build: add_inherited_rustflags: Apply code review suggestions

Co-authored-by: Jiahao XU <30436523+NobodyXu@users.noreply.github.com>

Author: mrkajetanp

src/flags.rs

@@ -26,22 +26,22 @@ impl RustcCodegenFlags {
     // Parse flags obtained from CARGO_ENCODED_RUSTFLAGS
     pub(crate) fn parse(rustflags_env: &str) -> Result<RustcCodegenFlags, Error> {
         fn is_flag_prefix(flag: &str) -> bool {
-            matches!(
-                flag,
-                "-Z" | "-C"
-                    | "--codegen"
-                    | "-L"
-                    | "-l"
-                    | "-o"
-                    | "-W"
-                    | "--warn"
-                    | "-A"
-                    | "--allow"
-                    | "-D"
-                    | "--deny"
-                    | "-F"
-                    | "--forbid"
-            )
+            [
+                "-Z",
+                "-C",
+                "--codegen",
+                "-L",
+                "-l",
+                "-o",
+                "-W",
+                "--warn",
+                "-A",
+                "--allow",
+                "-D",
+                "--deny",
+                "-F",
+                "--forbid",
+            ].contains(flag)
         }
 
         fn handle_flag_prefix<'a>(prev: &'a str, curr: &'a str) -> Cow<'a, str> {
@@ -64,17 +64,12 @@ impl RustcCodegenFlags {
 
         let mut prev_prefix = None;
         for curr in rustflags_env.split("\u{1f}") {
-            let prev = if let Some(prev) = prev_prefix {
-                prev_prefix = None;
-                prev
-            } else {
+            let prev = prev_prefix.take().unwrap_or("");
+            if prev.is_empty() && is_flag_prefix(curr) {
                 // Do not process prefixes on their own
-                if is_flag_prefix(curr) {
-                    prev_prefix = Some(curr);
-                    continue;
-                }
-                ""
-            };
+                prev_prefix = Some(curr);
+                continue;
+            }
 
             let rustc_flag = handle_flag_prefix(prev, curr);
             codegen_flags.set_rustc_flag(&rustc_flag)?;

src/lib.rs

@@ -2399,9 +2399,7 @@ impl Build {
 
         let env = env_os.to_string_lossy();
         let codegen_flags = RustcCodegenFlags::parse(&env)?;
-        let mut cc_flags_args: Vec<OsString> = vec![];
-        codegen_flags.cc_flags(self, path, *family, target, &mut cc_flags_args);
-        args.extend(cc_flags_args);
+        codegen_flags.cc_flags(self, path, *family, target, &mut args);
         Ok(())
     }