scripts: port rustdoc_test_builder.rs to Rust

The KUnit doctest generation uses `rustdoc --test-builder` to fetch
the doctests, which runs the test builder once per test.

By porting `rustdoc_test_builder.py` to Rust as a hostprog,
the overhead per test is reduced more than an order of magnitude
(e.g. ~50ms to ~1 ms).

Currently, there are about 100 doctests, which means the Python
script is already responsible for a few seconds of build time.
As more doctests continue to be added, this would become more
and more painful.

Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: ojeda

rust/Makefile

@@ -167,11 +167,13 @@ quiet_cmd_rustdoc_test_kernel = RUSTDOC TK $<
 		--extern build_error --extern macros \
 		--extern bindings \
 		--no-run --crate-name kernel -Zunstable-options \
-		--test-builder $(srctree)/scripts/rustdoc_test_builder.py \
+		--test-builder $(objtree)/scripts/rustdoc_test_builder \
 		$< $(rustdoc_test_kernel_quiet); \
 	$(srctree)/scripts/rustdoc_test_gen.py
 
-%/doctests_kernel_generated.rs %/doctests_kernel_generated_kunit.c: $(src)/kernel/lib.rs $(obj)/kernel.o FORCE
+%/doctests_kernel_generated.rs %/doctests_kernel_generated_kunit.c: \
+    $(src)/kernel/lib.rs $(obj)/kernel.o \
+    $(objtree)/scripts/rustdoc_test_builder FORCE
 	$(call if_changed,rustdoc_test_kernel)
 
 # We cannot use `-Zpanic-abort-tests` because some tests are dynamic,

scripts/.gitignore

@@ -6,6 +6,7 @@
 /kallsyms
 /module.lds
 /recordmcount
+/rustdoc_test_builder
 /sign-file
 /sorttable
 /unifdef

scripts/Makefile

@@ -10,9 +10,12 @@ hostprogs-always-$(CONFIG_BUILDTIME_TABLE_SORT)		+= sorttable
 hostprogs-always-$(CONFIG_ASN1)				+= asn1_compiler
 hostprogs-always-$(CONFIG_MODULE_SIG_FORMAT)		+= sign-file
 hostprogs-always-$(CONFIG_SYSTEM_EXTRA_CERTIFICATE)	+= insert-sys-cert
-hostprogs-always-$(CONFIG_RUST)				+= generate_rust_target
+hostprogs-always-$(CONFIG_RUST)				+= \
+	generate_rust_target \
+	rustdoc_test_builder
 
 generate_rust_target-rust := y
+rustdoc_test_builder-rust := y
 
 HOSTCFLAGS_sorttable.o = -I$(srctree)/tools/include
 HOSTLDLIBS_sorttable = -lpthread

scripts/rustdoc_test_builder.py

@@ -0,0 +1,43 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Test builder for `rustdoc`-generated tests.
+
+use std::fs::File;
+use std::io::{BufWriter, Read, Write};
+
+fn main() {
+    let mut stdin = std::io::stdin().lock();
+    let mut body = String::new();
+    stdin.read_to_string(&mut body).unwrap();
+
+    let name = body
+        .lines()
+        .find_map(|line| {
+            Some(
+                line.split_once("fn ")?
+                    .1
+                    .split_once("rust_kernel_")?
+                    .1
+                    .split_once("()")?
+                    .0,
+            )
+            .filter(|x| x.chars().all(|c| c.is_alphanumeric() || c == '_'))
+        })
+        .expect("No test name found.");
+
+    // Qualify `Result` to avoid the collision with our own `Result`
+    // coming from the prelude.
+    let body = body.replace(
+        &format!("rust_kernel_{name}() -> Result<(), impl core::fmt::Debug> {{"),
+        &format!("rust_kernel_{name}() -> core::result::Result<(), impl core::fmt::Debug> {{"),
+    );
+
+    let name = format!("rust_kernel_doctest_{name}");
+    let path = format!("rust/test/doctests/kernel/{name}");
+
+    write!(
+        BufWriter::new(File::create(path).unwrap()),
+        "{name}\n{body}"
+    )
+    .unwrap();
+}

scripts/rustdoc_test_builder.rs

@@ -3,7 +3,6 @@
 """rustdoc_test_gen - Generates KUnit tests from saved `rustdoc`-generated tests.
 """
 
-import json
 import os
 import pathlib
 
@@ -136,16 +135,16 @@ def main():
     c_test_cases = ""
     for filename in sorted(os.listdir(TESTS_DIR)):
         with open(TESTS_DIR / filename, "r") as fd:
-            test = json.load(fd)
+            (test_name, test_body) = fd.read().split('\n', 1)
             rust_tests += RUST_TEMPLATE_TEST.format(
-                test_name = test["name"],
-                test_body = test["body"]
+                test_name = test_name,
+                test_body = test_body
             )
             c_test_declarations += C_TEMPLATE_TEST_DECLARATION.format(
-                test_name = test["name"]
+                test_name = test_name
             )
             c_test_cases += C_TEMPLATE_TEST_CASE.format(
-                test_name = test["name"]
+                test_name = test_name
             )
 
     with open(RUST_FILE, "w") as fd: