Problem: amalgamated file is not guarded against double inclusion

src/cli.rs

@@ -179,6 +179,12 @@ pub struct Opts {
     /// original files.
     #[clap(long)]
     pub line_directives: bool,
+
+    /// Add #pragma once
+    ///
+    /// This will prepend #pragma once to the amalgamated file
+    #[clap(long)]
+    pub pragma_once: bool,
 }
 
 fn with_indices<'a, T>(

src/main.rs

@@ -71,12 +71,14 @@ fn run_with_writer(opts: &Opts, writer: impl Write) -> Result<()> {
         unresolvable_quote_include: opts.unresolvable_quote_include_handling(),
         unresolvable_system_include: opts.unresolvable_system_include_handling(),
     };
+    let pragma_once = opts.pragma_once;
     let mut processor = Processor::new(
         writer,
         resolver,
         opts.line_directives,
         filter,
         error_handling_opts,
+        pragma_once,
     );
     opts.files
         .iter()

src/process.rs

@@ -98,6 +98,8 @@ pub struct Processor<W> {
     expected_line: Option<LineRef>,
     error_handling_opts: ErrorHandlingOpts,
     regexes: Regexes,
+    guard_emitted: bool,
+    pragma_once: bool,
 }
 
 impl<W: Write> Processor<W> {
@@ -107,6 +109,7 @@ impl<W: Write> Processor<W> {
         line_directives: bool,
         inlining_filter: InliningFilter,
         error_handling_opts: ErrorHandlingOpts,
+        pragma_once: bool,
     ) -> Self {
         let expected_line = line_directives.then(|| LineRef {
             file_idx: EMPTY_STACK_IDX,
@@ -122,6 +125,8 @@ impl<W: Write> Processor<W> {
             expected_line,
             error_handling_opts,
             regexes: Regexes::new(),
+            guard_emitted: false,
+            pragma_once,
         }
     }
 
@@ -306,6 +311,10 @@ impl<W: Write> Processor<W> {
     }
 
     pub fn process(&mut self, source_file: &Path) -> Result<()> {
+        if self.pragma_once && !self.guard_emitted {
+            self.guard_emitted = true;
+            self.writer.write(b"#pragma once\n")?;
+        }
         info!("Processing source file {:?}", debug_file_name(source_file));
         let canonical_path = source_file.canonicalize().with_context(|| {
             format!(

tests/integration/misc.rs

@@ -53,3 +53,15 @@ fn multiple_source_files() -> Result<()> {
         .stdout("abc");
     Ok(())
 }
+
+#[test]
+fn pragma_once() -> Result<()> {
+    util::builder()
+        .source_file("test")?
+        .command()
+        .arg("--pragma-once")
+        .assert()
+        .success()
+        .stdout("#pragma once\ntest");
+    Ok(())
+}