add diagnostic items

Add diagnostic items to the following types:
  OsString (os_string_type)
  PathBuf (path_buf_type)
  Owned (to_owned_trait)

As well as the to_vec method on slice/[T]

compiler/rustc_span/src/symbol.rs

@@ -796,6 +796,7 @@ symbols! {
         options,
         or,
         or_patterns,
+        os_string_type,
         other,
         out,
         overlapping_marker_traits,
@@ -824,6 +825,7 @@ symbols! {
         pat2018,
         pat2021,
         path,
+        path_buf_type,
         pattern_parentheses,
         phantom_data,
         pin,
@@ -1084,6 +1086,7 @@ symbols! {
         slice,
         slice_alloc,
         slice_patterns,
+        slice_to_vec_method,
         slice_u8,
         slice_u8_alloc,
         slicing_syntax,
@@ -1159,6 +1162,7 @@ symbols! {
         then_with,
         thread,
         thread_local,
+        to_owned_trait,
         tool_attributes,
         tool_lints,
         trace_macros,

library/alloc/src/borrow.rs

@@ -32,6 +32,7 @@ where
 /// to `T`. The `ToOwned` trait generalizes `Clone` to construct owned data
 /// from any borrow of a given type.
 #[stable(feature = "rust1", since = "1.0.0")]
+#[cfg_attr(not(test), rustc_diagnostic_item = "to_owned_trait")]
 pub trait ToOwned {
     /// The resulting type after obtaining ownership.
     #[stable(feature = "rust1", since = "1.0.0")]

library/alloc/src/slice.rs

@@ -442,6 +442,7 @@ impl<T> [T] {
     /// // Here, `s` and `x` can be modified independently.
     /// ```
     #[rustc_conversion_suggestion]
+    #[rustc_diagnostic_item = "slice_to_vec_method"]
     #[stable(feature = "rust1", since = "1.0.0")]
     #[inline]
     pub fn to_vec(&self) -> Vec<T>

library/std/src/ffi/os_str.rs

@@ -71,6 +71,7 @@ use crate::sys_common::{AsInner, FromInner, IntoInner};
 /// [`CStr`]: crate::ffi::CStr
 /// [conversions]: super#conversions
 #[derive(Clone)]
+#[cfg_attr(not(test), rustc_diagnostic_item = "os_string_type")]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub struct OsString {
     inner: Buf,

library/std/src/path.rs

@@ -1066,6 +1066,7 @@ impl FusedIterator for Ancestors<'_> {}
 ///
 /// Which method works best depends on what kind of situation you're in.
 #[derive(Clone)]
+#[cfg_attr(not(test), rustc_diagnostic_item = "path_buf_type")]
 #[stable(feature = "rust1", since = "1.0.0")]
 // FIXME:
 // `PathBuf::as_mut_vec` current implementation relies