Rollup merge of rust-lang#147171 - Qelxiros:hashmap_diag, r=fee1-dead

recommend using a HashMap if a HashSet's second generic parameter doesn't implement BuildHasher

closes rust-lang#147147

~The suggestion span is wrong, but I'm not sure how to find the right one.~ fixed

I'm relatively new to the diagnostics ecosystem, so I'm not sure if `span_help` is the right choice. `span_suggestion_*` might be better, but the output from `x test` looks weird in that case.

Author: matthiaskrgr

core/src/hash/mod.rs

@@ -633,6 +633,7 @@ impl<H: Hasher + ?Sized> Hasher for &mut H {
 ///
 /// [`build_hasher`]: BuildHasher::build_hasher
 /// [`HashMap`]: ../../std/collections/struct.HashMap.html
+#[cfg_attr(not(test), rustc_diagnostic_item = "BuildHasher")]
 #[stable(since = "1.7.0", feature = "build_hasher")]
 pub trait BuildHasher {
     /// Type of the hasher that will be created.