new lint: needless traits in scope

CHANGELOG.md

@@ -4759,6 +4759,7 @@ Released 2018-09-13
 [`needless_range_loop`]: https://rust-lang.github.io/rust-clippy/master/index.html#needless_range_loop
 [`needless_return`]: https://rust-lang.github.io/rust-clippy/master/index.html#needless_return
 [`needless_splitn`]: https://rust-lang.github.io/rust-clippy/master/index.html#needless_splitn
+[`needless_traits_in_scope`]: https://rust-lang.github.io/rust-clippy/master/index.html#needless_traits_in_scope
 [`needless_update`]: https://rust-lang.github.io/rust-clippy/master/index.html#needless_update
 [`neg_cmp_op_on_partial_ord`]: https://rust-lang.github.io/rust-clippy/master/index.html#neg_cmp_op_on_partial_ord
 [`neg_multiply`]: https://rust-lang.github.io/rust-clippy/master/index.html#neg_multiply

clippy_lints/src/declared_lints.rs

@@ -446,6 +446,7 @@ pub(crate) static LINTS: &[&crate::LintInfo] = &[
     crate::needless_parens_on_range_literals::NEEDLESS_PARENS_ON_RANGE_LITERALS_INFO,
     crate::needless_pass_by_value::NEEDLESS_PASS_BY_VALUE_INFO,
     crate::needless_question_mark::NEEDLESS_QUESTION_MARK_INFO,
+    crate::needless_traits_in_scope::NEEDLESS_TRAITS_IN_SCOPE_INFO,
     crate::needless_update::NEEDLESS_UPDATE_INFO,
     crate::neg_cmp_op_on_partial_ord::NEG_CMP_OP_ON_PARTIAL_ORD_INFO,
     crate::neg_multiply::NEG_MULTIPLY_INFO,

clippy_lints/src/lib.rs

@@ -218,6 +218,7 @@ mod needless_late_init;
 mod needless_parens_on_range_literals;
 mod needless_pass_by_value;
 mod needless_question_mark;
+mod needless_traits_in_scope;
 mod needless_update;
 mod neg_cmp_op_on_partial_ord;
 mod neg_multiply;
@@ -934,6 +935,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
     store.register_late_pass(|_| Box::new(missing_assert_message::MissingAssertMessage));
     store.register_early_pass(|| Box::new(redundant_async_block::RedundantAsyncBlock));
     store.register_late_pass(|_| Box::new(let_with_type_underscore::UnderscoreTyped));
+    store.register_late_pass(|_| Box::new(needless_traits_in_scope::NeedlessTraitsInScope));
     // add lints here, do not remove this comment, it's used in `new_lint`
 }
 

clippy_lints/src/needless_traits_in_scope.rs

@@ -0,0 +1,76 @@
+use clippy_utils::diagnostics::span_lint_and_sugg;
+use rustc_errors::Applicability;
+use rustc_hir::*;
+use rustc_lint::{LateContext, LateLintPass};
+use rustc_session::{declare_lint_pass, declare_tool_lint};
+
+declare_clippy_lint! {
+    /// ### What it does
+    /// Find traits that are not explicitely used in scope (like `AsRef::as_ref()`)
+    /// but directly with the trait method (like `opt.as_ref()`).
+    ///
+    /// These traits can be imported anonymously with `use crate::Trait as _`.
+    /// This avoids name collision with other traits (possibly with the same name).
+    /// It also helps identify the traits in `use` statements.
+    ///
+    /// ### Why is this bad?
+    /// This needlessly brings a trait in trait's namespace.
+    ///
+    /// ### Example
+    /// ```rust
+    /// use std::io::Read;
+    /// fn main() {
+    ///   let mut b = "I'm your father!".as_bytes();
+    ///   let mut buffer = [0; 10];
+    ///   b.read(&mut buffer)
+    /// }
+    /// ```
+    /// Use instead:
+    /// ```rust
+    /// use std::io::Read as _;
+    /// fn main() {
+    ///   let mut b = "I'm your father!".as_bytes();
+    ///   let mut buffer = [0; 10];
+    ///   b.read(&mut buffer)
+    /// }
+    /// ```
+    #[clippy::version = "1.69.0"]
+    pub NEEDLESS_TRAITS_IN_SCOPE,
+    pedantic,
+    "trait is needlessly imported in trait's namespace, and can be anonymously imported"
+}
+declare_lint_pass!(NeedlessTraitsInScope => [NEEDLESS_TRAITS_IN_SCOPE]);
+
+impl<'tcx> LateLintPass<'tcx> for NeedlessTraitsInScope {
+    fn check_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx Item<'tcx>) {
+        // Only process `use` statements, ignore `UseKind::Glob`
+        let ItemKind::Use(use_path, UseKind::Single) = item.kind else {
+            return
+        };
+        // Check if the `use` is aliased with ` as `.
+        // If aliased, then do not process, it's probably for a good reason
+        if item.ident != use_path.segments.last().unwrap().ident {
+            return;
+        }
+        let path = use_path
+            .segments
+            .iter()
+            .map(|segment| segment.ident)
+            .fold(String::new(), |mut acc, ident| {
+                if !acc.is_empty() {
+                    acc += "::";
+                }
+                acc += ident.as_str();
+                acc
+            });
+        span_lint_and_sugg(
+            cx,
+            NEEDLESS_TRAITS_IN_SCOPE,
+            use_path.span,
+            "trait is needlessly imported in trait's namespace",
+            "you can import the trait anonymously",
+            format!("{path} as _"),
+            Applicability::MachineApplicable,
+        );
+    }
+}

tests/ui/needless_traits_in_scope.fixed

@@ -0,0 +1,40 @@
+// run-rustfix
+#![allow(unused)]
+#![warn(clippy::needless_traits_in_scope)]
+
+pub mod useless_trait_in_scope {
+    use std::io::Read as _;
+
+    pub fn warn() -> std::io::Result<usize> {
+        let mut b = "The trait is not used explicitely -> 'use std::io::Read' doesn't need to be in scope".as_bytes();
+        let mut buffer = [0; 10];
+        b.read(&mut buffer)
+    }
+}
+
+pub mod trait_not_in_scope {
+    use std::io::Read as _;
+
+    pub fn ok() -> std::io::Result<usize> {
+        let mut b = "The trait is not used explicitely, but 'use std::io::Read' is already not in scope".as_bytes();
+        let mut buffer = [0; 10];
+        b.read(&mut buffer)
+    }
+}
+
+// FIXME: when the trait is explicitely used, the lint should not trigger
+// pub mod useful_trait_in_scope {
+//     use std::io::Read;
+
+//     pub fn ok() -> std::io::Result<usize> {
+//         let mut b = "Trait is used explicitely -> 'use std::io::Read' is OK".as_bytes();
+//         let mut buffer = [0; 10];
+//         Read::read(&mut b, &mut buffer)
+//     }
+// }
+
+fn main() {
+    useless_trait_in_scope::warn();
+    trait_not_in_scope::ok();
+    // useful_trait_in_scope::ok();
+}

tests/ui/needless_traits_in_scope.rs

@@ -0,0 +1,40 @@
+// run-rustfix
+#![allow(unused)]
+#![warn(clippy::needless_traits_in_scope)]
+
+pub mod useless_trait_in_scope {
+    use std::io::Read;
+
+    pub fn warn() -> std::io::Result<usize> {
+        let mut b = "The trait is not used explicitely -> 'use std::io::Read' doesn't need to be in scope".as_bytes();
+        let mut buffer = [0; 10];
+        b.read(&mut buffer)
+    }
+}
+
+pub mod trait_not_in_scope {
+    use std::io::Read as _;
+
+    pub fn ok() -> std::io::Result<usize> {
+        let mut b = "The trait is not used explicitely, but 'use std::io::Read' is already not in scope".as_bytes();
+        let mut buffer = [0; 10];
+        b.read(&mut buffer)
+    }
+}
+
+// FIXME: when the trait is explicitely used, the lint should not trigger
+// pub mod useful_trait_in_scope {
+//     use std::io::Read;
+
+//     pub fn ok() -> std::io::Result<usize> {
+//         let mut b = "Trait is used explicitely -> 'use std::io::Read' is OK".as_bytes();
+//         let mut buffer = [0; 10];
+//         Read::read(&mut b, &mut buffer)
+//     }
+// }
+
+fn main() {
+    useless_trait_in_scope::warn();
+    trait_not_in_scope::ok();
+    // useful_trait_in_scope::ok();
+}

tests/ui/needless_traits_in_scope.stderr

@@ -0,0 +1,10 @@
+error: trait is needlessly imported in trait's namespace
+  --> $DIR/needless_traits_in_scope.rs:6:9
+   |
+LL |     use std::io::Read;
+   |         ^^^^^^^^^^^^^ help: you can import the trait anonymously: `std::io::Read as _`
+   |
+   = note: `-D clippy::needless-traits-in-scope` implied by `-D warnings`
+
+error: aborting due to previous error
+