add test

Author: RalfJung

tests/rustdoc-ui/target-feature-stability.rs

@@ -0,0 +1,18 @@
+//! This is a regression test for <https://github.com/rust-lang/rust/issues/137366>, ensuring
+//! that we can use the `neon` target feature on ARM-32 targets in rustdoc despite there
+//! being a "forbidden" feature of the same name for aarch64, and rustdoc merging the
+//! target features of all targets.
+//@ check-pass
+//@ compile-flags: --target armv7-unknown-linux-gnueabihf
+
+#![crate_type = "lib"]
+#![feature(no_core, lang_items)]
+#![feature(arm_target_feature)]
+#![no_core]
+
+#[lang = "sized"]
+pub trait Sized {}
+
+// `fp-armv8` is "forbidden" on aarch64 as we tie it to `neon`.
+#[target_feature(enable = "fp-armv8")]
+pub fn fun() {}