Add require_explicit_unstable_features option

rust/private/rust.bzl

@@ -753,6 +753,16 @@ _common_attrs = {
         cfg = "exec",
         providers = [[CrateInfo], [CrateGroupInfo]],
     ),
+    "require_explicit_unstable_features": attr.int(
+        doc = (
+            "Whether to require all unstable features to be explicitly opted in to using " +
+            "`-Zallow-features=...`. Possible values: [-1, 0, 1]. -1 means delegate to the " +
+            "toolchain.require_explicit_unstable_features boolean build setting; 0 means False; " +
+            "1 means True."
+        ),
+        values = [-1, 0, 1],
+        default = -1,
+    ),
     "rustc_env": attr.string_dict(
         doc = dedent("""\
             Dictionary of additional `"key": "value"` environment variables to set for rustc.

rust/private/rustc.bzl

@@ -835,6 +835,7 @@ def construct_arguments(
         build_metadata = False,
         force_depend_on_objects = False,
         skip_expanding_rustc_env = False,
+        require_explicit_unstable_features = False,
         error_format = None):
     """Builds an Args object containing common rustc flags
 
@@ -867,6 +868,7 @@ def construct_arguments(
         build_metadata (bool): Generate CLI arguments for building *only* .rmeta files. This requires use_json_output.
         force_depend_on_objects (bool): Force using `.rlib` object files instead of metadata (`.rmeta`) files even if they are available.
         skip_expanding_rustc_env (bool): Whether to skip expanding CrateInfo.rustc_env_attr
+        require_explicit_unstable_features (bool): Whether to require all unstable features to be explicitly opted in to using `-Zallow-features=...`.
         error_format (str, optional): Error format to pass to the `--error-format` command line argument. If set to None, uses the "_error_format" entry in `attr`.
 
     Returns:
@@ -895,6 +897,9 @@ def construct_arguments(
 
     process_wrapper_flags.add_all(build_flags_files, before_each = "--arg-file")
 
+    if require_explicit_unstable_features:
+        process_wrapper_flags.add("--require-explicit-unstable-features", "true")
+
     # Certain rust build processes expect to find files from the environment
     # variable `$CARGO_MANIFEST_DIR`. Examples of this include pest, tera,
     # asakuma.
@@ -1289,6 +1294,16 @@ def rustc_compile_action(
     if experimental_use_cc_common_link:
         emit = ["obj"]
 
+    # Determine whether to pass `--require-explicit-unstable-features true` to the process wrapper:
+    require_explicit_unstable_features = False
+    if hasattr(ctx.attr, "require_explicit_unstable_features"):
+        if ctx.attr.require_explicit_unstable_features == 0:
+            require_explicit_unstable_features = False
+        elif ctx.attr.require_explicit_unstable_features == 1:
+            require_explicit_unstable_features = True
+        elif ctx.attr.require_explicit_unstable_features == -1:
+            require_explicit_unstable_features = toolchain.require_explicit_unstable_features
+
     args, env_from_args = construct_arguments(
         ctx = ctx,
         attr = attr,
@@ -1311,6 +1326,7 @@ def rustc_compile_action(
         stamp = stamp,
         use_json_output = bool(build_metadata) or bool(rustc_output) or bool(rustc_rmeta_output),
         skip_expanding_rustc_env = skip_expanding_rustc_env,
+        require_explicit_unstable_features = require_explicit_unstable_features,
     )
 
     args_metadata = None
@@ -1337,6 +1353,7 @@ def rustc_compile_action(
             stamp = stamp,
             use_json_output = True,
             build_metadata = True,
+            require_explicit_unstable_features = require_explicit_unstable_features,
         )
 
     env = dict(ctx.configuration.default_shell_env)

rust/settings/BUILD.bazel

@@ -31,6 +31,7 @@ load(
     "no_std",
     "pipelined_compilation",
     "rename_first_party_crates",
+    "require_explicit_unstable_features",
     "rustc_output_diagnostics",
     "rustfmt_toml",
     "third_party_dir",
@@ -119,6 +120,8 @@ pipelined_compilation()
 
 rename_first_party_crates()
 
+require_explicit_unstable_features()
+
 rustc_output_diagnostics()
 
 rustfmt_toml()

rust/settings/settings.bzl

@@ -78,6 +78,19 @@ def rename_first_party_crates():
         build_setting_default = False,
     )
 
+def require_explicit_unstable_features():
+    """A flag controlling whether unstable features should be disallowed by default
+
+    If true, an empty `-Zallow-features=` will be added to the rustc command line whenever no other
+    `-Zallow-features=` is present in the rustc flags. The effect is to disallow all unstable
+    features by default, with the possibility to explicitly re-enable them selectively using
+    `-Zallow-features=...`.
+    """
+    bool_flag(
+        name = "require_explicit_unstable_features",
+        build_setting_default = False,
+    )
+
 def third_party_dir():
     """A flag specifying the location of vendored third-party rust crates within this \
     repository that must not be renamed when `rename_first_party_crates` is enabled.

rust/toolchain.bzl

@@ -538,6 +538,9 @@ def _generate_sysroot(
 def _experimental_use_cc_common_link(ctx):
     return ctx.attr.experimental_use_cc_common_link[BuildSettingInfo].value
 
+def _require_explicit_unstable_features(ctx):
+    return ctx.attr.require_explicit_unstable_features[BuildSettingInfo].value
+
 def _expand_flags(ctx, attr_name, targets, make_variables):
     targets = deduplicate(targets)
     expanded_flags = []
@@ -753,6 +756,7 @@ def _rust_toolchain_impl(ctx):
         target_json = target_json,
         target_os = target_os,
         target_triple = target_triple,
+        require_explicit_unstable_features = _require_explicit_unstable_features(ctx),
 
         # Experimental and incompatible flags
         _rename_first_party_crates = rename_first_party_crates,
@@ -904,6 +908,13 @@ rust_toolchain = rule(
         "per_crate_rustc_flags": attr.string_list(
             doc = "Extra flags to pass to rustc in non-exec configuration",
         ),
+        "require_explicit_unstable_features": attr.label(
+            default = Label(
+                "//rust/settings:require_explicit_unstable_features",
+            ),
+            doc = ("Label to a boolean build setting that controls whether all uses of unstable " +
+                   "Rust features must be explicitly opted in to using `-Zallow-features=...`."),
+        ),
         "rust_doc": attr.label(
             doc = "The location of the `rustdoc` binary. Can be a direct source or a filegroup containing one item.",
             allow_single_file = True,