diff --git a/CHANGELOG.md b/CHANGELOG.md
index e2b0069279b..267bc07d8fa 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -3194,6 +3194,7 @@ Released 2018-09-13
[`uninit_vec`]: https://rust-lang.github.io/rust-clippy/master/index.html#uninit_vec
[`unit_arg`]: https://rust-lang.github.io/rust-clippy/master/index.html#unit_arg
[`unit_cmp`]: https://rust-lang.github.io/rust-clippy/master/index.html#unit_cmp
+[`unit_hash`]: https://rust-lang.github.io/rust-clippy/master/index.html#unit_hash
[`unit_return_expecting_ord`]: https://rust-lang.github.io/rust-clippy/master/index.html#unit_return_expecting_ord
[`unnecessary_cast`]: https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_cast
[`unnecessary_filter_map`]: https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_filter_map
diff --git a/clippy_lints/src/lib.register_all.rs b/clippy_lints/src/lib.register_all.rs
index e08bfeec7ca..4e123f27ec0 100644
--- a/clippy_lints/src/lib.register_all.rs
+++ b/clippy_lints/src/lib.register_all.rs
@@ -283,6 +283,7 @@ store.register_group(true, "clippy::all", Some("clippy_all"), vec![
LintId::of(undropped_manually_drops::UNDROPPED_MANUALLY_DROPS),
LintId::of(unicode::INVISIBLE_CHARACTERS),
LintId::of(uninit_vec::UNINIT_VEC),
+ LintId::of(unit_hash::UNIT_HASH),
LintId::of(unit_return_expecting_ord::UNIT_RETURN_EXPECTING_ORD),
LintId::of(unit_types::UNIT_ARG),
LintId::of(unit_types::UNIT_CMP),
diff --git a/clippy_lints/src/lib.register_correctness.rs b/clippy_lints/src/lib.register_correctness.rs
index ff56a6081fb..4217fd3a3ea 100644
--- a/clippy_lints/src/lib.register_correctness.rs
+++ b/clippy_lints/src/lib.register_correctness.rs
@@ -64,6 +64,7 @@ store.register_group(true, "clippy::correctness", Some("clippy_correctness"), ve
LintId::of(undropped_manually_drops::UNDROPPED_MANUALLY_DROPS),
LintId::of(unicode::INVISIBLE_CHARACTERS),
LintId::of(uninit_vec::UNINIT_VEC),
+ LintId::of(unit_hash::UNIT_HASH),
LintId::of(unit_return_expecting_ord::UNIT_RETURN_EXPECTING_ORD),
LintId::of(unit_types::UNIT_CMP),
LintId::of(unnamed_address::FN_ADDRESS_COMPARISONS),
diff --git a/clippy_lints/src/lib.register_lints.rs b/clippy_lints/src/lib.register_lints.rs
index ff0a0f97397..249bf77a8f2 100644
--- a/clippy_lints/src/lib.register_lints.rs
+++ b/clippy_lints/src/lib.register_lints.rs
@@ -477,6 +477,7 @@ store.register_lints(&[
unicode::NON_ASCII_LITERAL,
unicode::UNICODE_NOT_NFC,
uninit_vec::UNINIT_VEC,
+ unit_hash::UNIT_HASH,
unit_return_expecting_ord::UNIT_RETURN_EXPECTING_ORD,
unit_types::LET_UNIT_VALUE,
unit_types::UNIT_ARG,
diff --git a/clippy_lints/src/lib.rs b/clippy_lints/src/lib.rs
index 17555515a70..fc15c591623 100644
--- a/clippy_lints/src/lib.rs
+++ b/clippy_lints/src/lib.rs
@@ -364,6 +364,7 @@ mod undocumented_unsafe_blocks;
mod undropped_manually_drops;
mod unicode;
mod uninit_vec;
+mod unit_hash;
mod unit_return_expecting_ord;
mod unit_types;
mod unnamed_address;
@@ -522,6 +523,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
store.register_late_pass(|| Box::new(collapsible_match::CollapsibleMatch));
store.register_late_pass(|| Box::new(unicode::Unicode));
store.register_late_pass(|| Box::new(uninit_vec::UninitVec));
+ store.register_late_pass(|| Box::new(unit_hash::UnitHash));
store.register_late_pass(|| Box::new(unit_return_expecting_ord::UnitReturnExpectingOrd));
store.register_late_pass(|| Box::new(strings::StringAdd));
store.register_late_pass(|| Box::new(implicit_return::ImplicitReturn));
diff --git a/clippy_lints/src/unit_hash.rs b/clippy_lints/src/unit_hash.rs
new file mode 100644
index 00000000000..a3a3f2d41c7
--- /dev/null
+++ b/clippy_lints/src/unit_hash.rs
@@ -0,0 +1,77 @@
+use clippy_utils::diagnostics::span_lint_and_then;
+use clippy_utils::source::snippet;
+use rustc_errors::Applicability;
+use rustc_hir::{Expr, ExprKind};
+use rustc_lint::{LateContext, LateLintPass};
+use rustc_session::{declare_lint_pass, declare_tool_lint};
+use rustc_span::sym;
+
+declare_clippy_lint! {
+ /// ### What it does
+ /// Detects `().hash(_)`.
+ ///
+ /// ### Why is this bad?
+ /// Hashing a unit value doesn't do anything as the implementation of `Hash` for `()` is a no-op.
+ ///
+ /// ### Example
+ /// ```rust
+ /// # use std::hash::Hash;
+ /// # use std::collections::hash_map::DefaultHasher;
+ /// # enum Foo { Empty, WithValue(u8) }
+ /// # use Foo::*;
+ /// # let mut state = DefaultHasher::new();
+ /// # let my_enum = Foo::Empty;
+ /// match my_enum {
+ /// Empty => ().hash(&mut state),
+ /// WithValue(x) => x.hash(&mut state),
+ /// }
+ /// ```
+ /// Use instead:
+ /// ```rust
+ /// # use std::hash::Hash;
+ /// # use std::collections::hash_map::DefaultHasher;
+ /// # enum Foo { Empty, WithValue(u8) }
+ /// # use Foo::*;
+ /// # let mut state = DefaultHasher::new();
+ /// # let my_enum = Foo::Empty;
+ /// match my_enum {
+ /// Empty => 0_u8.hash(&mut state),
+ /// WithValue(x) => x.hash(&mut state),
+ /// }
+ /// ```
+ pub UNIT_HASH,
+ correctness,
+ "hashing a unit value, which does nothing"
+}
+declare_lint_pass!(UnitHash => [UNIT_HASH]);
+
+impl LateLintPass<'tcx> for UnitHash {
+ fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'tcx>) {
+ if_chain! {
+ if let ExprKind::MethodCall(name_ident, _, args, _) = &expr.kind;
+ if name_ident.ident.name == sym::hash;
+ if let [recv, state_param] = args;
+ if cx.typeck_results().expr_ty(recv).is_unit();
+ then {
+ span_lint_and_then(
+ cx,
+ UNIT_HASH,
+ expr.span,
+ "this call to `hash` on the unit type will do nothing",
+ |diag| {
+ diag.span_suggestion(
+ expr.span,
+ "remove the call to `hash` or consider using",
+ format!(
+ "0_u8.hash({})",
+ snippet(cx, state_param.span, ".."),
+ ),
+ Applicability::MaybeIncorrect,
+ );
+ diag.note("the implementation of `Hash` for `()` is a no-op");
+ }
+ );
+ }
+ }
+ }
+}
diff --git a/tests/ui/unit_hash.rs b/tests/ui/unit_hash.rs
new file mode 100644
index 00000000000..989916c239b
--- /dev/null
+++ b/tests/ui/unit_hash.rs
@@ -0,0 +1,27 @@
+#![warn(clippy::unit_hash)]
+
+use std::collections::hash_map::DefaultHasher;
+use std::hash::Hash;
+
+enum Foo {
+ Empty,
+ WithValue(u8),
+}
+
+fn do_nothing() {}
+
+fn main() {
+ let mut state = DefaultHasher::new();
+ let my_enum = Foo::Empty;
+
+ match my_enum {
+ Foo::Empty => ().hash(&mut state),
+ Foo::WithValue(x) => x.hash(&mut state),
+ }
+
+ let res = ();
+ res.hash(&mut state);
+
+ #[allow(clippy::unit_arg)]
+ do_nothing().hash(&mut state);
+}
diff --git a/tests/ui/unit_hash.stderr b/tests/ui/unit_hash.stderr
new file mode 100644
index 00000000000..da276296e02
--- /dev/null
+++ b/tests/ui/unit_hash.stderr
@@ -0,0 +1,27 @@
+error: this call to `hash` on the unit type will do nothing
+ --> $DIR/unit_hash.rs:18:23
+ |
+LL | Foo::Empty => ().hash(&mut state),
+ | ^^^^^^^^^^^^^^^^^^^ help: remove the call to `hash` or consider using: `0_u8.hash(&mut state)`
+ |
+ = note: `-D clippy::unit-hash` implied by `-D warnings`
+ = note: the implementation of `Hash` for `()` is a no-op
+
+error: this call to `hash` on the unit type will do nothing
+ --> $DIR/unit_hash.rs:23:5
+ |
+LL | res.hash(&mut state);
+ | ^^^^^^^^^^^^^^^^^^^^ help: remove the call to `hash` or consider using: `0_u8.hash(&mut state)`
+ |
+ = note: the implementation of `Hash` for `()` is a no-op
+
+error: this call to `hash` on the unit type will do nothing
+ --> $DIR/unit_hash.rs:26:5
+ |
+LL | do_nothing().hash(&mut state);
+ | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: remove the call to `hash` or consider using: `0_u8.hash(&mut state)`
+ |
+ = note: the implementation of `Hash` for `()` is a no-op
+
+error: aborting due to 3 previous errors
+