needless_return suggest return unit type on void returns

closes #8177

previously, `needless_return` suggests an empty block `{}` to replace void `return` on match arms, this PR improve the suggestion by suggesting a unit instead.

changelog: `needless_return` suggests `()` instead of `{}` on match arms

clippy_lints/src/returns.rs

@@ -73,6 +73,7 @@ declare_clippy_lint! {
 enum RetReplacement {
     Empty,
     Block,
+    Unit,
 }
 
 declare_lint_pass!(Return => [LET_AND_RETURN, NEEDLESS_RETURN]);
@@ -212,7 +213,7 @@ fn check_final_expr<'tcx>(
         // (except for unit type functions) so we don't match it
         ExprKind::Match(_, arms, MatchSource::Normal) => {
             for arm in arms.iter() {
-                check_final_expr(cx, arm.body, Some(arm.body.span), RetReplacement::Block);
+                check_final_expr(cx, arm.body, Some(arm.body.span), RetReplacement::Unit);
             }
         },
         ExprKind::DropTemps(expr) => check_final_expr(cx, expr, None, RetReplacement::Empty),
@@ -259,6 +260,17 @@ fn emit_return_lint(cx: &LateContext<'_>, ret_span: Span, inner_span: Option<Spa
                     Applicability::MachineApplicable,
                 );
             },
+            RetReplacement::Unit => {
+                span_lint_and_sugg(
+                    cx,
+                    NEEDLESS_RETURN,
+                    ret_span,
+                    "unneeded `return` statement",
+                    "replace `return` with a unit value",
+                    "()".to_string(),
+                    Applicability::MachineApplicable,
+                );
+            },
         },
     }
 }

tests/ui/needless_return.fixed

@@ -71,7 +71,18 @@ fn test_void_if_fun(b: bool) {
 fn test_void_match(x: u32) {
     match x {
         0 => (),
-        _ => {},
+        _ => (),
+    }
+}
+
+fn test_nested_match(x: u32) {
+    match x {
+        0 => (),
+        1 => {
+            let _ = 42;
+            
+        },
+        _ => (),
     }
 }
 
@@ -182,7 +193,7 @@ async fn async_test_void_if_fun(b: bool) {
 async fn async_test_void_match(x: u32) {
     match x {
         0 => (),
-        _ => {},
+        _ => (),
     }
 }
 

tests/ui/needless_return.rs

@@ -75,6 +75,17 @@ fn test_void_match(x: u32) {
     }
 }
 
+fn test_nested_match(x: u32) {
+    match x {
+        0 => (),
+        1 => {
+            let _ = 42;
+            return;
+        },
+        _ => return,
+    }
+}
+
 fn read_line() -> String {
     use std::io::BufRead;
     let stdin = ::std::io::stdin();

tests/ui/needless_return.stderr

@@ -70,127 +70,139 @@ error: unneeded `return` statement
   --> $DIR/needless_return.rs:74:14
    |
 LL |         _ => return,
-   |              ^^^^^^ help: replace `return` with an empty block: `{}`
+   |              ^^^^^^ help: replace `return` with a unit value: `()`
 
 error: unneeded `return` statement
-  --> $DIR/needless_return.rs:89:9
-   |
-LL |         return String::from("test");
-   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: remove `return`: `String::from("test")`
-
-error: unneeded `return` statement
-  --> $DIR/needless_return.rs:91:9
-   |
-LL |         return String::new();
-   |         ^^^^^^^^^^^^^^^^^^^^^ help: remove `return`: `String::new()`
-
-error: unneeded `return` statement
-  --> $DIR/needless_return.rs:113:32
-   |
-LL |         bar.unwrap_or_else(|_| return)
-   |                                ^^^^^^ help: replace `return` with an empty block: `{}`
-
-error: unneeded `return` statement
-  --> $DIR/needless_return.rs:118:13
+  --> $DIR/needless_return.rs:83:13
    |
 LL |             return;
    |             ^^^^^^^ help: remove `return`
 
 error: unneeded `return` statement
-  --> $DIR/needless_return.rs:120:20
-   |
-LL |         let _ = || return;
-   |                    ^^^^^^ help: replace `return` with an empty block: `{}`
-
-error: unneeded `return` statement
-  --> $DIR/needless_return.rs:126:32
-   |
-LL |         res.unwrap_or_else(|_| return Foo)
-   |                                ^^^^^^^^^^ help: remove `return`: `Foo`
-
-error: unneeded `return` statement
-  --> $DIR/needless_return.rs:135:5
-   |
-LL |     return true;
-   |     ^^^^^^^^^^^^ help: remove `return`: `true`
-
-error: unneeded `return` statement
-  --> $DIR/needless_return.rs:139:5
-   |
-LL |     return true;
-   |     ^^^^^^^^^^^^ help: remove `return`: `true`
-
-error: unneeded `return` statement
-  --> $DIR/needless_return.rs:144:9
-   |
-LL |         return true;
-   |         ^^^^^^^^^^^^ help: remove `return`: `true`
-
-error: unneeded `return` statement
-  --> $DIR/needless_return.rs:146:9
-   |
-LL |         return false;
-   |         ^^^^^^^^^^^^^ help: remove `return`: `false`
-
-error: unneeded `return` statement
-  --> $DIR/needless_return.rs:152:17
-   |
-LL |         true => return false,
-   |                 ^^^^^^^^^^^^ help: remove `return`: `false`
-
-error: unneeded `return` statement
-  --> $DIR/needless_return.rs:154:13
-   |
-LL |             return true;
-   |             ^^^^^^^^^^^^ help: remove `return`: `true`
-
-error: unneeded `return` statement
-  --> $DIR/needless_return.rs:161:9
-   |
-LL |         return true;
-   |         ^^^^^^^^^^^^ help: remove `return`: `true`
-
-error: unneeded `return` statement
-  --> $DIR/needless_return.rs:163:16
-   |
-LL |     let _ = || return true;
-   |                ^^^^^^^^^^^ help: remove `return`: `true`
-
-error: unneeded `return` statement
-  --> $DIR/needless_return.rs:171:5
-   |
-LL |     return;
-   |     ^^^^^^^ help: remove `return`
-
-error: unneeded `return` statement
-  --> $DIR/needless_return.rs:176:9
-   |
-LL |         return;
-   |         ^^^^^^^ help: remove `return`
-
-error: unneeded `return` statement
-  --> $DIR/needless_return.rs:178:9
-   |
-LL |         return;
-   |         ^^^^^^^ help: remove `return`
-
-error: unneeded `return` statement
-  --> $DIR/needless_return.rs:185:14
+  --> $DIR/needless_return.rs:85:14
    |
 LL |         _ => return,
-   |              ^^^^^^ help: replace `return` with an empty block: `{}`
+   |              ^^^^^^ help: replace `return` with a unit value: `()`
 
 error: unneeded `return` statement
-  --> $DIR/needless_return.rs:200:9
+  --> $DIR/needless_return.rs:100:9
    |
 LL |         return String::from("test");
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: remove `return`: `String::from("test")`
 
 error: unneeded `return` statement
-  --> $DIR/needless_return.rs:202:9
+  --> $DIR/needless_return.rs:102:9
    |
 LL |         return String::new();
    |         ^^^^^^^^^^^^^^^^^^^^^ help: remove `return`: `String::new()`
 
-error: aborting due to 32 previous errors
+error: unneeded `return` statement
+  --> $DIR/needless_return.rs:124:32
+   |
+LL |         bar.unwrap_or_else(|_| return)
+   |                                ^^^^^^ help: replace `return` with an empty block: `{}`
+
+error: unneeded `return` statement
+  --> $DIR/needless_return.rs:129:13
+   |
+LL |             return;
+   |             ^^^^^^^ help: remove `return`
+
+error: unneeded `return` statement
+  --> $DIR/needless_return.rs:131:20
+   |
+LL |         let _ = || return;
+   |                    ^^^^^^ help: replace `return` with an empty block: `{}`
+
+error: unneeded `return` statement
+  --> $DIR/needless_return.rs:137:32
+   |
+LL |         res.unwrap_or_else(|_| return Foo)
+   |                                ^^^^^^^^^^ help: remove `return`: `Foo`
+
+error: unneeded `return` statement
+  --> $DIR/needless_return.rs:146:5
+   |
+LL |     return true;
+   |     ^^^^^^^^^^^^ help: remove `return`: `true`
+
+error: unneeded `return` statement
+  --> $DIR/needless_return.rs:150:5
+   |
+LL |     return true;
+   |     ^^^^^^^^^^^^ help: remove `return`: `true`
+
+error: unneeded `return` statement
+  --> $DIR/needless_return.rs:155:9
+   |
+LL |         return true;
+   |         ^^^^^^^^^^^^ help: remove `return`: `true`
+
+error: unneeded `return` statement
+  --> $DIR/needless_return.rs:157:9
+   |
+LL |         return false;
+   |         ^^^^^^^^^^^^^ help: remove `return`: `false`
+
+error: unneeded `return` statement
+  --> $DIR/needless_return.rs:163:17
+   |
+LL |         true => return false,
+   |                 ^^^^^^^^^^^^ help: remove `return`: `false`
+
+error: unneeded `return` statement
+  --> $DIR/needless_return.rs:165:13
+   |
+LL |             return true;
+   |             ^^^^^^^^^^^^ help: remove `return`: `true`
+
+error: unneeded `return` statement
+  --> $DIR/needless_return.rs:172:9
+   |
+LL |         return true;
+   |         ^^^^^^^^^^^^ help: remove `return`: `true`
+
+error: unneeded `return` statement
+  --> $DIR/needless_return.rs:174:16
+   |
+LL |     let _ = || return true;
+   |                ^^^^^^^^^^^ help: remove `return`: `true`
+
+error: unneeded `return` statement
+  --> $DIR/needless_return.rs:182:5
+   |
+LL |     return;
+   |     ^^^^^^^ help: remove `return`
+
+error: unneeded `return` statement
+  --> $DIR/needless_return.rs:187:9
+   |
+LL |         return;
+   |         ^^^^^^^ help: remove `return`
+
+error: unneeded `return` statement
+  --> $DIR/needless_return.rs:189:9
+   |
+LL |         return;
+   |         ^^^^^^^ help: remove `return`
+
+error: unneeded `return` statement
+  --> $DIR/needless_return.rs:196:14
+   |
+LL |         _ => return,
+   |              ^^^^^^ help: replace `return` with a unit value: `()`
+
+error: unneeded `return` statement
+  --> $DIR/needless_return.rs:211:9
+   |
+LL |         return String::from("test");
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: remove `return`: `String::from("test")`
+
+error: unneeded `return` statement
+  --> $DIR/needless_return.rs:213:9
+   |
+LL |         return String::new();
+   |         ^^^^^^^^^^^^^^^^^^^^^ help: remove `return`: `String::new()`
+
+error: aborting due to 34 previous errors