Auto merge of #3652 - Aehmlo:where_the_wild_things_are, r=flip1995

Add wildcard_match_arm lint

This lint prevents using a wildcard in a match arm. Implemented as a restriction currently, because this is pretty much an edge case. See #3649 for more information.

Didn't add any tests because I wasn't sure how, but if someone wants to point me in the right direction, I'd be happy to!

Author: bors

CHANGELOG.md

@@ -1027,6 +1027,7 @@ All notable changes to this project will be documented in this file.
 [`while_let_loop`]: https://rust-lang.github.io/rust-clippy/master/index.html#while_let_loop
 [`while_let_on_iterator`]: https://rust-lang.github.io/rust-clippy/master/index.html#while_let_on_iterator
 [`wildcard_dependencies`]: https://rust-lang.github.io/rust-clippy/master/index.html#wildcard_dependencies
+[`wildcard_match_arm`]: https://rust-lang.github.io/rust-clippy/master/index.html#wildcard_match_arm
 [`write_literal`]: https://rust-lang.github.io/rust-clippy/master/index.html#write_literal
 [`write_with_newline`]: https://rust-lang.github.io/rust-clippy/master/index.html#write_with_newline
 [`writeln_empty_string`]: https://rust-lang.github.io/rust-clippy/master/index.html#writeln_empty_string

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 292 lints included in this crate!](https://rust-lang.github.io/rust-clippy/master/index.html)
+[There are 293 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

@@ -495,6 +495,7 @@ pub fn register_plugins(reg: &mut rustc_plugin::Registry<'_>, conf: &Conf) {
         indexing_slicing::INDEXING_SLICING,
         inherent_impl::MULTIPLE_INHERENT_IMPL,
         literal_representation::DECIMAL_LITERAL_REPRESENTATION,
+        matches::WILDCARD_MATCH_ARM,
         mem_forget::MEM_FORGET,
         methods::CLONE_ON_REF_PTR,
         methods::OPTION_UNWRAP_USED,

clippy_lints/src/matches.rs

@@ -187,6 +187,25 @@ declare_clippy_lint! {
     "a match on an Option value instead of using `as_ref()` or `as_mut`"
 }
 
+/// **What it does:** Checks for wildcard matches using `_`.
+///
+/// **Why is this bad?** New variants added by library updates can be missed.
+///
+/// **Known problems:** None.
+///
+/// **Example:**
+/// ```rust
+/// match x {
+///     A => {},
+///     _ => {},
+/// }
+/// ```
+declare_clippy_lint! {
+    pub WILDCARD_MATCH_ARM,
+    restriction,
+    "a wildcard match arm using `_`"
+}
+
 #[allow(missing_copy_implementations)]
 pub struct MatchPass;
 
@@ -199,7 +218,8 @@ impl LintPass for MatchPass {
             SINGLE_MATCH_ELSE,
             MATCH_OVERLAPPING_ARM,
             MATCH_WILD_ERR_ARM,
-            MATCH_AS_REF
+            MATCH_AS_REF,
+            WILDCARD_MATCH_ARM
         )
     }
 
@@ -218,6 +238,7 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for MatchPass {
             check_match_bool(cx, ex, arms, expr);
             check_overlapping_arms(cx, ex, arms);
             check_wild_err_arm(cx, ex, arms);
+            check_wild_match(cx, arms);
             check_match_as_ref(cx, ex, arms, expr);
         }
         if let ExprKind::Match(ref ex, ref arms, _) = expr.node {
@@ -442,6 +463,21 @@ fn check_wild_err_arm(cx: &LateContext<'_, '_>, ex: &Expr, arms: &[Arm]) {
     }
 }
 
+fn check_wild_match(cx: &LateContext<'_, '_>, arms: &[Arm]) {
+    for arm in arms {
+        if is_wild(&arm.pats[0]) {
+            span_note_and_lint(
+                cx,
+                WILDCARD_MATCH_ARM,
+                arm.pats[0].span,
+                "wildcard match will miss any future added variants.",
+                arm.pats[0].span,
+                "to resolve, match each variant explicitly",
+            );
+        }
+    }
+}
+
 // If the block contains only a `panic!` macro (as expression or statement)
 fn is_panic_block(block: &Block) -> bool {
     match (&block.expr, block.stmts.len(), block.stmts.first()) {

tests/ui/wildcard_match_arm.rs

@@ -0,0 +1,36 @@
+#![deny(clippy::wildcard_match_arm)]
+
+#[derive(Clone, Copy, Debug, Eq, PartialEq)]
+enum Color {
+	Red,
+	Green,
+	Blue,
+	Rgb(u8, u8, u8),
+	Cyan,
+}
+
+impl Color {
+	fn is_monochrome(self) -> bool {
+		match self {
+			Color::Red | Color::Green | Color::Blue => true,
+			Color::Rgb(r, g, b) => r | g == 0 || r | b == 0 || g | b == 0,
+			Color::Cyan => false,
+		}
+	}
+}
+
+fn main() {
+	let color = Color::Rgb(0, 0, 127);
+	match color {
+		Color::Red => println!("Red"),
+		_ => eprintln!("Not red"),
+	};
+	match color {
+		Color::Red => {},
+		Color::Green => {},
+		Color::Blue => {},
+		Color::Cyan => {},
+		c if c.is_monochrome() => {},
+		Color::Rgb(_, _, _) => {},
+	};
+}

tests/ui/wildcard_match_arm.stderr

@@ -0,0 +1,15 @@
+error: wildcard match will miss any future added variants.
+  --> $DIR/wildcard_match_arm.rs:26:3
+   |
+LL |         _ => eprintln!("Not red"),
+   |         ^
+   |
+note: lint level defined here
+  --> $DIR/wildcard_match_arm.rs:1:9
+   |
+LL | #![deny(clippy::wildcard_match_arm)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^
+   = note: to resolve, match each variant explicitly
+
+error: aborting due to previous error
+