Added the new lint with some docs

Author: botahamec

CHANGELOG.md

@@ -3989,6 +3989,7 @@ Released 2018-09-13
 [`unstable_as_mut_slice`]: https://rust-lang.github.io/rust-clippy/master/index.html#unstable_as_mut_slice
 [`unstable_as_slice`]: https://rust-lang.github.io/rust-clippy/master/index.html#unstable_as_slice
 [`unused_async`]: https://rust-lang.github.io/rust-clippy/master/index.html#unused_async
+[`unused_box`]: https://rust-lang.github.io/rust-clippy/master/index.html#unused_box
 [`unused_collect`]: https://rust-lang.github.io/rust-clippy/master/index.html#unused_collect
 [`unused_io_amount`]: https://rust-lang.github.io/rust-clippy/master/index.html#unused_io_amount
 [`unused_label`]: https://rust-lang.github.io/rust-clippy/master/index.html#unused_label

clippy_lints/src/lib.register_lints.rs

@@ -561,6 +561,7 @@ store.register_lints(&[
     unnested_or_patterns::UNNESTED_OR_PATTERNS,
     unsafe_removed_from_name::UNSAFE_REMOVED_FROM_NAME,
     unused_async::UNUSED_ASYNC,
+    unused_box::UNUSED_BOX,
     unused_io_amount::UNUSED_IO_AMOUNT,
     unused_rounding::UNUSED_ROUNDING,
     unused_self::UNUSED_SELF,

clippy_lints/src/lib.register_nursery.rs

@@ -31,6 +31,7 @@ store.register_group(true, "clippy::nursery", Some("clippy_nursery"), vec![
     LintId::of(trait_bounds::TRAIT_DUPLICATION_IN_BOUNDS),
     LintId::of(trait_bounds::TYPE_REPETITION_IN_BOUNDS),
     LintId::of(transmute::TRANSMUTE_UNDEFINED_REPR),
+    LintId::of(unused_box::UNUSED_BOX),
     LintId::of(unused_rounding::UNUSED_ROUNDING),
     LintId::of(use_self::USE_SELF),
 ])

clippy_lints/src/lib.rs

@@ -393,6 +393,7 @@ mod unnecessary_wraps;
 mod unnested_or_patterns;
 mod unsafe_removed_from_name;
 mod unused_async;
+mod unused_box;
 mod unused_io_amount;
 mod unused_rounding;
 mod unused_self;
@@ -915,6 +916,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
     let verbose_bit_mask_threshold = conf.verbose_bit_mask_threshold;
     store.register_late_pass(move || Box::new(operators::Operators::new(verbose_bit_mask_threshold)));
     store.register_late_pass(|| Box::new(invalid_utf8_in_unchecked::InvalidUtf8InUnchecked));
+    store.register_late_pass(|| Box::new(unused_box::UnusedBox));
     // add lints here, do not remove this comment, it's used in `new_lint`
 }
 

clippy_lints/src/unused_box.rs

@@ -0,0 +1,32 @@
+use rustc_lint::LateLintPass;
+use rustc_session::{declare_lint_pass, declare_tool_lint};
+
+declare_clippy_lint! {
+    /// ### What it does
+    ///
+    /// Checks for a return type containing a `Box<T>` where `T` implements `Sized`
+    ///
+    /// ### Why is this bad?
+    ///
+    /// It's better to just return `T` in these cases
+    ///
+    /// ### Example
+    /// ```rust
+    /// fn foo() -> Box<String> {
+    ///     Box::new(String::from("Hello, world!"))
+    /// }
+    /// ```
+    /// Use instead:
+    /// ```rust
+    /// fn foo() -> String {
+    ///     String::from("Hello, world!")
+    /// }
+    /// ```
+    #[clippy::version = "1.64.0"]
+    pub UNUSED_BOX,
+    nursery,
+    "default lint description"
+}
+declare_lint_pass!(UnusedBox => [UNUSED_BOX]);
+
+impl LateLintPass<'_> for UnusedBox {}

tests/ui/unknown_clippy_lints.fixed

@@ -12,7 +12,7 @@
 // Shouldn't suggest rustc lint name(`dead_code`)
 #[warn(clippy::drop_copy)]
 // Shouldn't suggest removed/deprecated clippy lint name(`unused_collect`)
-#[warn(clippy::unused_self)]
+#[warn(clippy::unused_box)]
 // Shouldn't suggest renamed clippy lint name(`const_static_lifetime`)
 #[warn(clippy::redundant_static_lifetimes)]
 fn main() {}

tests/ui/unknown_clippy_lints.stderr

@@ -40,7 +40,7 @@ error: unknown lint: `clippy::unused_colle`
   --> $DIR/unknown_clippy_lints.rs:15:8
    |
 LL | #[warn(clippy::unused_colle)]
-   |        ^^^^^^^^^^^^^^^^^^^^ help: did you mean: `clippy::unused_self`
+   |        ^^^^^^^^^^^^^^^^^^^^ help: did you mean: `clippy::unused_box`
 
 error: unknown lint: `clippy::const_static_lifetim`
   --> $DIR/unknown_clippy_lints.rs:17:8

tests/ui/unused_box.rs

@@ -0,0 +1,5 @@
+#![warn(clippy::unused_box)]
+
+fn main() {
+    // test code goes here
+}

tests/ui/unused_box.stderr

@@ -0,0 +1,10 @@
+error: function returns `Box<usize>` when `usize` implements `Sized`
+  --> $DIR/unused_box.rs:3:13
+   |
+LL | fn foo() -> Box<usize> {
+   |             ^^^^^^^^^^ help: change the return type to: `usize`
+   |
+   = note: `-D clippy::unused-box` implied by `-D warnings`
+
+error: aborting due to previous error
+