proper applicability for obfuscated_if_else (#14061)

fix #14034

The currect implementation of `obfuscated_if_else` sometimes makes
incorrect suggestions when the original code have side effects (see the
example in the above issue). I think this can be fixed by changing the
applicability depending on whether it can have side effects or not.

changelog: [`obfuscated_if_else`]: change applicability when the
original code can have side effects

clippy_lints/src/methods/obfuscated_if_else.rs

@@ -1,5 +1,6 @@
 use super::OBFUSCATED_IF_ELSE;
 use clippy_utils::diagnostics::span_lint_and_sugg;
+use clippy_utils::eager_or_lazy::switch_to_eager_eval;
 use clippy_utils::source::snippet_with_applicability;
 use clippy_utils::sugg::Sugg;
 use rustc_errors::Applicability;
@@ -18,7 +19,12 @@ pub(super) fn check<'tcx>(
     let recv_ty = cx.typeck_results().expr_ty(then_recv);
 
     if recv_ty.is_bool() {
-        let mut applicability = Applicability::MachineApplicable;
+        let mut applicability = if switch_to_eager_eval(cx, then_arg) && switch_to_eager_eval(cx, unwrap_arg) {
+            Applicability::MachineApplicable
+        } else {
+            Applicability::MaybeIncorrect
+        };
+
         let if_then = match then_method_name {
             "then" if let ExprKind::Closure(closure) = then_arg.kind => {
                 let body = cx.tcx.hir().body(closure.body);

tests/ui/obfuscated_if_else.fixed

@@ -1,5 +1,5 @@
 #![warn(clippy::obfuscated_if_else)]
-#![allow(clippy::unnecessary_lazy_evaluations)]
+#![allow(clippy::unnecessary_lazy_evaluations, clippy::unit_arg, clippy::unused_unit)]
 
 fn main() {
     if true { "a" } else { "b" };
@@ -11,4 +11,8 @@ fn main() {
 
     let partial = (a == 1).then_some("a");
     partial.unwrap_or("b"); // not lint
+
+    let mut a = 0;
+    if true { a += 1 } else { () };
+    if true { () } else { a += 2 };
 }

tests/ui/obfuscated_if_else.rs

@@ -1,5 +1,5 @@
 #![warn(clippy::obfuscated_if_else)]
-#![allow(clippy::unnecessary_lazy_evaluations)]
+#![allow(clippy::unnecessary_lazy_evaluations, clippy::unit_arg, clippy::unused_unit)]
 
 fn main() {
     true.then_some("a").unwrap_or("b");
@@ -11,4 +11,8 @@ fn main() {
 
     let partial = (a == 1).then_some("a");
     partial.unwrap_or("b"); // not lint
+
+    let mut a = 0;
+    true.then_some(a += 1).unwrap_or(());
+    true.then_some(()).unwrap_or(a += 2);
 }

tests/ui/obfuscated_if_else.stderr

@@ -25,5 +25,17 @@ error: this method chain can be written more clearly with `if .. else ..`
 LL |     (a == 1).then(|| "a").unwrap_or("b");
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `if a == 1 { "a" } else { "b" }`
 
-error: aborting due to 4 previous errors
+error: this method chain can be written more clearly with `if .. else ..`
+  --> tests/ui/obfuscated_if_else.rs:16:5
+   |
+LL |     true.then_some(a += 1).unwrap_or(());
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `if true { a += 1 } else { () }`
+
+error: this method chain can be written more clearly with `if .. else ..`
+  --> tests/ui/obfuscated_if_else.rs:17:5
+   |
+LL |     true.then_some(()).unwrap_or(a += 2);
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `if true { () } else { a += 2 }`
+
+error: aborting due to 6 previous errors