.devcontainer: clang-based build inside the container.

This allows the `bsv.cc.compdb.generate` command in vscode to generate a
valid `compile_commands.json` database, usable by clangd. Otherwise,
GCC command flags used are unusable by `clangd` which we install into
the devcontainer.

.devcontainer/bazel-toolchains/BUILD

@@ -0,0 +1,27 @@
+# Based on https://bazel.build/versions/6.5.0/tutorials/ccp-toolchain-config
+
+load(":cc_toolchain_config.bzl", "cc_toolchain_config")
+
+package(default_visibility = ["//visibility:public"])
+
+cc_toolchain_suite(
+    name = "clang_suite",
+    toolchains = {
+        "k8": ":k8_toolchain",
+    },
+)
+filegroup(name = "empty")
+
+cc_toolchain(
+    name = "k8_toolchain",
+    toolchain_identifier = "k8-toolchain",
+    toolchain_config = ":k8_toolchain_config",
+    all_files = ":empty",
+    compiler_files = ":empty",
+    dwp_files = ":empty",
+    linker_files = ":empty",
+    objcopy_files = ":empty",
+    strip_files = ":empty",
+    supports_param_files = 0,
+)
+cc_toolchain_config(name = "k8_toolchain_config")

.devcontainer/bazel-toolchains/cc_toolchain_config.bzl

@@ -0,0 +1,96 @@
+# Based on https://bazel.build/versions/6.5.0/tutorials/ccp-toolchain-config
+load("@bazel_tools//tools/build_defs/cc:action_names.bzl", "ACTION_NAMES")
+load(
+    "@bazel_tools//tools/cpp:cc_toolchain_config_lib.bzl",
+    "feature",
+    "flag_group",
+    "flag_set",
+    "tool_path",
+)
+
+all_link_actions = [ # NEW
+    ACTION_NAMES.cpp_link_executable,
+    ACTION_NAMES.cpp_link_dynamic_library,
+    ACTION_NAMES.cpp_link_nodeps_dynamic_library,
+]
+
+def _impl(ctx):
+    tool_paths = [
+        tool_path(
+            name = "gcc",
+            path = "/usr/bin/clang",
+        ),
+        tool_path(
+            name = "ld",
+            path = "/usr/bin/ld",
+        ),
+        tool_path(
+            name = "ar",
+            path = "/usr/bin/ar",
+        ),
+        tool_path(
+            name = "cpp",
+            path = "/bin/false",
+        ),
+        tool_path(
+            name = "gcov",
+            path = "/bin/false",
+        ),
+        tool_path(
+            name = "nm",
+            path = "/bin/false",
+        ),
+        tool_path(
+            name = "objdump",
+            path = "/bin/false",
+        ),
+        tool_path(
+            name = "strip",
+            path = "/bin/false",
+        ),
+    ]
+
+    features = [
+        feature(
+            name = "default_linker_flags",
+            enabled = True,
+            flag_sets = [
+                flag_set(
+                    actions = all_link_actions,
+                    flag_groups = ([
+                        flag_group(
+                            flags = [
+                                "-lstdc++",
+                                "-lm",  # added because floorf from glibc was missing
+                            ],
+                        ),
+                    ]),
+                ),
+            ],
+        ),
+    ]
+
+
+    return cc_common.create_cc_toolchain_config_info(
+        ctx = ctx,
+        features = features,
+        cxx_builtin_include_directories = [
+            "/usr/lib/llvm-10/lib/clang/10.0.0/include",  # updated to current image's version
+            "/usr/include",
+        ],
+        toolchain_identifier = "local",
+        host_system_name = "local",
+        target_system_name = "local",
+        target_cpu = "k8",
+        target_libc = "unknown",
+        compiler = "clang",
+        abi_version = "unknown",
+        abi_libc_version = "unknown",
+        tool_paths = tool_paths,
+    )
+
+cc_toolchain_config = rule(
+    implementation = _impl,
+    attrs = {},
+    provides = [CcToolchainConfigInfo],
+)

.devcontainer/devcontainer.json

@@ -13,9 +13,12 @@
 		"vscode": {
 			// Set *default* container specific settings.json values on container create.
 			"settings": {
+				"bazel.commandLine.commandArgs": [
+					"--config=devcontainer"
+				],
 				"bazel.executable": "bazelisk",
 				"bsv.bazel.buildFlags": [
-					// "--config=remote"
+					"--config=devcontainer"
 				],
 				"bsv.cc.compdb.targets": [
 					"//:yatc"

tools/bazel.rc

@@ -40,3 +40,11 @@ build:remotecache --remote_executor=grpcs://remote.buildbuddy.io
 #build:remotecache --remote_download_toplevel # Helps remove network bottleneck if caching is enabled
 
 try-import %workspace%/.bazelrc.buildbuddysecrets
+
+# In devcontainer: use our custom-configured c++ toolchain valid inside the 20.04 devcontainer.
+build:devcontainer --crosstool_top=//.devcontainer/bazel-toolchains:clang_suite
+# Use --cpu as a differentiator.
+build:devcontainer --cpu=k8
+# Use the default Bazel C++ toolchain to build the tools used during the
+# build.
+build:devcontainer --host_crosstool_top=@bazel_tools//tools/cpp:toolchain