Add from_mel for Footprint with a Test (#5132)

We may want to construct a `Footprint` by taking the `max_encoded_len`
of a type, without having to construct the type.

This impl allows easy access to that.

---------

Co-authored-by: Bastian Köcher <[email protected]>
Co-authored-by: command-bot <>

prdoc/pr_5132.prdoc

@@ -0,0 +1,15 @@
+# Schema: Polkadot SDK PRDoc Schema (prdoc) v1.0.0
+# See doc at https://raw.githubusercontent.com/paritytech/polkadot-sdk/master/prdoc/schema_user.json
+
+title: Add `from_mel` for `Footprint`
+
+doc:
+  - audience: Runtime Dev
+    description: |
+      This introduces a new way to generate the `Footprint` type by calculating the max encoded
+      length of some generic type. This allows you to generate a `Footprint` of a type without
+      actually constructing that type.
+
+crates:
+  - name: frame-support
+    bump: patch

substrate/frame/support/src/traits/storage.rs

@@ -170,13 +170,20 @@ pub struct Footprint {
 }
 
 impl Footprint {
+	/// Construct a footprint directly from `items` and `len`.
 	pub fn from_parts(items: usize, len: usize) -> Self {
 		Self { count: items as u64, size: len as u64 }
 	}
 
+	/// Construct a footprint with one item, and size equal to the encoded size of `e`.
 	pub fn from_encodable(e: impl Encode) -> Self {
 		Self::from_parts(1, e.encoded_size())
 	}
+
+	/// Construct a footprint with one item, and size equal to the max encoded length of `E`.
+	pub fn from_mel<E: MaxEncodedLen>() -> Self {
+		Self::from_parts(1, E::max_encoded_len())
+	}
 }
 
 /// A storage price that increases linearly with the number of elements and their size.
@@ -288,7 +295,8 @@ impl_incrementable!(u8, u16, u32, u64, u128, i8, i16, i32, i64, i128);
 #[cfg(test)]
 mod tests {
 	use super::*;
-	use sp_core::ConstU64;
+	use crate::BoundedVec;
+	use sp_core::{ConstU32, ConstU64};
 
 	#[test]
 	fn linear_storage_price_works() {
@@ -305,4 +313,17 @@ mod tests {
 
 		assert_eq!(p(u64::MAX, u64::MAX), u64::MAX);
 	}
+
+	#[test]
+	fn footprint_from_mel_works() {
+		let footprint = Footprint::from_mel::<(u8, BoundedVec<u8, ConstU32<9>>)>();
+		let expected_size = BoundedVec::<u8, ConstU32<9>>::max_encoded_len() as u64;
+		assert_eq!(expected_size, 10);
+		assert_eq!(footprint, Footprint { count: 1, size: expected_size + 1 });
+
+		let footprint = Footprint::from_mel::<(u8, BoundedVec<u8, ConstU32<999>>)>();
+		let expected_size = BoundedVec::<u8, ConstU32<999>>::max_encoded_len() as u64;
+		assert_eq!(expected_size, 1001);
+		assert_eq!(footprint, Footprint { count: 1, size: expected_size + 1 });
+	}
 }