Rollup merge of rust-lang#94550 - GuillaumeGomez:HKF-macros, r=notriddle

rustdoc: Add test for higher kinded functions generated by macros

Fixes rust-lang#75564.

The problem has been solved apparently so adding a test to prevent a regression.

r? ```@notriddle```

src/test/rustdoc/macro-higher-kinded-function.rs

@@ -0,0 +1,21 @@
+#![crate_name = "foo"]
+
+pub struct TyCtxt<'tcx>(&'tcx u8);
+
+macro_rules! gen {
+    ($(($name:ident, $tcx:lifetime, [$k:ty], [$r:ty]))*) => {
+        pub struct Providers {
+            $(pub $name: for<$tcx> fn(TyCtxt<$tcx>, $k) -> $r,)*
+        }
+    }
+}
+
+// @has 'foo/struct.Providers.html'
+// @has - '//*[@class="docblock item-decl"]//code' "pub a: for<'tcx> fn(_: TyCtxt<'tcx>, _: u8) -> i8,"
+// @has - '//*[@class="docblock item-decl"]//code' "pub b: for<'tcx> fn(_: TyCtxt<'tcx>, _: u16) -> i16,"
+// @has - '//*[@id="structfield.a"]/code' "a: for<'tcx> fn(_: TyCtxt<'tcx>, _: u8) -> i8"
+// @has - '//*[@id="structfield.b"]/code' "b: for<'tcx> fn(_: TyCtxt<'tcx>, _: u16) -> i16"
+gen! {
+    (a, 'tcx, [u8], [i8])
+    (b, 'tcx, [u16], [i16])
+}