Auto merge of rust-lang#96520 - lcnr:general-incoherent-impls, r=petrochenkov

generalize "incoherent impls" impl for user defined types

To allow the move of `trait Error` into core.

continues the work from rust-lang#94963, finishes rust-lang/compiler-team#487

r? `@petrochenkov` cc `@yaahc`

Author: bors

core/src/ffi/c_str.rs

@@ -77,7 +77,7 @@ use crate::str;
 #[derive(Hash)]
 #[cfg_attr(not(test), rustc_diagnostic_item = "CStr")]
 #[unstable(feature = "core_c_str", issue = "94079")]
-#[cfg_attr(not(bootstrap), lang = "CStr")]
+#[cfg_attr(not(bootstrap), rustc_has_incoherent_inherent_impls)]
 // FIXME:
 // `fn from` in `impl From<&CStr> for Box<CStr>` current implementation relies
 // on `CStr` being layout-compatible with `[u8]`.