support redirecting subprocesses' stdout/stderr to print!() so it gets captured for rust tests

crates/fayalite/Cargo.toml

@@ -21,6 +21,7 @@ fayalite-proc-macros.workspace = true
 hashbrown.workspace = true
 num-bigint.workspace = true
 num-traits.workspace = true
+os_pipe.workspace = true
 serde_json.workspace = true
 serde.workspace = true
 which.workspace = true

crates/fayalite/src/cli.rs

@@ -5,6 +5,7 @@ use crate::{
     firrtl,
     intern::Interned,
     module::Module,
+    util::streaming_read_utf8::streaming_read_utf8,
 };
 use clap::{
     builder::{OsStringValueParser, TypedValueParser},
@@ -51,6 +52,8 @@ pub struct BaseArgs {
     /// the stem of the generated main output file, e.g. to get foo.v, pass --file-stem=foo
     #[arg(long)]
     pub file_stem: Option<String>,
+    #[arg(skip = false)]
+    pub redirect_output_for_rust_test: bool,
 }
 
 impl BaseArgs {
@@ -60,6 +63,34 @@ impl BaseArgs {
             top_fir_file_stem: self.file_stem.clone(),
         }
     }
+    /// handles possibly redirecting the command's output for Rust tests
+    pub fn run_external_command(
+        &self,
+        mut command: process::Command,
+    ) -> io::Result<process::ExitStatus> {
+        if self.redirect_output_for_rust_test {
+            let (reader, writer) = os_pipe::pipe()?;
+            let mut reader = io::BufReader::new(reader);
+            command.stderr(writer.try_clone()?);
+            command.stdout(writer); // must not leave writer around after spawning child
+            command.stdin(process::Stdio::null());
+            let mut child = command.spawn()?;
+            drop(command); // close writers
+            Ok(loop {
+                let status = child.try_wait()?;
+                streaming_read_utf8(&mut reader, |s| {
+                    // use print! so output goes to Rust test output capture
+                    print!("{s}");
+                    io::Result::Ok(())
+                })?;
+                if let Some(status) = status {
+                    break status;
+                }
+            })
+        } else {
+            command.status()
+        }
+    }
 }
 
 #[derive(Parser, Debug, Clone)]
@@ -188,7 +219,7 @@ impl VerilogArgs {
         }
         cmd.args(&self.firtool_extra_args);
         cmd.current_dir(&self.firrtl.base.output);
-        let status = cmd.status()?;
+        let status = self.firrtl.base.run_external_command(cmd)?;
         if status.success() {
             Ok(output)
         } else {

crates/fayalite/src/util.rs

@@ -6,6 +6,7 @@ mod const_cmp;
 mod const_usize;
 mod misc;
 mod scoped_ref;
+pub(crate) mod streaming_read_utf8;
 
 #[doc(inline)]
 pub use const_bool::{ConstBool, ConstBoolDispatch, ConstBoolDispatchTag, GenericConstBool};

crates/fayalite/src/util/streaming_read_utf8.rs

@@ -0,0 +1,29 @@
+use std::{
+    io::{self, BufRead},
+    str,
+};
+
+pub(crate) fn streaming_read_utf8<R: BufRead, E: From<io::Error>>(
+    reader: R,
+    mut callback: impl FnMut(&str) -> Result<(), E>,
+) -> Result<(), E> {
+    let mut buf = [0; 4];
+    let mut buf_len = 0;
+    for byte in reader.bytes() {
+        buf[buf_len] = byte?;
+        buf_len += 1;
+        match str::from_utf8(&buf[..buf_len]) {
+            Ok(buf) => {
+                callback(buf)?;
+                buf_len = 0;
+            }
+            Err(e) => {
+                if e.error_len().is_some() {
+                    callback("\u{FFFD}")?; // replacement character
+                    buf_len = 0;
+                }
+            }
+        }
+    }
+    Ok(())
+}