rustdoc --test: Prevent reaching the maximum size of command-line b…

…y using files for arguments if there are too many

src/librustdoc/doctest.rs

@@ -23,6 +23,7 @@ use rustc_target::spec::{Target, TargetTriple};
 use tempfile::Builder as TempFileBuilder;
 
 use std::env;
+use std::fs::File;
 use std::io::{self, Write};
 use std::panic;
 use std::path::{Path, PathBuf};
@@ -31,6 +32,8 @@ use std::str;
 use std::sync::atomic::{AtomicUsize, Ordering};
 use std::sync::{Arc, Mutex};
 
+use tempfile::tempdir;
+
 use crate::clean::{types::AttributesExt, Attributes};
 use crate::config::Options as RustdocOptions;
 use crate::html::markdown::{self, ErrorCodes, Ignore, LangString};
@@ -348,13 +351,49 @@ fn run_test(
         .unwrap_or_else(|| rustc_interface::util::rustc_path().expect("found rustc"));
     let mut compiler = wrapped_rustc_command(&rustdoc_options.test_builder_wrappers, rustc_binary);
     compiler.arg("--crate-type").arg("bin");
-    for cfg in &rustdoc_options.cfgs {
-        compiler.arg("--cfg").arg(&cfg);
+
+    let mut needs_unstable_options = false;
+    let mut written_cfg_args = false;
+    #[allow(unused_variables)]
+    let tmp_dir;
+
+    // If there are too many `cfg` arguments, instead of risking reaching `Command`'s limit on
+    // the number of arguments, we put them into a file which we then pass as `@` argument.
+    if rustdoc_options.cfgs.len() + rustdoc_options.check_cfgs.len() > 1_000
+        && let Ok(temp_dir) = tempdir()
+        && let file_path = temp_dir.path().join("cfgs")
+        && let Ok(mut file) = File::create(&file_path)
+    {
+        let mut content = Vec::new();
+        for cfg in &rustdoc_options.cfgs {
+            content.push(format!("--cfg={cfg}"));
+        }
+        if !rustdoc_options.check_cfgs.is_empty() {
+            needs_unstable_options = true;
+            for check_cfg in &rustdoc_options.check_cfgs {
+                content.push(format!("--check-cfg={check_cfg}"));
+            }
+        }
+        let content = content.join("\n");
+        #[allow(unused_assignments)] // Needed for `tmp_dir = temp_dir`.
+        if file.write(content.as_bytes()).is_ok() {
+            // To prevent removing the temporary directory too soon.
+            tmp_dir = temp_dir;
+            written_cfg_args = true;
+            compiler.arg(&format!("@{}", file_path.display()));
+        }
     }
-    if !rustdoc_options.check_cfgs.is_empty() {
-        compiler.arg("-Z").arg("unstable-options");
-        for check_cfg in &rustdoc_options.check_cfgs {
-            compiler.arg("--check-cfg").arg(&check_cfg);
+    // If there were not enough `cfg` argument to make a file, or the file creation failed,
+    // we use the usual `Command` API.
+    if !written_cfg_args {
+        for cfg in &rustdoc_options.cfgs {
+            compiler.arg("--cfg").arg(&cfg);
+        }
+        if !rustdoc_options.check_cfgs.is_empty() {
+            needs_unstable_options = true;
+            for check_cfg in &rustdoc_options.check_cfgs {
+                compiler.arg("--check-cfg").arg(&check_cfg);
+            }
         }
     }
     if let Some(sysroot) = rustdoc_options.maybe_sysroot {
@@ -370,9 +409,14 @@ fn run_test(
     if rustdoc_options.json_unused_externs.is_enabled() && !lang_string.compile_fail {
         compiler.arg("--error-format=json");
         compiler.arg("--json").arg("unused-externs");
-        compiler.arg("-Z").arg("unstable-options");
         compiler.arg("-W").arg("unused_crate_dependencies");
+        needs_unstable_options = true;
     }
+
+    if needs_unstable_options {
+        compiler.arg("-Z").arg("unstable-options");
+    }
+
     for lib_str in &rustdoc_options.lib_strs {
         compiler.arg("-L").arg(&lib_str);
     }