rustdoc: Support argument files

Factors out the `rustc_driver` logic that handles argument files
so that rustdoc supports them as well, e.g.:

    rustdoc @argfile

This is needed to be able to generate docs for projects that
already use argument files when compiling them, e.g. projects
that pass a huge number of `--cfg` arguments.

The feature was stabilized for `rustc` in #66172.

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

compiler/rustc_driver/src/args.rs

@@ -3,7 +3,7 @@ use std::fmt;
 use std::fs;
 use std::io;
 
-pub fn arg_expand(arg: String) -> Result<Vec<String>, Error> {
+fn arg_expand(arg: String) -> Result<Vec<String>, Error> {
     if let Some(path) = arg.strip_prefix('@') {
         let file = match fs::read_to_string(path) {
             Ok(file) => file,
@@ -18,6 +18,20 @@ pub fn arg_expand(arg: String) -> Result<Vec<String>, Error> {
     }
 }
 
+pub fn arg_expand_all(at_args: &[String]) -> Vec<String> {
+    let mut args = Vec::new();
+    for arg in at_args {
+        match arg_expand(arg.clone()) {
+            Ok(arg) => args.extend(arg),
+            Err(err) => rustc_session::early_error(
+                rustc_session::config::ErrorOutputType::default(),
+                &format!("Failed to load argument file: {}", err),
+            ),
+        }
+    }
+    args
+}
+
 #[derive(Debug)]
 pub enum Error {
     Utf8Error(Option<String>),