Ignore underscore-prefixed args for needless_pass_by_value lint

When a user explicitly tags a param as unused (yet?), there is no need to raise another lint on it.

clippy_lints/src/needless_pass_by_value.rs

@@ -129,7 +129,7 @@ impl<'tcx> LateLintPass<'tcx> for NeedlessPassByValue {
             })
             .collect::<Vec<_>>();
 
-        // Collect moved variables and spans which will need dereferencings from the
+        // Collect moved variables and spans which will need dereferencing from the
         // function body.
         let MovedVariablesCtxt { moved_vars } = {
             let mut ctx = MovedVariablesCtxt::default();
@@ -148,12 +148,13 @@ impl<'tcx> LateLintPass<'tcx> for NeedlessPassByValue {
                 return;
             }
 
-            // Ignore `self`s.
+            // Ignore `self`s and params whose variable name starts with an underscore
-            if idx == 0 {
             if let PatKind::Binding(.., ident, _) = arg.pat.kind {
-                    if ident.name == kw::SelfLower {
+                if idx == 0 && ident.name == kw::SelfLower {
                     continue;
                 }
+                if ident.name.as_str().starts_with('_') {
+                    continue;
                 }
             }
 

tests/ui/needless_pass_by_value.rs

@@ -84,7 +84,7 @@ trait Serialize {}
 impl<'a, T> Serialize for &'a T where T: Serialize {}
 impl Serialize for i32 {}
 
-fn test_blanket_ref<T: Foo, S: Serialize>(_foo: T, _serializable: S) {}
+fn test_blanket_ref<T: Foo, S: Serialize>(vals: T, serializable: S) {}
 //~^ ERROR: this argument is passed by value, but not consumed in the function body
 
 fn issue_2114(s: String, t: String, u: Vec<i32>, v: Vec<i32>) {
@@ -116,7 +116,7 @@ impl<T: Serialize, U> S<T, U> {
     ) {
     }
 
-    fn baz(&self, _u: U, _s: Self) {}
+    fn baz(&self, uu: U, ss: Self) {}
     //~^ ERROR: this argument is passed by value, but not consumed in the function body
     //~| ERROR: this argument is passed by value, but not consumed in the function body
 }
@@ -162,13 +162,13 @@ fn test_destructure_copy(x: CopyWrapper, y: CopyWrapper, z: CopyWrapper) {
 // The following 3 lines should not cause an ICE. See #2831
 trait Bar<'a, A> {}
 impl<'b, T> Bar<'b, T> for T {}
-fn some_fun<'b, S: Bar<'b, ()>>(_item: S) {}
+fn some_fun<'b, S: Bar<'b, ()>>(items: S) {}
 //~^ ERROR: this argument is passed by value, but not consumed in the function body
 
 // Also this should not cause an ICE. See #2831
 trait Club<'a, A> {}
 impl<T> Club<'static, T> for T {}
-fn more_fun(_item: impl Club<'static, i32>) {}
+fn more_fun(items: impl Club<'static, i32>) {}
 //~^ ERROR: this argument is passed by value, but not consumed in the function body
 
 fn is_sync<T>(_: T)
@@ -177,6 +177,10 @@ where
 {
 }
 
+struct Obj(String);
+
+fn prefix_test(_unused_with_prefix: Obj) {}
+
 fn main() {
     // This should not cause an ICE either
     // https://github.com/rust-lang/rust-clippy/issues/3144

tests/ui/needless_pass_by_value.stderr

@@ -46,7 +46,7 @@ LL | fn test_destructure(x: Wrapper, y: Wrapper, z: Wrapper) {
 error: this argument is passed by value, but not consumed in the function body
   --> tests/ui/needless_pass_by_value.rs:87:49
    |
-LL | fn test_blanket_ref<T: Foo, S: Serialize>(_foo: T, _serializable: S) {}
+LL | fn test_blanket_ref<T: Foo, S: Serialize>(vals: T, serializable: S) {}
    |                                                 ^ help: consider taking a reference instead: `&T`
 
 error: this argument is passed by value, but not consumed in the function body
@@ -106,13 +106,13 @@ LL |         t: String,
 error: this argument is passed by value, but not consumed in the function body
   --> tests/ui/needless_pass_by_value.rs:119:23
    |
-LL |     fn baz(&self, _u: U, _s: Self) {}
+LL |     fn baz(&self, uu: U, ss: Self) {}
    |                       ^ help: consider taking a reference instead: `&U`
 
 error: this argument is passed by value, but not consumed in the function body
   --> tests/ui/needless_pass_by_value.rs:119:30
    |
-LL |     fn baz(&self, _u: U, _s: Self) {}
+LL |     fn baz(&self, uu: U, ss: Self) {}
    |                              ^^^^ help: consider taking a reference instead: `&Self`
 
 error: this argument is passed by value, but not consumed in the function body
@@ -166,13 +166,13 @@ LL | struct CopyWrapper(u32);
 error: this argument is passed by value, but not consumed in the function body
   --> tests/ui/needless_pass_by_value.rs:165:40
    |
-LL | fn some_fun<'b, S: Bar<'b, ()>>(_item: S) {}
+LL | fn some_fun<'b, S: Bar<'b, ()>>(items: S) {}
    |                                        ^ help: consider taking a reference instead: `&S`
 
 error: this argument is passed by value, but not consumed in the function body
   --> tests/ui/needless_pass_by_value.rs:171:20
    |
-LL | fn more_fun(_item: impl Club<'static, i32>) {}
+LL | fn more_fun(items: impl Club<'static, i32>) {}
    |                    ^^^^^^^^^^^^^^^^^^^^^^^ help: consider taking a reference instead: `&impl Club<'static, i32>`
 
 error: aborting due to 22 previous errors