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src/sys/lib/cm_rust/README.md

cm_rust library

This library contains common component manager representations of the concepts that appear in component manifests (.cml files and binary .cm files).

When converting from fuchsia.component.decl FIDL data types, cm_rust will perform validation using cm_fidl_validator. The native Rust representations in cm_rust can be more ergonomic and has stronger guarantees than the equivalents generated from FIDL. For example, some table fields are no longer optional after validation and converted to cm_rust types.

Relationship to .cml

.cml files go through a series of transformations before they are consumed by component manager at runtime.

  • JSON5 .cml files are transformed by cmc into .cm files, a fuchsia.component.decl/Component FIDL object stored in FIDL persistence convention:
    • It applies some local validation on the .cml content.
    • It uses cm_fidl_validator to validate the resulting Component declaration.
    • This step does not involve cm_rust.
  • When instantiating a component at runtime, component manager will transform the fuchsia.component.decl/Component FIDL into the native Rust representation as defined in cm_rust:
    • It uses cm_rust to transform the Component Rust FIDL binding type into a native Rust representation.
    • In doing so, it uses cm_fidl_validator to validate the Component declaration. This may seem duplicative but is necessary because we should not trust that the .cm file was properly validated.
    • Most other business logic is based on the cm_rust types.

Each of these stages perform some level of validation, which includes validating things like entity names, paths, URLs, etc. All of these types require the same validation and should be represented by the types in this library.

When adding a basic, common type to the CML syntax, consider whether that type should be added here, so that every stage of the transformation pipeline can benefit.

serde integration

When built for host, these types come with serde serialization and deserialization implementations that perform the required validation. serde is not supported on device builds.