[unnecessary_lazy_eval]: reduce applicability if closure has return type annotation

clippy_lints/src/methods/unnecessary_lazy_eval.rs

@@ -2,6 +2,7 @@ use clippy_utils::diagnostics::span_lint_and_then;
 use clippy_utils::source::snippet;
 use clippy_utils::ty::is_type_diagnostic_item;
 use clippy_utils::{eager_or_lazy, is_from_proc_macro, usage};
+use hir::FnRetTy;
 use rustc_errors::Applicability;
 use rustc_hir as hir;
 use rustc_lint::LateContext;
@@ -27,7 +28,7 @@ pub(super) fn check<'tcx>(
     let is_bool = cx.typeck_results().expr_ty(recv).is_bool();
 
     if is_option || is_result || is_bool {
-        if let hir::ExprKind::Closure(&hir::Closure { body, .. }) = arg.kind {
+        if let hir::ExprKind::Closure(&hir::Closure { body, fn_decl, .. }) = arg.kind {
             let body = cx.tcx.hir().body(body);
             let body_expr = &body.value;
 
@@ -48,7 +49,14 @@ pub(super) fn check<'tcx>(
                     .iter()
                     // bindings are checked to be unused above
                     .all(|param| matches!(param.pat.kind, hir::PatKind::Binding(..) | hir::PatKind::Wild))
-                {
+                    && matches!(
+                        fn_decl.output,
+                        FnRetTy::DefaultReturn(_)
+                            | FnRetTy::Return(hir::Ty {
+                                kind: hir::TyKind::Infer,
+                                ..
+                            })
+                    ) {
                     Applicability::MachineApplicable
                 } else {
                     // replacing the lambda may break type inference

tests/ui/unnecessary_lazy_eval.fixed

@@ -5,6 +5,7 @@
 #![allow(clippy::map_identity)]
 #![allow(clippy::needless_borrow)]
 #![allow(clippy::unnecessary_literal_unwrap)]
+#![allow(clippy::unit_arg)]
 
 use std::ops::Deref;
 
@@ -76,6 +77,8 @@ fn main() {
     let _ = opt.ok_or(2);
     let _ = nested_tuple_opt.unwrap_or(Some((1, 2)));
     let _ = cond.then_some(astronomers_pi);
+    let _ = true.then_some({});
+    let _ = true.then_some({});
 
     // Should lint - Builtin deref
     let r = &1;

tests/ui/unnecessary_lazy_eval.rs

@@ -5,6 +5,7 @@
 #![allow(clippy::map_identity)]
 #![allow(clippy::needless_borrow)]
 #![allow(clippy::unnecessary_literal_unwrap)]
+#![allow(clippy::unit_arg)]
 
 use std::ops::Deref;
 
@@ -76,6 +77,8 @@ fn main() {
     let _ = opt.ok_or_else(|| 2);
     let _ = nested_tuple_opt.unwrap_or_else(|| Some((1, 2)));
     let _ = cond.then(|| astronomers_pi);
+    let _ = true.then(|| -> _ {});
+    let _ = true.then(|| {});
 
     // Should lint - Builtin deref
     let r = &1;