Allow to set a query's result as a side effect.

compiler/rustc_macros/src/query.rs

@@ -114,6 +114,9 @@ struct QueryModifiers {
 
     /// Always remap the ParamEnv's constness before hashing.
     remap_env_constness: Option<Ident>,
+
+    /// Generate a `feed` method to set the query's value from another query.
+    feedable: Option<Ident>,
 }
 
 fn parse_query_modifiers(input: ParseStream<'_>) -> Result<QueryModifiers> {
@@ -128,6 +131,7 @@ fn parse_query_modifiers(input: ParseStream<'_>) -> Result<QueryModifiers> {
     let mut depth_limit = None;
     let mut separate_provide_extern = None;
     let mut remap_env_constness = None;
+    let mut feedable = None;
 
     while !input.is_empty() {
         let modifier: Ident = input.parse()?;
@@ -187,6 +191,8 @@ fn parse_query_modifiers(input: ParseStream<'_>) -> Result<QueryModifiers> {
             try_insert!(separate_provide_extern = modifier);
         } else if modifier == "remap_env_constness" {
             try_insert!(remap_env_constness = modifier);
+        } else if modifier == "feedable" {
+            try_insert!(feedable = modifier);
         } else {
             return Err(Error::new(modifier.span(), "unknown query modifier"));
         }
@@ -206,6 +212,7 @@ fn parse_query_modifiers(input: ParseStream<'_>) -> Result<QueryModifiers> {
         depth_limit,
         separate_provide_extern,
         remap_env_constness,
+        feedable,
     })
 }
 
@@ -296,6 +303,7 @@ pub fn rustc_queries(input: TokenStream) -> TokenStream {
     let mut query_stream = quote! {};
     let mut query_description_stream = quote! {};
     let mut query_cached_stream = quote! {};
+    let mut feedable_queries = quote! {};
 
     for query in queries.0 {
         let Query { name, arg, modifiers, .. } = &query;
@@ -350,6 +358,13 @@ pub fn rustc_queries(input: TokenStream) -> TokenStream {
             [#attribute_stream] fn #name(#arg) #result,
         });
 
+        if modifiers.feedable.is_some() {
+            feedable_queries.extend(quote! {
+                #(#doc_comments)*
+                [#attribute_stream] fn #name(#arg) #result,
+            });
+        }
+
         add_query_desc_cached_impl(&query, &mut query_description_stream, &mut query_cached_stream);
     }
 
@@ -363,7 +378,11 @@ pub fn rustc_queries(input: TokenStream) -> TokenStream {
                 }
             }
         }
-
+        macro_rules! rustc_feedable_queries {
+            ( $macro:ident! ) => {
+                $macro!(#feedable_queries);
+            }
+        }
         pub mod descs {
             use super::*;
             #query_description_stream