Adjust artificial-block.rs

- Document `artificial-block.rs`
- Move `artificial-block.rs` under `tests/ui/reachable`

Author: jieyouxu

Diff for: tests/ui/artificial-block.rs

@@ -0,0 +1,30 @@
+//! Check that we don't get compile errors on unreachable code after the `{ return 3; }` artificial
+//! block below. This test is run-pass to also exercise the codegen, but it might be possible to
+//! reduce to build-pass or even check-pass.
+//!
+//! This test was introduced as part of commit `a833f152baa17460e8414355e832d30d5161f8e8` which
+//! removes an "artificial block". See also commit `3d738e9e0634a4cd6239d1317bd7dad53be68dc8` for
+//! more elaboration, reproduced below (this is outdated for *today*'s rustc as of 2024-12-10, but
+//! is helpful to understand the original intention):
+//!
+//! > Return a fresh, unreachable context after ret, break, and cont
+//! >
+//! > This ensures we don't get compile errors on unreachable code (see
+//! > test/run-pass/artificial-block.rs for an example of sane code that wasn't compiling). In the
+//! > future, we might want to warn about non-trivial code appearing in an unreachable context,
+//! > and/or avoid generating unreachable code altogether (though I'm sure LLVM will weed it out as
+//! > well).
+//!
+//! Since then, `ret` became `return`, `int` became `isize` and `assert` became a macro.
+
+//@ run-pass
+
+fn f() -> isize {
+    {
+        return 3;
+    }
+}
+
+fn main() {
+    assert_eq!(f(), 3);
+}