Make the "detect-old-time" UI test more representative

The test code did have an inference failure, but that would have failed
on Rust 1.79 and earlier too. Now it is rewritten to be specifically
affected by 1.80's `impl FromIterator<_> for Box<str>`.

tests/ui/inference/detect-old-time-version-format_description-parse.rs

@@ -1,8 +1,13 @@
 #![crate_name = "time"]
+#![crate_type = "lib"]
 
-fn main() {
-    let items = Box::new(vec![]); //~ ERROR E0282
+// This code compiled without error in Rust 1.79, but started failing in 1.80
+// after the addition of several `impl FromIterator<_> for Box<str>`.
+
+pub fn parse() -> Option<Vec<()>> {
+    let iter = std::iter::once(Some(())).map(|o| o.map(Into::into));
+    let items = iter.collect::<Option<Box<_>>>()?; //~ ERROR E0282
+    //~^ NOTE this is an inference error on crate `time` caused by an API change in Rust 1.80.0; update `time` to version `>=0.3.35`
+    Some(items.into())
     //~^ NOTE type must be known at this point
-    //~| NOTE this is an inference error on crate `time` caused by an API change in Rust 1.80.0; update `time` to version `>=0.3.35`
-    items.into();
 }

tests/ui/inference/detect-old-time-version-format_description-parse.stderr

@@ -1,8 +1,11 @@
-error[E0282]: type annotations needed for `Box<Vec<_>>`
-  --> $DIR/detect-old-time-version-format_description-parse.rs:4:9
+error[E0282]: type annotations needed for `Box<_>`
+  --> $DIR/detect-old-time-version-format_description-parse.rs:9:9
    |
-LL |     let items = Box::new(vec![]);
-   |         ^^^^^   ---------------- type must be known at this point
+LL |     let items = iter.collect::<Option<Box<_>>>()?;
+   |         ^^^^^
+LL |
+LL |     Some(items.into())
+   |                ---- type must be known at this point
    |
    = note: this is an inference error on crate `time` caused by an API change in Rust 1.80.0; update `time` to version `>=0.3.35` by calling `cargo update`