Use lang-item instead of db lookup for FnTrait kind

Author: ShoyuVanilla

crates/hir-ty/src/infer/closure.rs

@@ -177,15 +177,7 @@ impl InferenceContext<'_> {
     }
 
     fn fn_trait_kind_from_trait_id(&self, trait_id: hir_def::TraitId) -> Option<FnTrait> {
-        utils::fn_traits(self.db.upcast(), self.owner.module(self.db.upcast()).krate())
-            .enumerate()
-            .find_map(|(i, t)| (t == trait_id).then_some(i))
-            .map(|i| match i {
-                0 => FnTrait::Fn,
-                1 => FnTrait::FnMut,
-                2 => FnTrait::FnOnce,
-                _ => unreachable!(),
-            })
+        FnTrait::from_lang_item(self.db.lang_attr(trait_id.into())?)
     }
 }
 

crates/hir-ty/src/traits.rs

@@ -217,6 +217,15 @@ impl FnTrait {
         }
     }
 
+    pub const fn from_lang_item(lang_item: LangItem) -> Option<Self> {
+        match lang_item {
+            LangItem::FnOnce => Some(FnTrait::FnOnce),
+            LangItem::FnMut => Some(FnTrait::FnMut),
+            LangItem::Fn => Some(FnTrait::Fn),
+            _ => None,
+        }
+    }
+
     pub const fn to_chalk_ir(self) -> rust_ir::ClosureKind {
         match self {
             FnTrait::FnOnce => rust_ir::ClosureKind::FnOnce,