1 files changed, 148 insertions, 0 deletions
diff --git a/src/tools/clippy/clippy_lints/src/unused_unit.rs b/src/tools/clippy/clippy_lints/src/unused_unit.rs
new file mode 100644
index 000000000..52585e595
--- /dev/null
+++ b/src/tools/clippy/clippy_lints/src/unused_unit.rs
@@ -0,0 +1,148 @@
+use clippy_utils::diagnostics::span_lint_and_sugg;
+use clippy_utils::source::{position_before_rarrow, snippet_opt};
+use if_chain::if_chain;
+use rustc_ast::ast;
+use rustc_ast::visit::FnKind;
+use rustc_errors::Applicability;
+use rustc_lint::{EarlyContext, EarlyLintPass};
+use rustc_session::{declare_lint_pass, declare_tool_lint};
+use rustc_span::source_map::Span;
+use rustc_span::BytePos;
+
+declare_clippy_lint! {
+    /// ### What it does
+    /// Checks for unit (`()`) expressions that can be removed.
+    ///
+    /// ### Why is this bad?
+    /// Such expressions add no value, but can make the code
+    /// less readable. Depending on formatting they can make a `break` or `return`
+    /// statement look like a function call.
+    ///
+    /// ### Example
+    /// ```rust
+    /// fn return_unit() -> () {
+    ///     ()
+    /// }
+    /// ```
+    /// is equivalent to
+    /// ```rust
+    /// fn return_unit() {}
+    /// ```
+    #[clippy::version = "1.31.0"]
+    pub UNUSED_UNIT,
+    style,
+    "needless unit expression"
+}
+
+declare_lint_pass!(UnusedUnit => [UNUSED_UNIT]);
+
+impl EarlyLintPass for UnusedUnit {
+    fn check_fn(&mut self, cx: &EarlyContext<'_>, kind: FnKind<'_>, span: Span, _: ast::NodeId) {
+        if_chain! {
+            if let ast::FnRetTy::Ty(ref ty) = kind.decl().output;
+            if let ast::TyKind::Tup(ref vals) = ty.kind;
+            if vals.is_empty() && !ty.span.from_expansion() && get_def(span) == get_def(ty.span);
+            then {
+                lint_unneeded_unit_return(cx, ty, span);
+            }
+        }
+    }
+
+    fn check_block(&mut self, cx: &EarlyContext<'_>, block: &ast::Block) {
+        if_chain! {
+            if let Some(stmt) = block.stmts.last();
+            if let ast::StmtKind::Expr(ref expr) = stmt.kind;
+            if is_unit_expr(expr);
+            let ctxt = block.span.ctxt();
+            if stmt.span.ctxt() == ctxt && expr.span.ctxt() == ctxt;
+            then {
+                let sp = expr.span;
+                span_lint_and_sugg(
+                    cx,
+                    UNUSED_UNIT,
+                    sp,
+                    "unneeded unit expression",
+                    "remove the final `()`",
+                    String::new(),
+                    Applicability::MachineApplicable,
+                );
+            }
+        }
+    }
+
+    fn check_expr(&mut self, cx: &EarlyContext<'_>, e: &ast::Expr) {
+        match e.kind {
+            ast::ExprKind::Ret(Some(ref expr)) | ast::ExprKind::Break(_, Some(ref expr)) => {
+                if is_unit_expr(expr) && !expr.span.from_expansion() {
+                    span_lint_and_sugg(
+                        cx,
+                        UNUSED_UNIT,
+                        expr.span,
+                        "unneeded `()`",
+                        "remove the `()`",
+                        String::new(),
+                        Applicability::MachineApplicable,
+                    );
+                }
+            },
+            _ => (),
+        }
+    }
+
+    fn check_poly_trait_ref(&mut self, cx: &EarlyContext<'_>, poly: &ast::PolyTraitRef, _: &ast::TraitBoundModifier) {
+        let segments = &poly.trait_ref.path.segments;
+
+        if_chain! {
+            if segments.len() == 1;
+            if ["Fn", "FnMut", "FnOnce"].contains(&segments[0].ident.name.as_str());
+            if let Some(args) = &segments[0].args;
+            if let ast::GenericArgs::Parenthesized(generic_args) = &**args;
+            if let ast::FnRetTy::Ty(ty) = &generic_args.output;
+            if ty.kind.is_unit();
+            then {
+                lint_unneeded_unit_return(cx, ty, generic_args.span);
+            }
+        }
+    }
+}
+
+// get the def site
+#[must_use]
+fn get_def(span: Span) -> Option<Span> {
+    if span.from_expansion() {
+        Some(span.ctxt().outer_expn_data().def_site)
+    } else {
+        None
+    }
+}
+
+// is this expr a `()` unit?
+fn is_unit_expr(expr: &ast::Expr) -> bool {
+    if let ast::ExprKind::Tup(ref vals) = expr.kind {
+        vals.is_empty()
+    } else {
+        false
+    }
+}
+
+fn lint_unneeded_unit_return(cx: &EarlyContext<'_>, ty: &ast::Ty, span: Span) {
+    let (ret_span, appl) =
+        snippet_opt(cx, span.with_hi(ty.span.hi())).map_or((ty.span, Applicability::MaybeIncorrect), |fn_source| {
+            position_before_rarrow(&fn_source).map_or((ty.span, Applicability::MaybeIncorrect), |rpos| {
+                (
+                    #[expect(clippy::cast_possible_truncation)]
+                    ty.span.with_lo(BytePos(span.lo().0 + rpos as u32)),
+                    Applicability::MachineApplicable,
+                )
+            })
+        });
+    span_lint_and_sugg(
+        cx,
+        UNUSED_UNIT,
+        ret_span,
+        "unneeded unit return type",
+        "remove the `-> ()`",
+        String::new(),
+        appl,
+    );
+}