Fix if_then_some_else_none sugg missing closure intro

Fixes rust-lang#13407

Author: lukaslueg

clippy_lints/src/if_then_some_else_none.rs

@@ -105,6 +105,8 @@ impl<'tcx> LateLintPass<'tcx> for IfThenSomeElseNone {
                             snippet_with_context(cx, first_stmt.span.until(then_arg.span), ctxt, "..", &mut app);
                         let closure = if method_name == "then" { "|| " } else { "" };
                         format!("{closure} {{ {block_snippet}; {arg_snip} }}")
+                    } else if method_name == "then" {
+                        (std::borrow::Cow::Borrowed("|| ") + arg_snip).into_owned()
                     } else {
                         arg_snip.into_owned()
                     };

tests/ui/if_then_some_else_none.fixed

@@ -113,6 +113,10 @@ fn issue11394(b: bool, v: Result<(), ()>) -> Result<(), ()> {
     Ok(())
 }
 
+fn issue13407(s: &str) -> Option<bool> {
+    (s == "1").then(|| true)
+}
+
 const fn issue12103(x: u32) -> Option<u32> {
     // Should not issue an error in `const` context
     if x > 42 { Some(150) } else { None }

tests/ui/if_then_some_else_none.rs

@@ -131,6 +131,10 @@ fn issue11394(b: bool, v: Result<(), ()>) -> Result<(), ()> {
     Ok(())
 }
 
+fn issue13407(s: &str) -> Option<bool> {
+    if s == "1" { Some(true) } else { None }
+}
+
 const fn issue12103(x: u32) -> Option<u32> {
     // Should not issue an error in `const` context
     if x > 42 { Some(150) } else { None }

tests/ui/if_then_some_else_none.stderr

@@ -52,5 +52,11 @@ LL | |         None
 LL | |     };
    | |_____^ help: try: `foo().then(||  { println!("true!"); 150 })`
 
-error: aborting due to 5 previous errors
+error: this could be simplified with `bool::then`
+  --> tests/ui/if_then_some_else_none.rs:135:5
+   |
+LL |     if s == "1" { Some(true) } else { None }
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `(s == "1").then(|| true)`
+
+error: aborting due to 6 previous errors