IndexedFields::by_id() can be const (#32430)

make it clear that `IndexedFields::by_id()` does no allocations by making it const. this is nice when figuring out the memory footprint of readsets

GitOrigin-RevId: d83b2bfe4e3ff96962a72bd46d3dac5041d2acfa

crates/common/src/bootstrap_model/index/database_index/indexed_fields.rs

@@ -35,8 +35,8 @@ use crate::{
 pub struct IndexedFields(WithHeapSize<Vec<FieldPath>>);
 
 impl IndexedFields {
-    pub fn by_id() -> Self {
-        IndexedFields(vec![].into())
+    pub const fn by_id() -> Self {
+        IndexedFields(WithHeapSize::new_vec())
     }
 
     pub fn creation_time() -> Self {

crates/value/src/heap_size.rs

@@ -238,6 +238,15 @@ impl<T: HeapSize> From<Vec<T>> for WithHeapSize<Vec<T>> {
     }
 }
 
+impl<T: HeapSize> WithHeapSize<Vec<T>> {
+    pub const fn new_vec() -> Self {
+        Self {
+            inner: Vec::new(),
+            elements_heap_size: 0,
+        }
+    }
+}
+
 impl<T: HeapSize> From<WithHeapSize<Vec<T>>> for Vec<T> {
     fn from(value: WithHeapSize<Vec<T>>) -> Self {
         value.inner