Remove the _description_fns module.

`rustc_queries` generates a macro and two modules. One of the modules
looks like this:
```
mod _description_fns {
    ...

    #[allow(unused_variables)]
    pub fn hir_module_items<'tcx>(tcx: TyCtxt<'tcx>, key: LocalModDefId) -> String {
	format!("getting HIR module items in `{}`", tcx.def_path_str(key))
    }

    ...
}
```
Members of this module are then used in `TaggedQueryKey::description`.

This commit removes the `_description_fns` module entirely. For each
query we now instead generate a description closure that is used
instead. This closure is passed in the modifiers list.

This change simplifies `rustc_queries` quite a bit. It requires adding
another query modifier, but query modifiers are how other query-specific
details are already passed to the declarative macros, so it's more
consistent.

compiler/rustc_macros/src/query.rs

@@ -243,7 +243,7 @@ fn make_modifiers_stream(query: &Query) -> proc_macro2::TokenStream {
         arena_cache,
         cache_on_disk,
         depth_limit,
-        desc: _,
+        desc,
         eval_always,
         feedable,
         no_force,
@@ -256,7 +256,18 @@ fn make_modifiers_stream(query: &Query) -> proc_macro2::TokenStream {
     let arena_cache = arena_cache.is_some();
     let cache_on_disk = cache_on_disk.is_some();
     let depth_limit = depth_limit.is_some();
-    // `desc` is not handled here
+    let desc = {
+        // Put a description closure in the `desc` modifier.
+        let key_pat = &query.key_pat;
+        let key_ty = &query.key_ty;
+        let desc_expr_list = &desc.expr_list;
+        quote! {
+            {
+                #[allow(unused_variables)]
+                |tcx: TyCtxt<'tcx>, #key_pat: #key_ty| format!(#desc_expr_list)
+            }
+        }
+    };
     let eval_always = eval_always.is_some();
     let feedable = feedable.is_some();
     let no_force = no_force.is_some();
@@ -276,7 +287,7 @@ fn make_modifiers_stream(query: &Query) -> proc_macro2::TokenStream {
         arena_cache: #arena_cache,
         cache_on_disk: #cache_on_disk,
         depth_limit: #depth_limit,
-        // `desc` is not handled here
+        desc: #desc,
         eval_always: #eval_always,
         feedable: #feedable,
         no_force: #no_force,
@@ -314,37 +325,6 @@ fn doc_comment_from_desc(list: &Punctuated<Expr, token::Comma>) -> Result<Attrib
     Ok(parse_quote! { #[doc = #doc_string] })
 }
 
-/// Contains token streams that are used to accumulate per-query helper
-/// functions, to be used by the final output of `rustc_queries!`.
-///
-/// Helper items typically have the same name as the query they relate to,
-/// and expect to be interpolated into a dedicated module.
-#[derive(Default)]
-struct HelperTokenStreams {
-    description_fns_stream: proc_macro2::TokenStream,
-}
-
-fn make_helpers_for_query(query: &Query, streams: &mut HelperTokenStreams) {
-    let Query { name, key_pat, key_ty, modifiers, .. } = &query;
-
-    // Replace span for `name` to make rust-analyzer ignore it.
-    let mut erased_name = name.clone();
-    erased_name.set_span(Span::call_site());
-
-    let Desc { expr_list, .. } = &modifiers.desc;
-
-    let desc = quote! {
-        #[allow(unused_variables)]
-        pub fn #erased_name<'tcx>(tcx: TyCtxt<'tcx>, #key_pat: #key_ty) -> String {
-            format!(#expr_list)
-        }
-    };
-
-    streams.description_fns_stream.extend(quote! {
-        #desc
-    });
-}
-
 /// Add hints for rust-analyzer
 fn add_to_analyzer_stream(query: &Query, analyzer_stream: &mut proc_macro2::TokenStream) {
     // Add links to relevant modifiers
@@ -419,7 +399,6 @@ pub(super) fn rustc_queries(input: TokenStream) -> TokenStream {
 
     let mut query_stream = quote! {};
     let mut non_query_stream = quote! {};
-    let mut helpers = HelperTokenStreams::default();
     let mut analyzer_stream = quote! {};
     let mut errors = quote! {};
 
@@ -472,11 +451,8 @@ fn #name(#key_ty) #return_ty
         }
 
         add_to_analyzer_stream(&query, &mut analyzer_stream);
-        make_helpers_for_query(&query, &mut helpers);
     }
 
-    let HelperTokenStreams { description_fns_stream } = helpers;
-
     TokenStream::from(quote! {
         /// Higher-order macro that invokes the specified macro with (a) a list of all query
         /// signatures (including modifiers), and (b) a list of non-query names. This allows
@@ -500,17 +476,6 @@ mod _analyzer_hints {
             #analyzer_stream
         }
 
-        /// Functions that format a human-readable description of each query
-        /// and its key, as specified by the `desc` query modifier.
-        ///
-        /// (The leading `_` avoids collisions with actual query names when
-        /// expanded in `rustc_middle::queries`, and makes this macro-generated
-        /// module easier to search for.)
-        pub mod _description_fns {
-            use super::*;
-            #description_fns_stream
-        }
-
         #errors
     })
 }

compiler/rustc_middle/src/query/plumbing.rs

@@ -301,6 +301,7 @@ fn $name:ident($($K:tt)*) -> $V:ty
                     arena_cache: $arena_cache:literal,
                     cache_on_disk: $cache_on_disk:literal,
                     depth_limit: $depth_limit:literal,
+                    desc: $desc:expr,
                     eval_always: $eval_always:literal,
                     feedable: $feedable:literal,
                     no_force: $no_force:literal,
@@ -435,8 +436,7 @@ pub fn query_name(&self) -> &'static str {
             pub fn description(&self, tcx: TyCtxt<'tcx>) -> String {
                 let (name, description) = ty::print::with_no_queries!(match self {
                     $(
-                        TaggedQueryKey::$name(key) =>
-                            (stringify!($name), _description_fns::$name(tcx, *key)),
+                        TaggedQueryKey::$name(key) => (stringify!($name), ($desc)(tcx, *key)),
                     )*
                 });
                 if tcx.sess.verbose_internals() {

compiler/rustc_query_impl/src/dep_kind_vtables.rs

@@ -135,6 +135,7 @@ fn $name:ident($K:ty) -> $V:ty
                     arena_cache: $arena_cache:literal,
                     cache_on_disk: $cache_on_disk:literal,
                     depth_limit: $depth_limit:literal,
+                    desc: $desc:expr,
                     eval_always: $eval_always:literal,
                     feedable: $feedable:literal,
                     no_force: $no_force:literal,

compiler/rustc_query_impl/src/query_impl.rs

@@ -19,6 +19,7 @@ fn $name:ident($K:ty) -> $V:ty
                     arena_cache: $arena_cache:literal,
                     cache_on_disk: $cache_on_disk:literal,
                     depth_limit: $depth_limit:literal,
+                    desc: $desc:expr,
                     eval_always: $eval_always:literal,
                     feedable: $feedable:literal,
                     no_force: $no_force:literal,