Add rust_system_toolchain_repository for pre-installed toolchains

Introduce a repository rule that probes the system for an installed Rust
toolchain and generates BUILD targets with sysroot_path set. This follows
the cc_configure pattern from rules_cc: the repository rule runs at fetch
time, discovers the toolchain, and generates appropriate targets.

Changes:
- New rust/private/system_rust_configure.bzl with:
  - rust_system_toolchain_repository repository rule
  - system_rust_ext module extension for bzlmod
  - System probing (rustc --print sysroot, --version, --print target-libdir)
  - BUILD file generation with rust_toolchain + toolchain targets
- rust/toolchain.bzl: Add sysroot_path attribute to rust_toolchain. When
  set, sysroot files are excluded from action inputs and --sysroot is
  passed to rustc pointing at the system installation.
- rust/private/rustc.bzl: Use system rustc path when sysroot_path is set
- rust/private/clippy.bzl: Use system clippy-driver when sysroot_path is set
- rust/private/rustdoc.bzl: Use system rustdoc when sysroot_path is set
- rust/private/unpretty.bzl: Use system rustc when sysroot_path is set
- cargo/private/cargo_build_script.bzl: Use system tool paths for CARGO,
  RUSTC, RUSTDOC env vars when sysroot_path is set

This replaces the previous system_sysroot boolean approach with a cleaner
separation: the repository rule handles system probing, and the toolchain
rule simply accepts a sysroot_path string for path redirection.

Author: rejuvenile

cargo/private/cargo_build_script.bzl

@@ -416,7 +416,10 @@ def _cargo_build_script_impl(ctx):
         env.update(ctx.configuration.default_shell_env)
 
     if toolchain.cargo:
-        env["CARGO"] = "${pwd}/%s" % toolchain.cargo.path
+        if toolchain.sysroot_path:
+            env["CARGO"] = toolchain.sysroot_path + "/bin/cargo"
+        else:
+            env["CARGO"] = "${pwd}/%s" % toolchain.cargo.path
 
     env.update({
         "CARGO_CRATE_NAME": name_to_crate_name(pkg_name),
@@ -425,8 +428,8 @@ def _cargo_build_script_impl(ctx):
         "HOST": toolchain.exec_triple.str,
         "NUM_JOBS": "1",
         "OPT_LEVEL": compilation_mode_opt_level,
-        "RUSTC": toolchain.rustc.path,
-        "RUSTDOC": toolchain.rust_doc.path,
+        "RUSTC": (toolchain.sysroot_path + "/bin/rustc") if toolchain.sysroot_path else toolchain.rustc.path,
+        "RUSTDOC": (toolchain.sysroot_path + "/bin/rustdoc") if toolchain.sysroot_path else toolchain.rust_doc.path,
         "TARGET": toolchain.target_flag_value,
         # OUT_DIR is set by the runner itself, rather than on the action.
     })

rust/private/clippy.bzl

@@ -160,12 +160,14 @@ def rust_clippy_action(ctx, clippy_executable, process_wrapper, crate_info, conf
         crate_info_dict["rustc_output"] = clippy_diagnostics_file
         crate_info = rust_common.create_crate_info(**crate_info_dict)
 
+    clippy_tool_path = (toolchain.sysroot_path + "/bin/clippy-driver") if toolchain.sysroot_path else clippy_executable.path
+
     args, env = construct_arguments(
         ctx = ctx,
         attr = ctx.rule.attr,
         file = ctx.file,
         toolchain = toolchain,
-        tool_path = clippy_executable.path,
+        tool_path = clippy_tool_path,
         cc_toolchain = cc_toolchain,
         feature_configuration = feature_configuration,
         crate_info = crate_info,

rust/private/rustc.bzl

@@ -1193,8 +1193,10 @@ def construct_arguments(
             {},
         ))
 
-    # Ensure the sysroot is set for the target platform
-    if toolchain._toolchain_generated_sysroot:
+    # Ensure the sysroot is set for the target platform.
+    # When sysroot_path is set (system toolchain), always pass --sysroot so rustc
+    # finds the standard library at the pre-installed location.
+    if toolchain.sysroot_path or toolchain._toolchain_generated_sysroot:
         rustc_flags.add(toolchain.sysroot, format = "--sysroot=%s")
 
     if toolchain._rename_first_party_crates:
@@ -1400,12 +1402,16 @@ def rustc_compile_action(
         elif ctx.attr.require_explicit_unstable_features == -1:
             require_explicit_unstable_features = toolchain.require_explicit_unstable_features
 
+    # When using a system sysroot, use the system rustc path instead of the
+    # sysroot-symlinked binary.
+    rustc_tool_path = (toolchain.sysroot_path + "/bin/rustc") if toolchain.sysroot_path else toolchain.rustc.path
+
     args, env_from_args = construct_arguments(
         ctx = ctx,
         attr = attr,
         file = ctx.file,
         toolchain = toolchain,
-        tool_path = toolchain.rustc.path,
+        tool_path = rustc_tool_path,
         cc_toolchain = cc_toolchain,
         emit = emit,
         feature_configuration = feature_configuration,
@@ -1432,7 +1438,7 @@ def rustc_compile_action(
             attr = attr,
             file = ctx.file,
             toolchain = toolchain,
-            tool_path = toolchain.rustc.path,
+            tool_path = rustc_tool_path,
             cc_toolchain = cc_toolchain,
             emit = emit,
             feature_configuration = feature_configuration,

rust/private/rustdoc.bzl

@@ -126,12 +126,19 @@ def rustdoc_compile_action(
     # arguments expecting to do so.
     rustdoc_crate_info = _strip_crate_info_output(crate_info)
 
+    if toolchain.sysroot_path:
+        rustdoc_tool_path = toolchain.sysroot_path + "/bin/rustdoc"
+    elif is_test:
+        rustdoc_tool_path = toolchain.rust_doc.short_path
+    else:
+        rustdoc_tool_path = toolchain.rust_doc.path
+
     args, env = construct_arguments(
         ctx = ctx,
         attr = ctx.attr,
         file = ctx.file,
         toolchain = toolchain,
-        tool_path = toolchain.rust_doc.short_path if is_test else toolchain.rust_doc.path,
+        tool_path = rustdoc_tool_path,
         cc_toolchain = cc_toolchain,
         feature_configuration = feature_configuration,
         crate_info = rustdoc_crate_info,