toolchain: extra_rustc_flags after target rustc flags

Summary: So that if you pass something on the command line, it takes precedence over any package/target flags

Reviewed By: dtolnay

Differential Revision: D54976353

fbshipit-source-id: de0c21b48e4512f9efd270280310c9ccb514cd43

prelude/rust/build.bzl

@@ -1015,6 +1015,7 @@ def _compute_common_args(
         _rustc_flags(toolchain_info.rustc_check_flags) if is_check else [],
         _rustc_flags(toolchain_info.rustc_coverage_flags) if ctx.attrs.coverage else [],
         _rustc_flags(ctx.attrs.rustc_flags),
+        _rustc_flags(toolchain_info.extra_rustc_flags),
         cmd_args(ctx.attrs.features, format = '--cfg=feature="{}"'),
         dep_args,
     )

prelude/rust/rust_toolchain.bzl

@@ -40,6 +40,9 @@ rust_toolchain_attrs = {
     "rustc_target_triple": provider_field(str | None, default = None),
     # Baseline compiler config
     "rustc_flags": provider_field(list[typing.Any], default = []),
+    # Rustc flags, except that they are applied on the command line after the
+    # target's rustc flags
+    "extra_rustc_flags": provider_field(list[typing.Any], default = []),
     # Extra flags when building binaries
     "rustc_binary_flags": provider_field(list[typing.Any], default = []),
     # Extra flags for doing check builds