Struct MigrateForeignAssetPrecompileMappings 

pub struct MigrateForeignAssetPrecompileMappings<T, I, W>(/* private fields */);

Migration to backfill foreign asset precompile mappings for existing assets.

This migration populates the bidirectional mapping between foreign asset IDs (e.g., XCM Locations) and sequential u32 indices in pallet_assets_precompiles for all existing foreign assets.

The mapping enables EVM precompile addresses for foreign assets, where the u32 index is embedded in the first 4 bytes of the 20-byte Ethereum address.

Background

Foreign assets are identified by types (like XCM Location) that do not fit in 4 bytes. In order to facilitate EVM precompile addresses for these assets, a mapping is maintained between a sequential u32 index and the actual foreign asset ID. The pallet maintains a bidirectional mapping:

• AssetIndexToForeignAssetId: u32 index -> Foreign Asset ID
• ForeignAssetIdToAssetIndex: Foreign Asset ID -> u32 index

While new foreign assets automatically get mapped via the AssetsCallback hook, this migration ensures existing foreign assets (created before the mapping was introduced) are also added to the mapping with sequential indices.

Type Parameters

• T: The runtime configuration implementing both pallet_assets::Config<I> and pallet::Config
• I: The pallet_assets instance identifier (e.g., ForeignAssetsInstance)
• W: The weight info implementation for the migration benchmarks

Usage in Runtime

Add this to the runtime’s Migrations tuple in lib.rs:

pub type Migrations = (
    // ... other migrations ...
    pallet_assets_precompiles::MigrateForeignAssetPrecompileMappings<Runtime, ForeignAssetsInstance, ()>,
);

Safety

• Non-destructive: Does not modify any asset data, only adds mappings
• Sequential indices: Each migrated asset gets the next available index

NB: If the SCALE encoding of AssetId (e.g. xcm::v5::Location) ever changes, the keys in ForeignAssetIdToAssetIndex must be migrated in-place (decode old encoding → re-encode with new encoding → reinsert with the same index). Simply clearing and repopulating would reassign indices, breaking EVM contracts that cached precompile addresses derived from those indices.

Trait Implementations

impl<T, I, W> SteppedMigration for MigrateForeignAssetPrecompileMappings<T, I, W>

where T: Config<I> + Config<ForeignAssetId = <T as Config<I>>::AssetId>, I: 'static, W: WeightInfo,

type Cursor = <T as Config<I>>::AssetId

The cursor type that stores the progress (aka. state) of this migration.

type Identifier = MigrationId<32>

The unique identifier type of this migration.

fn id() -> Self::Identifier

The unique identifier of this migration.

fn step( cursor: Option<Self::Cursor>, meter: &mut WeightMeter, ) -> Result<Option<Self::Cursor>, SteppedMigrationError>

Try to migrate as much as possible with the given weight.

fn pre_upgrade() -> Result<Vec<u8>, TryRuntimeError>

Hook for testing that is run before the migration is started.

fn post_upgrade(state: Vec<u8>) -> Result<(), TryRuntimeError>

Hook for testing that is run after the migration is completed.

fn max_steps() -> Option<u32>

The maximum number of steps that this migration can take.

fn migrating_prefixes() -> Option<impl IntoIterator<Item = Vec<u8>>>

Returns the prefixes of the keys to be migrated or None if the migration does not know.

fn transactional_step( cursor: Option<Self::Cursor>, meter: &mut WeightMeter, ) -> Result<Option<Self::Cursor>, SteppedMigrationError>

Same as [Self::step], but rolls back pending changes in the error case.