Merge #1895

1895: Handle associated type shorthand (`T::Item`) (Second attempt) r=flodiebold a=flodiebold

This is only allowed for generic parameters (including `Self` in traits), and
special care needs to be taken to not run into cycles while resolving it,
because we use the where clauses of the generic parameter to find candidates for
the trait containing the associated type, but the where clauses may themselves
contain instances of short-hand associated types.

In some cases this is even fine, e.g. we might have `T: Trait<U::Item>, U:
Iterator`. If there is a cycle, we'll currently panic, which isn't great, but
better than overflowing the stack...

Co-authored-by: Florian Diebold <[email protected]>

1 files changed, 8 insertions, 3 deletions

diff --git a/crates/ra_hir/src/generics.rs b/crates/ra_hir/src/generics.rs
index 77fb76bfc..ccb777492 100644
--- a/crates/ra_hir/src/generics.rs
+++ b/crates/ra_hir/src/generics.rs
@@ -26,8 +26,9 @@ pub struct GenericParam {
 }
 
 /// Data about the generic parameters of a function, struct, impl, etc.
-#[derive(Clone, PartialEq, Eq, Debug, Default)]
+#[derive(Clone, PartialEq, Eq, Debug)]
 pub struct GenericParams {
+    pub(crate) def: GenericDef,
     pub(crate) parent_params: Option<Arc<GenericParams>>,
     pub(crate) params: Vec<GenericParam>,
     pub(crate) where_predicates: Vec<WherePredicate>,
@@ -69,7 +70,6 @@ impl GenericParams {
         db: &(impl DefDatabase + AstDatabase),
         def: GenericDef,
     ) -> Arc<GenericParams> {
-        let mut generics = GenericParams::default();
         let parent = match def {
             GenericDef::Function(it) => it.container(db).map(GenericDef::from),
             GenericDef::TypeAlias(it) => it.container(db).map(GenericDef::from),
@@ -77,7 +77,12 @@ impl GenericParams {
             GenericDef::Adt(_) | GenericDef::Trait(_) => None,
             GenericDef::ImplBlock(_) => None,
         };
-        generics.parent_params = parent.map(|p| db.generic_params(p));
+        let mut generics = GenericParams {
+            def,
+            params: Vec::new(),
+            parent_params: parent.map(|p| db.generic_params(p)),
+            where_predicates: Vec::new(),
+        };
         let start = generics.parent_params.as_ref().map(|p| p.params.len()).unwrap_or(0) as u32;
         // FIXME: add `: Sized` bound for everything except for `Self` in traits
         match def {