New lint [error_impl_error]

CHANGELOG.md

@@ -4819,6 +4819,7 @@ Released 2018-09-13
 [`equatable_if_let`]: https://rust-lang.github.io/rust-clippy/master/index.html#equatable_if_let
 [`erasing_op`]: https://rust-lang.github.io/rust-clippy/master/index.html#erasing_op
 [`err_expect`]: https://rust-lang.github.io/rust-clippy/master/index.html#err_expect
+[`error_impl_error`]: https://rust-lang.github.io/rust-clippy/master/index.html#error_impl_error
 [`eval_order_dependence`]: https://rust-lang.github.io/rust-clippy/master/index.html#eval_order_dependence
 [`excessive_nesting`]: https://rust-lang.github.io/rust-clippy/master/index.html#excessive_nesting
 [`excessive_precision`]: https://rust-lang.github.io/rust-clippy/master/index.html#excessive_precision

clippy_lints/src/declared_lints.rs

@@ -155,6 +155,7 @@ pub(crate) static LINTS: &[&crate::LintInfo] = &[
     crate::enum_variants::MODULE_INCEPTION_INFO,
     crate::enum_variants::MODULE_NAME_REPETITIONS_INFO,
     crate::equatable_if_let::EQUATABLE_IF_LET_INFO,
+    crate::error_impl_error::ERROR_IMPL_ERROR_INFO,
     crate::escape::BOXED_LOCAL_INFO,
     crate::eta_reduction::REDUNDANT_CLOSURE_INFO,
     crate::eta_reduction::REDUNDANT_CLOSURE_FOR_METHOD_CALLS_INFO,

clippy_lints/src/error_impl_error.rs

@@ -0,0 +1,76 @@
+use clippy_utils::{
+    diagnostics::{span_lint, span_lint_hir_and_then},
+    path_res,
+    ty::implements_trait,
+};
+use rustc_hir::{def_id::DefId, Item, ItemKind, Node};
+use rustc_hir_analysis::hir_ty_to_ty;
+use rustc_lint::{LateContext, LateLintPass};
+use rustc_session::{declare_lint_pass, declare_tool_lint};
+use rustc_span::sym;
+
+declare_clippy_lint! {
+    /// ### What it does
+    /// Checks for types named `Error` that implement `Error`.
+    ///
+    /// ### Why is this bad?
+    /// It can become confusing when a codebase has 20 types all named `Error`, requiring either
+    /// aliasing them in the `use` statement them or qualifying them like `my_module::Error`. This
+    /// severely hinders readability.
+    ///
+    /// ### Example
+    /// ```rust,ignore
+    /// #[derive(Debug)]
+    /// pub enum Error { ... }
+    ///
+    /// impl std::fmt::Display for Error { ... }
+    ///
+    /// impl std::error::Error for Error { ... }
+    /// ```
+    #[clippy::version = "1.72.0"]
+    pub ERROR_IMPL_ERROR,
+    restriction,
+    "types named `Error` that implement `Error`"
+}
+declare_lint_pass!(ErrorImplError => [ERROR_IMPL_ERROR]);
+
+impl<'tcx> LateLintPass<'tcx> for ErrorImplError {
+    fn check_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx Item<'tcx>) {
+        let Some(error_def_id) = cx.tcx.get_diagnostic_item(sym::Error) else {
+            return;
+        };
+
+        match item.kind {
+            ItemKind::TyAlias(ty, _) if implements_trait(cx, hir_ty_to_ty(cx.tcx, ty), error_def_id, &[])
+                && item.ident.name == sym::Error =>
+            {
+                span_lint(
+                    cx,
+                    ERROR_IMPL_ERROR,
+                    item.ident.span,
+                    "type alias named `Error` that implements `Error`",
+                );
+            },
+            ItemKind::Impl(imp) if let Some(trait_def_id) = imp.of_trait.and_then(|t| t.trait_def_id())
+                && error_def_id == trait_def_id
+                && let Some(def_id) = path_res(cx, imp.self_ty).opt_def_id().and_then(DefId::as_local)
+                && let hir_id = cx.tcx.hir().local_def_id_to_hir_id(def_id)
+                && let Node::Item(ty_item) = cx.tcx.hir().get(hir_id)
+                && ty_item.ident.name == sym::Error =>
+            {
+                span_lint_hir_and_then(
+                    cx,
+                    ERROR_IMPL_ERROR,
+                    hir_id,
+                    ty_item.ident.span,
+                    "type named `Error` that implements `Error`",
+                    |diag| {
+                        diag.span_note(item.span, "`Error` was implemented here");
+                    }
+                );
+            }
+            _ => {},
+        }
+        {}
+    }
+}

clippy_lints/src/lib.rs

@@ -120,6 +120,7 @@ mod entry;
 mod enum_clike;
 mod enum_variants;
 mod equatable_if_let;
+mod error_impl_error;
 mod escape;
 mod eta_reduction;
 mod excessive_bools;
@@ -1080,6 +1081,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
     store.register_late_pass(move |_| Box::new(tuple_array_conversions::TupleArrayConversions { msrv: msrv() }));
     store.register_late_pass(|_| Box::new(manual_float_methods::ManualFloatMethods));
     store.register_late_pass(|_| Box::new(four_forward_slashes::FourForwardSlashes));
+    store.register_late_pass(|_| Box::new(error_impl_error::ErrorImplError));
     // add lints here, do not remove this comment, it's used in `new_lint`
 }
 

tests/ui/error_impl_error.rs

@@ -0,0 +1,71 @@
+#![allow(unused)]
+#![warn(clippy::error_impl_error)]
+#![no_main]
+
+mod a {
+    #[derive(Debug)]
+    struct Error;
+
+    impl std::fmt::Display for Error {
+        fn fmt(&self, _: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+            todo!()
+        }
+    }
+
+    impl std::error::Error for Error {}
+}
+
+mod b {
+    #[derive(Debug)]
+    enum Error {}
+
+    impl std::fmt::Display for Error {
+        fn fmt(&self, _: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+            todo!()
+        }
+    }
+
+    impl std::error::Error for Error {}
+}
+
+mod c {
+    union Error {
+        a: u32,
+        b: u32,
+    }
+
+    impl std::fmt::Debug for Error {
+        fn fmt(&self, _: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+            todo!()
+        }
+    }
+
+    impl std::fmt::Display for Error {
+        fn fmt(&self, _: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+            todo!()
+        }
+    }
+
+    impl std::error::Error for Error {}
+}
+
+mod d {
+    type Error = std::fmt::Error;
+}
+
+mod e {
+    #[derive(Debug)]
+    struct MyError;
+
+    impl std::fmt::Display for MyError {
+        fn fmt(&self, _: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+            todo!()
+        }
+    }
+
+    impl std::error::Error for MyError {}
+}
+
+mod f {
+    type MyError = std::fmt::Error;
+}

tests/ui/error_impl_error.stderr

@@ -0,0 +1,45 @@
+error: type named `Error` that implements `Error`
+  --> $DIR/error_impl_error.rs:7:12
+   |
+LL |     struct Error;
+   |            ^^^^^
+   |
+note: `Error` was implemented here
+  --> $DIR/error_impl_error.rs:15:5
+   |
+LL |     impl std::error::Error for Error {}
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   = note: `-D clippy::error-impl-error` implied by `-D warnings`
+
+error: type named `Error` that implements `Error`
+  --> $DIR/error_impl_error.rs:20:10
+   |
+LL |     enum Error {}
+   |          ^^^^^
+   |
+note: `Error` was implemented here
+  --> $DIR/error_impl_error.rs:28:5
+   |
+LL |     impl std::error::Error for Error {}
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: type named `Error` that implements `Error`
+  --> $DIR/error_impl_error.rs:32:11
+   |
+LL |     union Error {
+   |           ^^^^^
+   |
+note: `Error` was implemented here
+  --> $DIR/error_impl_error.rs:49:5
+   |
+LL |     impl std::error::Error for Error {}
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: type alias named `Error` that implements `Error`
+  --> $DIR/error_impl_error.rs:53:10
+   |
+LL |     type Error = std::fmt::Error;
+   |          ^^^^^
+
+error: aborting due to 4 previous errors
+