More principled sources for enums and fields

1 files changed, 49 insertions, 0 deletions

diff --git a/crates/ra_hir_def/src/trace.rs b/crates/ra_hir_def/src/trace.rs
new file mode 100644
index 000000000..fc26f5a48
--- /dev/null
+++ b/crates/ra_hir_def/src/trace.rs
@@ -0,0 +1,49 @@
+//! Trace is a pretty niche data structure which is used when lowering a CST
+//! into HIR.
+//!
+//! Lowering process calculates two bits of information:
+//! * the lowered syntax itself
+//! * a mapping between lowered syntax and original syntax
+//!
+//! Due to the way salsa works, the mapping is usually hot lava, as it contains
+//! absolute offsets. The `Trace` structure (inspired, at least in name, by
+//! Kotlin's `BindingTrace`) allows use the same code to compute both
+//! projections.
+use ra_arena::{map::ArenaMap, Arena, ArenaId, RawId};
+
+pub(crate) struct Trace<ID: ArenaId, T, V> {
+    for_arena: bool,
+    arena: Arena<ID, T>,
+    map: ArenaMap<ID, V>,
+    len: u32,
+}
+
+impl<ID: ra_arena::ArenaId, T, V> Trace<ID, T, V> {
+    pub(crate) fn new_for_arena() -> Trace<ID, T, V> {
+        Trace { for_arena: true, arena: Arena::default(), map: ArenaMap::default(), len: 0 }
+    }
+
+    pub(crate) fn new_for_map() -> Trace<ID, T, V> {
+        Trace { for_arena: false, arena: Arena::default(), map: ArenaMap::default(), len: 0 }
+    }
+
+    pub(crate) fn alloc(&mut self, value: impl Fn() -> V, data: impl Fn() -> T) {
+        if self.for_arena {
+            self.arena.alloc(data());
+        } else {
+            let id = ID::from_raw(RawId::from(self.len));
+            self.len += 1;
+            self.map.insert(id, value());
+        }
+    }
+
+    pub(crate) fn into_arena(self) -> Arena<ID, T> {
+        assert!(self.for_arena);
+        self.arena
+    }
+
+    pub(crate) fn into_map(self) -> ArenaMap<ID, V> {
+        assert!(!self.for_arena);
+        self.map
+    }
+}