Perform target-machine command-line quoting as bytes, not characters

This works because the only characters we care about escaping are ASCII
characters, so forwarding non-ASCII UTF-8 bytes as-is still produces correct
results.

Author: Zalathar

compiler/rustc_codegen_llvm/src/back/command_line_args.rs

@@ -6,32 +6,34 @@ mod tests;
 ///
 /// The result is intended to be informational, for embedding in debug metadata,
 /// and might not be properly quoted/escaped for actual command-line use.
-pub(crate) fn quote_command_line_args(args: &[String]) -> String {
+pub(crate) fn quote_command_line_args(args: &[String]) -> Vec<u8> {
     // Start with a decent-sized buffer, since rustc invocations tend to be long.
-    let mut buf = String::with_capacity(128);
+    let mut buf = Vec::with_capacity(128);
 
     for arg in args {
         if !buf.is_empty() {
-            buf.push(' ');
+            buf.push(b' ');
         }
 
         print_arg_quoted(&mut buf, arg);
     }
 
+    debug_assert!(str::from_utf8(&buf).is_ok());
+
     buf
 }
 
 /// Equivalent to LLVM's `sys::printArg` with quoting always enabled
 /// (see llvm/lib/Support/Program.cpp).
-fn print_arg_quoted(buf: &mut String, arg: &str) {
+fn print_arg_quoted(buf: &mut Vec<u8>, arg: &str) {
     buf.reserve(arg.len() + 2);
 
-    buf.push('"');
-    for ch in arg.chars() {
-        if matches!(ch, '"' | '\\' | '$') {
-            buf.push('\\');
+    buf.push(b'"');
+    for &byte in arg.as_bytes() {
+        if matches!(byte, b'"' | b'\\' | b'$') {
+            buf.push(b'\\');
         }
-        buf.push(ch);
+        buf.push(byte);
     }
-    buf.push('"');
+    buf.push(b'"');
 }

compiler/rustc_codegen_llvm/src/back/command_line_args/tests.rs

@@ -20,6 +20,6 @@ fn quote_command_line_args() {
     for &Case { args, expected } in cases {
         let args = args.iter().copied().map(str::to_owned).collect::<Vec<_>>();
         let actual = quote_command_line_args(&args);
-        assert_eq!(actual, expected, "args {args:?}");
+        assert_eq!(str::from_utf8(&actual), Ok(expected), "args {args:?}");
     }
 }

compiler/rustc_codegen_llvm/src/back/owned_target_machine.rs

@@ -39,7 +39,7 @@ impl OwnedTargetMachine {
         debug_info_compression: &CStr,
         use_emulated_tls: bool,
         argv0: &str,
-        command_line_args: &str,
+        command_line_args: &[u8],
         use_wasm_eh: bool,
     ) -> Result<Self, LlvmError<'static>> {
         // SAFETY: llvm::LLVMRustCreateTargetMachine copies pointed to data