Rollup merge of rust-lang#5408 - dtolnay:matchbool, r=flip1995

Downgrade match_bool to pedantic

I don't quite buy the justification in https://rust-lang.github.io/rust-clippy/. The justification is:

> It makes the code less readable.

In the Rust codebases I've worked in, I have found people were comfortable using `match bool` (selectively) to make code more readable. For example, initializing struct fields is a place where the indentation of `match` can work better than the indentation of `if`:

```rust
let _ = Struct {
    v: {
        ...
    },
    w: match doing_w {
        true => ...,
        false => ...,
    },
    x: Nested {
        c: ...,
        b: ...,
        a: ...,
    },
    y: if doing_y {
        ...
    } else { // :(
        ...
    },
    z: ...,
};
```

Or sometimes people prefer something a bit less pithy than `if` when the meaning of the bool doesn't read off clearly from the condition:

```rust
if set.insert(...) {
    ... // ???
} else {
    ...
}

match set.insert(...) {
    // set.insert returns false if already present
    false => ...,
    true => ...,
}
```

Or `match` can be a better fit when the bool is playing the role more of a value than a branch condition:

```rust
impl ErrorCodes {
    pub fn from(b: bool) -> Self {
        match b {
            true => ErrorCodes::Yes,
            false => ErrorCodes::No,
        }
    }
}
```

And then there's plain old it's-1-line-shorter, which means we get 25% more content on a screen when stacking a sequence of conditions:

```rust
let old_noun = match old_binding.is_import() {
    true => "import",
    false => "definition",
};
let new_participle = match new_binding.is_import() {
    true => "imported",
    false => "defined",
};
```

Bottom line is I think this lint fits the bill better as a pedantic lint; I don't think linting on this by default is justified.

changelog: Remove match_bool from default set of enabled lints

clippy_lints/src/lib.rs

@@ -1134,6 +1134,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&loops::EXPLICIT_INTO_ITER_LOOP),
         LintId::of(&loops::EXPLICIT_ITER_LOOP),
         LintId::of(&macro_use::MACRO_USE_IMPORTS),
+        LintId::of(&matches::MATCH_BOOL),
         LintId::of(&matches::SINGLE_MATCH_ELSE),
         LintId::of(&methods::FILTER_MAP),
         LintId::of(&methods::FILTER_MAP_NEXT),
@@ -1279,7 +1280,6 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&map_unit_fn::RESULT_MAP_UNIT_FN),
         LintId::of(&matches::INFALLIBLE_DESTRUCTURING_MATCH),
         LintId::of(&matches::MATCH_AS_REF),
-        LintId::of(&matches::MATCH_BOOL),
         LintId::of(&matches::MATCH_OVERLAPPING_ARM),
         LintId::of(&matches::MATCH_REF_PATS),
         LintId::of(&matches::MATCH_SINGLE_BINDING),
@@ -1470,7 +1470,6 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&main_recursion::MAIN_RECURSION),
         LintId::of(&map_clone::MAP_CLONE),
         LintId::of(&matches::INFALLIBLE_DESTRUCTURING_MATCH),
-        LintId::of(&matches::MATCH_BOOL),
         LintId::of(&matches::MATCH_OVERLAPPING_ARM),
         LintId::of(&matches::MATCH_REF_PATS),
         LintId::of(&matches::MATCH_WILD_ERR_ARM),

clippy_lints/src/matches.rs

@@ -138,7 +138,7 @@ declare_clippy_lint! {
     /// }
     /// ```
     pub MATCH_BOOL,
-    style,
+    pedantic,
     "a `match` on a boolean expression instead of an `if..else` block"
 }
 

src/lintlist/mod.rs

@@ -1139,7 +1139,7 @@ pub static ref ALL_LINTS: Vec<Lint> = vec![
     },
     Lint {
         name: "match_bool",
-        group: "style",
+        group: "pedantic",
         desc: "a `match` on a boolean expression instead of an `if..else` block",
         deprecation: None,
         module: "matches",

tests/ui/implicit_return.fixed

@@ -21,7 +21,6 @@ fn test_if_block() -> bool {
     }
 }
 
-#[allow(clippy::match_bool)]
 #[rustfmt::skip]
 fn test_match(x: bool) -> bool {
     match x {
@@ -30,7 +29,7 @@ fn test_match(x: bool) -> bool {
     }
 }
 
-#[allow(clippy::match_bool, clippy::needless_return)]
+#[allow(clippy::needless_return)]
 fn test_match_with_unreachable(x: bool) -> bool {
     match x {
         true => return false,

tests/ui/implicit_return.rs

@@ -21,7 +21,6 @@ fn test_if_block() -> bool {
     }
 }
 
-#[allow(clippy::match_bool)]
 #[rustfmt::skip]
 fn test_match(x: bool) -> bool {
     match x {
@@ -30,7 +29,7 @@ fn test_match(x: bool) -> bool {
     }
 }
 
-#[allow(clippy::match_bool, clippy::needless_return)]
+#[allow(clippy::needless_return)]
 fn test_match_with_unreachable(x: bool) -> bool {
     match x {
         true => return false,

tests/ui/implicit_return.stderr

@@ -19,49 +19,49 @@ LL |         false
    |         ^^^^^ help: add `return` as shown: `return false`
 
 error: missing `return` statement
-  --> $DIR/implicit_return.rs:28:17
+  --> $DIR/implicit_return.rs:27:17
    |
 LL |         true => false,
    |                 ^^^^^ help: add `return` as shown: `return false`
 
 error: missing `return` statement
-  --> $DIR/implicit_return.rs:29:20
+  --> $DIR/implicit_return.rs:28:20
    |
 LL |         false => { true },
    |                    ^^^^ help: add `return` as shown: `return true`
 
 error: missing `return` statement
-  --> $DIR/implicit_return.rs:44:9
+  --> $DIR/implicit_return.rs:43:9
    |
 LL |         break true;
    |         ^^^^^^^^^^ help: change `break` to `return` as shown: `return true`
 
 error: missing `return` statement
-  --> $DIR/implicit_return.rs:52:13
+  --> $DIR/implicit_return.rs:51:13
    |
 LL |             break true;
    |             ^^^^^^^^^^ help: change `break` to `return` as shown: `return true`
 
 error: missing `return` statement
-  --> $DIR/implicit_return.rs:61:13
+  --> $DIR/implicit_return.rs:60:13
    |
 LL |             break true;
    |             ^^^^^^^^^^ help: change `break` to `return` as shown: `return true`
 
 error: missing `return` statement
-  --> $DIR/implicit_return.rs:79:18
+  --> $DIR/implicit_return.rs:78:18
    |
 LL |     let _ = || { true };
    |                  ^^^^ help: add `return` as shown: `return true`
 
 error: missing `return` statement
-  --> $DIR/implicit_return.rs:80:16
+  --> $DIR/implicit_return.rs:79:16
    |
 LL |     let _ = || true;
    |                ^^^^ help: add `return` as shown: `return true`
 
 error: missing `return` statement
-  --> $DIR/implicit_return.rs:88:5
+  --> $DIR/implicit_return.rs:87:5
    |
 LL |     format!("test {}", "test")
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^ help: add `return` as shown: `return format!("test {}", "test")`

tests/ui/match_bool.rs

@@ -1,3 +1,5 @@
+#![deny(clippy::match_bool)]
+
 fn match_bool() {
     let test: bool = true;
 

tests/ui/match_bool.stderr

@@ -1,24 +1,28 @@
 error: this boolean expression can be simplified
-  --> $DIR/match_bool.rs:29:11
+  --> $DIR/match_bool.rs:31:11
    |
 LL |     match test && test {
    |           ^^^^^^^^^^^^ help: try: `test`
    |
    = note: `-D clippy::nonminimal-bool` implied by `-D warnings`
 
 error: you seem to be trying to match on a boolean expression
-  --> $DIR/match_bool.rs:4:5
+  --> $DIR/match_bool.rs:6:5
    |
 LL | /     match test {
 LL | |         true => 0,
 LL | |         false => 42,
 LL | |     };
    | |_____^ help: consider using an `if`/`else` expression: `if test { 0 } else { 42 }`
    |
-   = note: `-D clippy::match-bool` implied by `-D warnings`
+note: the lint level is defined here
+  --> $DIR/match_bool.rs:1:9
+   |
+LL | #![deny(clippy::match_bool)]
+   |         ^^^^^^^^^^^^^^^^^^
 
 error: you seem to be trying to match on a boolean expression
-  --> $DIR/match_bool.rs:10:5
+  --> $DIR/match_bool.rs:12:5
    |
 LL | /     match option == 1 {
 LL | |         true => 1,
@@ -27,7 +31,7 @@ LL | |     };
    | |_____^ help: consider using an `if`/`else` expression: `if option == 1 { 1 } else { 0 }`
 
 error: you seem to be trying to match on a boolean expression
-  --> $DIR/match_bool.rs:15:5
+  --> $DIR/match_bool.rs:17:5
    |
 LL | /     match test {
 LL | |         true => (),
@@ -45,7 +49,7 @@ LL |     };
    |
 
 error: you seem to be trying to match on a boolean expression
-  --> $DIR/match_bool.rs:22:5
+  --> $DIR/match_bool.rs:24:5
    |
 LL | /     match test {
 LL | |         false => {
@@ -63,7 +67,7 @@ LL |     };
    |
 
 error: you seem to be trying to match on a boolean expression
-  --> $DIR/match_bool.rs:29:5
+  --> $DIR/match_bool.rs:31:5
    |
 LL | /     match test && test {
 LL | |         false => {
@@ -81,15 +85,15 @@ LL |     };
    |
 
 error: equal expressions as operands to `&&`
-  --> $DIR/match_bool.rs:29:11
+  --> $DIR/match_bool.rs:31:11
    |
 LL |     match test && test {
    |           ^^^^^^^^^^^^
    |
    = note: `#[deny(clippy::eq_op)]` on by default
 
 error: you seem to be trying to match on a boolean expression
-  --> $DIR/match_bool.rs:36:5
+  --> $DIR/match_bool.rs:38:5
    |
 LL | /     match test {
 LL | |         false => {

tests/ui/needless_return.fixed

@@ -1,6 +1,6 @@
 // run-rustfix
 
-#![allow(unused, clippy::needless_bool, clippy::match_bool)]
+#![allow(unused, clippy::needless_bool)]
 #![allow(clippy::if_same_then_else, clippy::single_match)]
 #![warn(clippy::needless_return)]
 

tests/ui/needless_return.rs

@@ -1,6 +1,6 @@
 // run-rustfix
 
-#![allow(unused, clippy::needless_bool, clippy::match_bool)]
+#![allow(unused, clippy::needless_bool)]
 #![allow(clippy::if_same_then_else, clippy::single_match)]
 #![warn(clippy::needless_return)]