Rollup merge of rust-lang#131095 - GuillaumeGomez:switch-to-env-variables, r=notriddle

Use environment variables instead of command line arguments for merged doctests

Fixes [rust-lang#130796](rust-lang#130796).

Before merging this one, let's first ensure it has a smaller impact compared to command line arguments (results can be seen [here](rust-lang#130285)). I'll start a crater run once CI passed.

cc `@ehuss`
r? `@notriddle`

Author: compiler-errors

src/librustdoc/doctest.rs

@@ -643,8 +643,7 @@ fn run_test(
     } else {
         cmd = Command::new(&output_file);
         if doctest.is_multiple_tests {
-            cmd.arg("*doctest-bin-path");
-            cmd.arg(&output_file);
+            cmd.env("RUSTDOC_DOCTEST_BIN_PATH", &output_file);
         }
     }
     if let Some(run_directory) = &rustdoc_options.test_run_directory {

src/librustdoc/doctest/runner.rs

@@ -112,8 +112,7 @@ mod __doctest_mod {{
     use std::path::PathBuf;
 
     pub static BINARY_PATH: OnceLock<PathBuf> = OnceLock::new();
-    pub const RUN_OPTION: &str = \"*doctest-inner-test\";
-    pub const BIN_OPTION: &str = \"*doctest-bin-path\";
+    pub const RUN_OPTION: &str = \"RUSTDOC_DOCTEST_RUN_NB_TEST\";
 
     #[allow(unused)]
     pub fn doctest_path() -> Option<&'static PathBuf> {{
@@ -123,8 +122,8 @@ mod __doctest_mod {{
     #[allow(unused)]
     pub fn doctest_runner(bin: &std::path::Path, test_nb: usize) -> Result<(), String> {{
         let out = std::process::Command::new(bin)
-            .arg(self::RUN_OPTION)
-            .arg(test_nb.to_string())
+            .env(self::RUN_OPTION, test_nb.to_string())
+            .args(std::env::args().skip(1).collect::<Vec<_>>())
             .output()
             .expect(\"failed to run command\");
         if !out.status.success() {{
@@ -138,36 +137,27 @@ mod __doctest_mod {{
 #[rustc_main]
 fn main() -> std::process::ExitCode {{
 const TESTS: [test::TestDescAndFn; {nb_tests}] = [{ids}];
-let bin_marker = std::ffi::OsStr::new(__doctest_mod::BIN_OPTION);
 let test_marker = std::ffi::OsStr::new(__doctest_mod::RUN_OPTION);
 let test_args = &[{test_args}];
+const ENV_BIN: &'static str = \"RUSTDOC_DOCTEST_BIN_PATH\";
 
-let mut args = std::env::args_os().skip(1);
-while let Some(arg) = args.next() {{
-    if arg == bin_marker {{
-        let Some(binary) = args.next() else {{
-            panic!(\"missing argument after `{{}}`\", __doctest_mod::BIN_OPTION);
-        }};
-        if crate::__doctest_mod::BINARY_PATH.set(binary.into()).is_err() {{
-            panic!(\"`{{}}` option was used more than once\", bin_marker.to_string_lossy());
-        }}
-        return std::process::Termination::report(test::test_main(test_args, Vec::from(TESTS), None));
-    }} else if arg == test_marker {{
-        let Some(nb_test) = args.next() else {{
-            panic!(\"missing argument after `{{}}`\", __doctest_mod::RUN_OPTION);
-        }};
-        if let Some(nb_test) = nb_test.to_str().and_then(|nb| nb.parse::<usize>().ok()) {{
-            if let Some(test) = TESTS.get(nb_test) {{
-                if let test::StaticTestFn(f) = test.testfn {{
-                    return std::process::Termination::report(f());
-                }}
+if let Ok(binary) = std::env::var(ENV_BIN) {{
+    let _ = crate::__doctest_mod::BINARY_PATH.set(binary.into());
+    unsafe {{ std::env::remove_var(ENV_BIN); }}
+    return std::process::Termination::report(test::test_main(test_args, Vec::from(TESTS), None));
+}} else if let Ok(nb_test) = std::env::var(__doctest_mod::RUN_OPTION) {{
+    if let Ok(nb_test) = nb_test.parse::<usize>() {{
+        if let Some(test) = TESTS.get(nb_test) {{
+            if let test::StaticTestFn(f) = test.testfn {{
+                return std::process::Termination::report(f());
             }}
         }}
-        panic!(\"Unexpected value after `{{}}`\", __doctest_mod::RUN_OPTION);
     }}
+    panic!(\"Unexpected value for `{{}}`\", __doctest_mod::RUN_OPTION);
 }}
 
-eprintln!(\"WARNING: No argument provided so doctests will be run in the same process\");
+eprintln!(\"WARNING: No rustdoc doctest environment variable provided so doctests will be run in \
+the same process\");
 std::process::Termination::report(test::test_main(test_args, Vec::from(TESTS), None))
 }}",
             nb_tests = self.nb_tests,