Try to add bare-metal and RISC-V support

Pursuand to bazelbuild#256.

- Add "cpu" constraint fulfillments for "riscv32imac" and "riscv64imac",
  matching the CPU strings available in the Rust toolchain for
  bare-metal builds.
- Add mappings for the "none" OS constraint, which is available upstream
  in bazelbuild/platforms.

rust/platform/cpu/BUILD

@@ -3,3 +3,15 @@ constraint_value(
     constraint_setting = "@platforms//cpu",
     visibility = ["//visibility:public"],
 )
+
+constraint_value(
+    name = "riscv32imac",
+    constraint_setting = "@platforms//cpu",
+    visibility = ["//visibility:public"],
+)
+
+constraint_value(
+    name = "riscv64imac",
+    constraint_setting = "@platforms//cpu",
+    visibility = ["//visibility:public"],
+)

rust/platform/platform.bzl

@@ -30,10 +30,13 @@ _SUPPORTED_T2_PLATFORM_TRIPLES = [
     "aarch64-apple-ios",
     "aarch64-linux-android",
     "aarch64-unknown-linux-gnu",
+    "aarch64-unknown-none",
     "arm-unknown-linux-gnueabi",
     "i686-linux-android",
     "i686-unknown-freebsd",
     "powerpc-unknown-linux-gnu",
+    "riscv32imac-unknown-none-elf",
+    "riscv64imac-unknown-none-elf",
     "s390x-unknown-linux-gnu",
     "wasm32-unknown-unknown",
     "wasm32-wasi",
@@ -49,6 +52,8 @@ _SUPPORTED_CPU_ARCH = [
     "powerpc",
     "s390x",
     "x86_64",
+    "riscv32imac",
+    "riscv64imac",
 ]
 
 _SUPPORTED_SYSTEMS = [

rust/platform/triple_mappings.bzl

@@ -30,6 +30,7 @@ _SYSTEM_TO_BUILTIN_SYS_SUFFIX = {
     "windows": "windows",
     "ios": "ios",
     "android": "android",
+    "none": "none",
     "emscripten": None,
     "unknown": None,
     "wasi": None,
@@ -53,6 +54,7 @@ _SYSTEM_TO_BINARY_EXT = {
     # windows target
     "unknown": ".wasm",
     "wasi": ".wasm",
+    "none": "",
 }
 
 _SYSTEM_TO_STATICLIB_EXT = {
@@ -64,6 +66,9 @@ _SYSTEM_TO_STATICLIB_EXT = {
     "emscripten": ".js",
     "unknown": "",
     "wasi": "",
+    # This reflects Rust's behavior for the aarch64-unknown-none target,
+    # which produces .a files when invoked to build a staticlib.
+    "none": ".a",
 }
 
 _SYSTEM_TO_DYLIB_EXT = {
@@ -73,8 +78,12 @@ _SYSTEM_TO_DYLIB_EXT = {
     "ios": ".dylib",
     "windows": ".dll",
     "emscripten": ".js",
+    # This is currently a hack allowing us to have the proper
+    # generated extension for the wasm target, similarly to the
+    # windows target
     "unknown": ".wasm",
     "wasi": ".wasm",
+    "none": "",
 }
 
 # See https://github.com/rust-lang/rust/blob/master/src/libstd/build.rs
@@ -116,15 +125,21 @@ _SYSTEM_TO_STDLIB_LINKFLAGS = {
     "cloudabi": ["-lunwind", "-lc", "-lcompiler_rt"],
     "unknown": [],
     "wasi": [],
+    # We don't actually expect the stdlib to build for the "none" system,
+    # but we don't need extra flags for it.
+    "none": [],
 }
 
 def cpu_arch_to_constraints(cpu_arch):
-    plat_suffix = _CPU_ARCH_TO_BUILTIN_PLAT_SUFFIX[cpu_arch]
+    plat_suffix = _CPU_ARCH_TO_BUILTIN_PLAT_SUFFIX.get(cpu_arch)
+
+    if plat_suffix:
+        return ["@platforms//cpu:{}".format(plat_suffix)]
 
-    if not plat_suffix:
-        fail("CPU architecture \"{}\" is not supported by rules_rust".format(cpu_arch))
+    if cpu_arch.startswith("riscv"):
+        return ["@io_bazel_rules_rust//rust/platform/cpu:{}".format(cpu_arch)]
 
-    return ["@platforms//cpu:{}".format(plat_suffix)]
+    fail("CPU architecture \"{}\" is not supported by rules_rust".format(cpu_arch))
 
 def vendor_to_constraints(vendor):
     # TODO(acmcarther): Review: