feat: Add BPF triple constraint mapping

Add support for tier 3 targets bpfeb-unknown-none and bpfel-unknown-none
(see
https://github.com/rust-lang/rust/blob/f5e2df7/src/doc/rustc/src/platform-support.md?plain=1#L311-L312).

Author: tamird

rust/platform/BUILD.bazel

@@ -6,6 +6,8 @@ package(default_visibility = ["//visibility:public"])
 declare_config_settings()
 
 # WASI Preview version constraint settings
+#
+# TODO(https://github.com/bazelbuild/platforms/pull/123): Replace with upstream.
 constraint_setting(
     name = "wasi_version",
     default_constraint_value = ":wasi_preview_1",
@@ -21,6 +23,23 @@ constraint_value(
     constraint_setting = ":wasi_version",
 )
 
+# BPF execution constraints
+#
+# TODO(https://github.com/bazelbuild/platforms/pull/131): Replace with upstream.
+constraint_setting(
+    name = "bpf_arch",
+)
+
+constraint_value(
+    name = "bpfeb",
+    constraint_setting = ":bpf_arch",
+)
+
+constraint_value(
+    name = "bpfel",
+    constraint_setting = ":bpf_arch",
+)
+
 package_group(
     name = "function_transition_allowlist",
     packages = [

rust/platform/triple_mappings.bzl

@@ -75,6 +75,8 @@ SUPPORTED_T2_PLATFORM_TRIPLES = {
 
 _T3_PLATFORM_TRIPLES = {
     "aarch64-unknown-nto-qnx710": _support(std = True, host_tools = False),
+    "bpfeb-unknown-none": _support(std = False, host_tools = False),
+    "bpfel-unknown-none": _support(std = False, host_tools = False),
     "wasm64-unknown-unknown": _support(std = False, host_tools = False),
 }
 
@@ -416,6 +418,16 @@ def triple_to_constraint_set(target_triple):
     Returns:
         list: A list of constraints (each represented by a list of strings)
     """
+    if target_triple == "bpfeb-unknown-none":
+        return [
+            "@platforms//os:none",
+            "@rules_rust//rust/platform:bpfeb",
+        ]
+    if target_triple == "bpfel-unknown-none":
+        return [
+            "@platforms//os:none",
+            "@rules_rust//rust/platform:bpfel",
+        ]
     if target_triple == "wasm32-wasi":
         return [
             "@platforms//cpu:wasm32",

test/unit/platform_triple/BUILD.bazel

@@ -1,6 +1,11 @@
+load(":bpf_platform_test.bzl", "bpf_platform_test_suite")
 load(":platform_triple_test.bzl", "platform_triple_test_suite")
 load(":wasi_platform_test.bzl", "wasi_platform_test_suite")
 
+bpf_platform_test_suite(
+    name = "bpf_platform_test_suite",
+)
+
 platform_triple_test_suite(
     name = "platform_triple_test_suite",
 )

test/unit/platform_triple/bpf_platform_test.bzl

@@ -0,0 +1,52 @@
+"""Tests for BPF platform constraint mappings"""
+
+load("@bazel_skylib//lib:unittest.bzl", "asserts", "unittest")
+load("//rust/platform:triple_mappings.bzl", "triple_to_constraint_set")
+
+def _bpf_platform_constraints_test_impl(ctx):
+    env = unittest.begin(ctx)
+
+    bpfeb_constraints = triple_to_constraint_set("bpfeb-unknown-none")
+    asserts.equals(
+        env,
+        [
+            "@platforms//os:none",
+            "@rules_rust//rust/platform:bpfeb",
+        ],
+        bpfeb_constraints,
+        "bpfeb-unknown-none should map to the BPF big-endian CPU and 'none' OS constraints",
+    )
+
+    bpfel_constraints = triple_to_constraint_set("bpfel-unknown-none")
+    asserts.equals(
+        env,
+        [
+            "@platforms//os:none",
+            "@rules_rust//rust/platform:bpfel",
+        ],
+        bpfel_constraints,
+        "bpfel-unknown-none should map to the BPF little-endian CPU and 'none' OS constraints",
+    )
+
+    return unittest.end(env)
+
+bpf_platform_constraints_test = unittest.make(_bpf_platform_constraints_test_impl)
+
+def bpf_platform_test_suite(name, **kwargs):
+    """Define a test suite for BPF platform constraint mappings.
+
+    Args:
+        name (str): The name of the test suite.
+        **kwargs (dict): Additional keyword arguments for the test_suite.
+    """
+    bpf_platform_constraints_test(
+        name = "bpf_platform_constraints_test",
+    )
+
+    native.test_suite(
+        name = name,
+        tests = [
+            ":bpf_platform_constraints_test",
+        ],
+        **kwargs
+    )

test/unit/platform_triple/platform_triple_test.bzl

@@ -125,6 +125,10 @@ def _construct_known_triples_test_impl(ctx):
     _assert_parts(env, triple("thumbv7em-none-eabi"), "thumbv7em", None, "none", "eabi")
     _assert_parts(env, triple("thumbv8m.main-none-eabi"), "thumbv8m.main", None, "none", "eabi")
 
+    # BPF targets
+    _assert_parts(env, triple("bpfeb-unknown-none"), "bpfeb", "unknown", "none", None)
+    _assert_parts(env, triple("bpfel-unknown-none"), "bpfel", "unknown", "none", None)
+
     # Test all WASM32 targets
     _assert_parts(env, triple("wasm32-unknown-unknown"), "wasm32", "unknown", "unknown", None)
     _assert_parts(env, triple("wasm32-unknown-emscripten"), "wasm32", "unknown", "emscripten", None)