Move UnitHash into Methods lint pass

clippy_lints/src/methods/unit_hash.rs

@@ -0,0 +1,29 @@
+use clippy_utils::diagnostics::span_lint_and_then;
+use clippy_utils::is_trait_method;
+use clippy_utils::source::snippet;
+use rustc_errors::Applicability;
+use rustc_hir::Expr;
+use rustc_lint::LateContext;
+use rustc_span::sym;
+
+use super::UNIT_HASH;
+
+pub(super) fn check<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>, recv: &'tcx Expr<'_>, arg: &'tcx Expr<'_>) {
+    if is_trait_method(cx, expr, sym::Hash) && cx.typeck_results().expr_ty(recv).is_unit() {
+        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, arg.span, ".."),),
+                    Applicability::MaybeIncorrect,
+                );
+                diag.note("the implementation of `Hash` for `()` is a no-op");
+            },
+        );
+    }
+}