Per-class tracking to achieve higher level of compilation avoidance

[oliviernotteghem]

This change adds support for per-class compilation avoidance, enabling fine-grained dependency tracking during Kotlin compilation. When enabled, the jdeps output records exactly
which classes from each dependency JAR are used, along with SHA-256 hashes of their bytecode.

Motivation

Traditional compilation avoidance works at the JAR level—if any file in a dependency JAR changes, all dependents must recompile. Per-class tracking improves this by allowing
build systems to skip recompilation when a dependency JAR changes but the specific classes actually used remain unchanged (same hashes).

Configuration

Two new toolchain attributes:

define_kt_toolchain(
name = "my_toolchain",
experimental_track_class_usage = "on", # Track class usage with hashes
experimental_track_resource_usage = "on", # Track Android R class references
)

Changes

Proto definitions:

• deps.proto: Added UsedClass message with fullyQualifiedName, internalPath, and hash fields; added usedResources field to Dependencies
• kotlin_model.proto: Added track_class_usage and track_resource_usage fields

JDeps plugin:

• Added configuration keys and CLI options for the new tracking flags
• BaseJdepsGenExtension: Added getHashFromJarEntry() to compute SHA-256 hashes; updated doWriteJdeps() to write UsedClass entries when tracking is enabled
• JdepsGenExtension (K1): Added resource tracking for Android R class references
• K2 extensions: Added usedResources support

Builder:

• Added TRACK_CLASS_USAGE and TRACK_RESOURCE_USAGE flags to KotlinBuilder
• Pass tracking flags to jdeps plugin in KotlinJvmTaskExecutor

Starlark rules:

• toolchains.bzl: Added experimental_track_class_usage and experimental_track_resource_usage attributes
• compile.bzl: Pass tracking flags to builder

Output Format

When enabled, jdeps includes per-class entries:

dependency {
path: "bazel-out/.../libparser.jar" kind: EXPLICIT usedClasses { fullyQualifiedName: "com.lib.Parser" internalPath: "com/lib/Parser.class"
hash: <32 bytes SHA-256>
} }

Testing

Added KotlinBuilderJvmClassTrackingTest with tests verifying:

• Tracking is disabled by default (no UsedClass entries)
• Tracking records classes with hashes when enabled
• Multiple classes are tracked correctly
• Supertype dependencies are included
• Hashes are deterministic across compilations

[agluszak]

this is a divergence vs Bazel jdeps

[oliviernotteghem]

See my comment below (where the same proto changes are done in the mentioned PR).

[agluszak]

turning this on doesn't really enable (as in: cause) any compilation avoidance, right?

[oliviernotteghem]

@agluszak : you're absolutely right. To enable consumption of the class usage data and enable more granular compilation avoidance you need also this change on Bazel core bazelbuild/bazel#16457