From fd036a4f154c46253443b3a79b6f4400c40e87b1 Mon Sep 17 00:00:00 2001
From: Niko Matsakis <niko@alum.mit.edu>
Date: Mon, 6 Jul 2020 21:48:14 +0000
Subject: [PATCH] do not generate `in_db_mut` when it is inappropriate

---
 components/salsa-macros/src/query_group.rs | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/components/salsa-macros/src/query_group.rs b/components/salsa-macros/src/query_group.rs
index 307584fb..917dea2d 100644
--- a/components/salsa-macros/src/query_group.rs
+++ b/components/salsa-macros/src/query_group.rs
@@ -364,7 +364,9 @@ pub(crate) fn query_group(args: TokenStream, input: TokenStream) -> TokenStream
         output.extend(quote! {
             #[derive(Default, Debug)]
             #trait_vis struct #qt;
+        });
 
+        output.extend(quote! {
             impl #qt {
                 /// Get access to extra methods pertaining to this query. For
                 /// example, you can use this to run the GC (`sweep`) across a
@@ -375,7 +377,12 @@ pub(crate) fn query_group(args: TokenStream, input: TokenStream) -> TokenStream
                 {
                     salsa::plumbing::get_query_table::<#qt>(db)
                 }
+            }
+        });
 
+        if query.storage.supports_mut() {}
+        output.extend(quote! {
+            impl #qt {
                 /// Like `in_db`, but gives access to methods for setting the
                 /// value of an input. Not applicable to derived queries.
                 ///
@@ -651,6 +658,7 @@ enum QueryStorage {
 }
 
 impl QueryStorage {
+    /// Do we need a `QueryFunction` impl for this type of query?
     fn needs_query_function(&self) -> bool {
         match self {
             QueryStorage::Input
@@ -660,4 +668,16 @@ impl QueryStorage {
             QueryStorage::Memoized | QueryStorage::Dependencies => true,
         }
     }
+
+    /// Does this type of query support `&mut` operations?
+    fn supports_mut(&self) -> bool {
+        match self {
+            QueryStorage::Input => true,
+            QueryStorage::Interned
+            | QueryStorage::InternedLookup { .. }
+            | QueryStorage::Transparent
+            | QueryStorage::Memoized
+            | QueryStorage::Dependencies => false,
+        }
+    }
 }