Rollup merge of #82261 - ojeda:rustdoc-argfile, r=jyn514

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.

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>),

compiler/rustc_driver/src/lib.rs

@@ -55,7 +55,7 @@ use std::process::{self, Command, Stdio};
 use std::str;
 use std::time::Instant;
 
-mod args;
+pub mod args;
 pub mod pretty;
 
 /// Exit status code used for successful compilation and help output.
@@ -188,16 +188,8 @@ fn run_compiler(
         Box<dyn FnOnce(&config::Options) -> Box<dyn CodegenBackend> + Send>,
     >,
 ) -> interface::Result<()> {
-    let mut args = Vec::new();
-    for arg in at_args {
-        match args::arg_expand(arg.clone()) {
-            Ok(arg) => args.extend(arg),
-            Err(err) => early_error(
-                ErrorOutputType::default(),
-                &format!("Failed to load argument file: {}", err),
-            ),
-        }
-    }
+    let args = args::arg_expand_all(at_args);
+
     let diagnostic_output = emitter.map_or(DiagnosticOutput::Default, DiagnosticOutput::Raw);
     let matches = match handle_options(&args) {
         Some(matches) => matches,