scope`]: https://rust-lang.github.io/rust-clippy/master/index.html#small_underscore_scope
 [`str_to_string`]: https://rust-lang.github.io/rust-clippy/master/index.html#str_to_string
 [`string_add`]: https://rust-lang.github.io/rust-clippy/master/index.html#string_add
 [`string_add_assign`]: https://rust-lang.github.io/rust-clippy/master/index.html#string_add_assign

README.md

@@ -7,7 +7,7 @@
 
 A collection of lints to catch common mistakes and improve your [Rust](https://github.com/rust-lang/rust) code.
 
-[There are 299 lints included in this crate!](https://rust-lang.github.io/rust-clippy/master/index.html)
+[There are 300 lints included in this crate!](https://rust-lang.github.io/rust-clippy/master/index.html)
 
 We have a bunch of lint categories to allow you to choose how much Clippy is supposed to ~~annoy~~ help you:
 

clippy_lints/src/lib.rs

@@ -252,6 +252,7 @@ pub mod returns;
 pub mod serde_api;
 pub mod shadow;
 pub mod slow_vector_initialization;
+pub mod small_underscore_scope;
 pub mod strings;
 pub mod suspicious_trait_impl;
 pub mod swap;
@@ -574,6 +575,7 @@ pub fn register_plugins(reg: &mut rustc_plugin::Registry<'_>, conf: &Conf) {
     reg.register_late_lint_pass(box missing_const_for_fn::MissingConstForFn);
     reg.register_late_lint_pass(box transmuting_null::TransmutingNull);
     reg.register_late_lint_pass(box path_buf_push_overwrite::PathBufPushOverwrite);
+    reg.register_early_lint_pass(box small_underscore_scope::SmallUnderscoreScope);
 
     reg.register_lint_group("clippy::restriction", Some("clippy_restriction"), vec![
         arithmetic::FLOAT_ARITHMETIC,
@@ -634,6 +636,7 @@ pub fn register_plugins(reg: &mut rustc_plugin::Registry<'_>, conf: &Conf) {
         non_expressive_names::SIMILAR_NAMES,
         replace_consts::REPLACE_CONSTS,
         shadow::SHADOW_UNRELATED,
+        small_underscore_scope::SMALL_UNDERSCORE_SCOPE,
         strings::STRING_ADD_ASSIGN,
         types::CAST_POSSIBLE_TRUNCATION,
         types::CAST_POSSIBLE_WRAP,

clippy_lints/src/small_underscore_scope.rs

@@ -0,0 +1,69 @@
+use crate::utils::span_lint_and_sugg;
+use rustc::lint::{EarlyContext, EarlyLintPass, LintArray, LintPass};
+use rustc::{declare_lint_pass, declare_tool_lint};
+use rustc_errors::Applicability;
+use syntax::ast::*;
+use syntax::source_map::Span;
+
+declare_clippy_lint! {
+    /// **What is does:** Checks for wildcard patterns inside a struct or tuple which could instead encompass the entire pattern.
+    ///
+    /// **Why is this bad?** The extra binding information is meaningless and makes the code harder to read.
+    ///
+    /// **Known problems:** None.
+    ///
+    /// **Example:**
+    ///
+    /// ```rust,ignore
+    /// // Bad
+    /// let t = (1, 2);
+    /// let (_, _) = t;
+    ///
+    /// // Good
+    /// let t = (1, 2);
+    /// let _ = t;
+    /// ```
+    pub SMALL_UNDERSCORE_SCOPE,
+    style,
+    "wildcard binding occurs inside a struct, but the wildcard could be the entire binding"
+}
+
+declare_lint_pass!(SmallUnderscoreScope => [SMALL_UNDERSCORE_SCOPE]);
+
+impl EarlyLintPass for SmallUnderscoreScope {
+    fn check_pat(&mut self, cx: &EarlyContext<'_>, pat: &Pat, _: &mut bool) {
+        match pat.node {
+            PatKind::TupleStruct(_, ref pats, _) | PatKind::Tuple(ref pats, _) => {
+                // `Foo(..)` | `(..)` | `Foo(_, _)` | `(_, _)`
+                if pats.is_empty()
+                    || pats.iter().all(|pat| match pat.node {
+                        PatKind::Wild => true,
+                        _ => false,
+                    })
+                {
+                    emit_lint(cx, pat.span);
+                }
+            },
+            PatKind::Struct(_, ref pats, _) => {
+                // `Bar { .. }`
+                // The `Bar { x: _, y: _ }` is covered by `unneeded_field_pattern`, which suggests `Bar { .. }`
+                if pats.is_empty() {
+                    emit_lint(cx, pat.span);
+                }
+            },
+            _ => (),
+        }
+    }
+}
+
+fn emit_lint(cx: &EarlyContext<'_>, sp: Span) {
+    span_lint_and_sugg(
+        cx,
+        SMALL_UNDERSCORE_SCOPE,
+        sp,
+        "this wildcard binding could have a wider scope",
+        "try",
+        "_".to_string(),
+        Applicability::MachineApplicable,
+    )
+}

tests/ui/small_underscore_scope.fixed

@@ -0,0 +1,27 @@
+// run-rustfix
+
+#![warn(clippy::small_underscore_scope)]
+
+struct NewType(u8);
+
+#[allow(dead_code)]
+struct Foo {
+    x: u8,
+    y: u8,
+}
+
+fn main() {
+    let n = NewType(24);
+    let _ = n;
+    let _ = n;
+
+    let t = (4, 5);
+    let _ = t;
+    let _ = t;
+
+    let t2 = (6, (7, 8));
+    let (_x, _) = t2;
+
+    let f = Foo { x: 3, y: 7 };
+    let _ = f;
+}

tests/ui/small_underscore_scope.rs

@@ -0,0 +1,27 @@
+// run-rustfix
+
+#![warn(clippy::small_underscore_scope)]
+
+struct NewType(u8);
+
+#[allow(dead_code)]
+struct Foo {
+    x: u8,
+    y: u8,
+}
+
+fn main() {
+    let n = NewType(24);
+    let NewType(_) = n;
+    let NewType(..) = n;
+
+    let t = (4, 5);
+    let (_, _) = t;
+    let (..) = t;
+
+    let t2 = (6, (7, 8));
+    let (_x, (_, _)) = t2;
+
+    let f = Foo { x: 3, y: 7 };
+    let Foo { .. } = f;
+}

tests/ui/small_underscore_scope.stderr

@@ -0,0 +1,40 @@
+error: this wildcard binding could have a wider scope
+  --> $DIR/small_underscore_scope.rs:15:9
+   |
+LL |     let NewType(_) = n;
+   |         ^^^^^^^^^^ help: try: `_`
+   |
+   = note: `-D clippy::small-underscore-scope` implied by `-D warnings`
+
+error: this wildcard binding could have a wider scope
+  --> $DIR/small_underscore_scope.rs:16:9
+   |
+LL |     let NewType(..) = n;
+   |         ^^^^^^^^^^^ help: try: `_`
+
+error: this wildcard binding could have a wider scope
+  --> $DIR/small_underscore_scope.rs:19:9
+   |
+LL |     let (_, _) = t;
+   |         ^^^^^^ help: try: `_`
+
+error: this wildcard binding could have a wider scope
+  --> $DIR/small_underscore_scope.rs:20:9
+   |
+LL |     let (..) = t;
+   |         ^^^^ help: try: `_`
+
+error: this wildcard binding could have a wider scope
+  --> $DIR/small_underscore_scope.rs:23:14
+   |
+LL |     let (_x, (_, _)) = t2;
+   |              ^^^^^^ help: try: `_`
+
+error: this wildcard binding could have a wider scope
+  --> $DIR/small_underscore_scope.rs:26:9
+   |
+LL |     let Foo { .. } = f;
+   |         ^^^^^^^^^^ help: try: `_`
+
+error: aborting due to 6 previous errors
+