Chalk `HasInterner` derive macro doesn't work because it expands to named constant

[flodiebold]

Reproduction:

use std::marker::PhantomData;

use chalk_ir::interner::{HasInterner, Interner};
use chalk_derive::HasInterner;

#[derive(HasInterner)]
struct WithInterner<I: Interner> {
    _phantom: PhantomData<I>,
}

fn test<I: Interner>(i: I) {
    let x: <WithInterner<I> as HasInterner>::Interner = i;
}

With chalk_ir and chalk_derive dependencies. x should be type I, which works if I write the HasInterner impl by hand, but not in the above code.

It turns out that the synstructure derive macro expands to an impl in a named constant:

#[allow(non_upper_case_globals)]
#[doc(hidden)]
const _DERIVE_chalk_ir_interner_HasInterner_FOR_WithInterner: () = {
    impl<I: Interner> ::chalk_ir::interner::HasInterner for WithInterner<I> {
        type Interner = I;
    }
};

Which we don't support. Maybe we can change synstructure to use unnamed consts, or build a hack to treat _DERIVE consts like unnamed ones 😬

[flodiebold]

This patch does indeed fix it:

diff --git a/crates/hir_def/src/item_tree/lower.rs b/crates/hir_def/src/item_tree/lower.rs
index aa2dbaba1..48e6bb4a6 100644
--- a/crates/hir_def/src/item_tree/lower.rs
+++ b/crates/hir_def/src/item_tree/lower.rs
@@ -448,7 +448,10 @@ impl<'a> Ctx<'a> {
     }
 
     fn lower_const(&mut self, konst: &ast::Const) -> FileItemTreeId<Const> {
-        let name = konst.name().map(|it| it.as_name());
+        let mut name = konst.name().map(|it| it.as_name());
+        if name.as_ref().map_or(false, |n| n.to_string().starts_with("_DERIVE_")) {
+            name = None;
+        }
         let type_ref = self.lower_type_ref_opt(konst.ty());
         let visibility = self.lower_visibility(konst);
         let ast_id = self.source_ast_id_map.ast_id(konst);

[lnicola]

CC mystor/synstructure#43

[flodiebold]

Ah, nice