The crate_root property will be used to specify the entry point for a
rustc compilation. This will allow the srcs property to be used to
collect all src inputs to rustc rather than just the entry point.
Bug: 286077158
Test: m libnum_traits
Change-Id: I1a167182305dcc11cc927d562ceed622153111d3
Sandboxing the rust rules requires having explicit inputs for the rust
toolchain. This module type makes the rustc prebuilt and its
dependencies explicit as a module that can be depended on by all rust
rules.
Bug: 286077158
Test: rust sandboxing topic + go test
Change-Id: If2b80b32e329e6a6ee11ba824de868cf04714553
Adds a new rust_toolchain_library module, a module which can compute the
source path from the toolchain root.
Bug: 140642453
Test: m
Change-Id: I2d90722880279533eb93d2d946ac8c31fc0b1835
