pallet-contracts: Rent projection RPC (#4754)

* Initial approach

* Introduce the pallet-contracts-common crate

* Add rent::compute_rent_projection

* Wire everything together

* Fix build error.

* Rename EvictionDate → EvictionAt.

* Clean.

* Renaming and cleaning.

* Add documentation for rent_projection RPC.

* Add documentation for rent_projection runtime API.

* Refactor rent_budget.

Merge it with subsistence_treshold.

* Bump impl_version

* Constrain RPC impl with Block::Header::Number.

* Rename pallet-contracts-common into -primitives

* Add a comment for `compute_rent_projection` on the usage

* Small tidying

substrate/frame/contracts/src/lib.rs

@@ -128,6 +128,7 @@ use frame_support::{
 use frame_support::traits::{OnFreeBalanceZero, OnUnbalanced, Currency, Get, Time, Randomness};
 use frame_system::{self as system, ensure_signed, RawOrigin, ensure_root};
 use sp_core::storage::well_known_keys::CHILD_STORAGE_KEY_PREFIX;
+use pallet_contracts_primitives::{RentProjection, ContractAccessError};
 
 pub type CodeHash<T> = <T as frame_system::Trait>::Hash;
 pub type TrieId = Vec<u8>;
@@ -680,14 +681,6 @@ decl_module! {
 	}
 }
 
-/// The possible errors that can happen querying the storage of a contract.
-pub enum GetStorageError {
-	/// The given address doesn't point on a contract.
-	ContractDoesntExist,
-	/// The specified contract is a tombstone and thus cannot have any storage.
-	IsTombstone,
-}
-
 /// Public APIs provided by the contracts module.
 impl<T: Trait> Module<T> {
 	/// Perform a call to a specified contract.
@@ -710,11 +703,11 @@ impl<T: Trait> Module<T> {
 	pub fn get_storage(
 		address: T::AccountId,
 		key: [u8; 32],
-	) -> sp_std::result::Result<Option<Vec<u8>>, GetStorageError> {
+	) -> sp_std::result::Result<Option<Vec<u8>>, ContractAccessError> {
 		let contract_info = <ContractInfoOf<T>>::get(&address)
-			.ok_or(GetStorageError::ContractDoesntExist)?
+			.ok_or(ContractAccessError::DoesntExist)?
 			.get_alive()
-			.ok_or(GetStorageError::IsTombstone)?;
+			.ok_or(ContractAccessError::IsTombstone)?;
 
 		let maybe_value = AccountDb::<T>::get_storage(
 			&DirectAccountDb,
@@ -724,6 +717,12 @@ impl<T: Trait> Module<T> {
 		);
 		Ok(maybe_value)
 	}
+
+	pub fn rent_projection(
+		address: T::AccountId,
+	) -> sp_std::result::Result<RentProjection<T::BlockNumber>, ContractAccessError> {
+		rent::compute_rent_projection::<T>(&address)
+	}
 }
 
 impl<T: Trait> Module<T> {