Merge pull request #18707 from ChayimFriedman2/subst

feat: Show substitution where hovering over generic things

crates/hir/src/lib.rs

@@ -3574,6 +3574,61 @@ impl GenericDef {
     }
 }
 
+// We cannot call this `Substitution` unfortunately...
+#[derive(Debug)]
+pub struct GenericSubstitution {
+    def: GenericDefId,
+    subst: Substitution,
+    env: Arc<TraitEnvironment>,
+}
+
+impl GenericSubstitution {
+    fn new(def: GenericDefId, subst: Substitution, env: Arc<TraitEnvironment>) -> Self {
+        Self { def, subst, env }
+    }
+
+    pub fn types(&self, db: &dyn HirDatabase) -> Vec<(Symbol, Type)> {
+        let container = match self.def {
+            GenericDefId::ConstId(id) => Some(id.lookup(db.upcast()).container),
+            GenericDefId::FunctionId(id) => Some(id.lookup(db.upcast()).container),
+            GenericDefId::TypeAliasId(id) => Some(id.lookup(db.upcast()).container),
+            _ => None,
+        };
+        let container_type_params = container
+            .and_then(|container| match container {
+                ItemContainerId::ImplId(container) => Some(container.into()),
+                ItemContainerId::TraitId(container) => Some(container.into()),
+                _ => None,
+            })
+            .map(|container| {
+                db.generic_params(container)
+                    .iter_type_or_consts()
+                    .filter_map(|param| match param.1 {
+                        TypeOrConstParamData::TypeParamData(param) => Some(param.name.clone()),
+                        TypeOrConstParamData::ConstParamData(_) => None,
+                    })
+                    .collect::<Vec<_>>()
+            });
+        let generics = db.generic_params(self.def);
+        let type_params = generics.iter_type_or_consts().filter_map(|param| match param.1 {
+            TypeOrConstParamData::TypeParamData(param) => Some(param.name.clone()),
+            TypeOrConstParamData::ConstParamData(_) => None,
+        });
+        // The `Substitution` is first self then container, we want the reverse order.
+        let self_params = self.subst.type_parameters(Interner).zip(type_params);
+        let container_params = self.subst.as_slice(Interner)[generics.len()..]
+            .iter()
+            .filter_map(|param| param.ty(Interner).cloned())
+            .zip(container_type_params.into_iter().flatten());
+        container_params
+            .chain(self_params)
+            .filter_map(|(ty, name)| {
+                Some((name?.symbol().clone(), Type { ty, env: self.env.clone() }))
+            })
+            .collect()
+    }
+}
+
 /// A single local definition.
 #[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
 pub struct Local {