crates/fayalite/src/prelude.rs

@@ -37,7 +37,7 @@ pub use crate::{
         value::{SimOnly, SimOnlyValue, SimValue, ToSimValue, ToSimValueWithType},
     },
     source_location::SourceLocation,
-    testing::{FormalMode, assert_formal},
+    testing::{FormalMode, assert_formal, checked_vcd_output},
     ty::{AsMask, CanonicalType, TraceAsString, Type},
     util::{ConstUsize, GenericConstUsize},
     wire::Wire,

crates/fayalite/src/testing.rs

@@ -12,11 +12,13 @@ use crate::{
     bundle::BundleType,
     firrtl::ExportOptions,
     module::Module,
-    util::HashMap,
+    sim::{Simulation, vcd::VcdWriterDecls},
+    util::{HashMap, RcWriter},
 };
 use serde::{Deserialize, Serialize};
 use std::{
     fmt::{self, Write},
+    panic::Location,
     path::{Path, PathBuf},
     process::Command,
     sync::{Mutex, OnceLock},
@@ -222,3 +224,190 @@ pub fn assert_formal<M: AsRef<Module<T>>, T: BundleType>(
     )
     .expect("testing::assert_formal() failed");
 }
+
+pub struct CheckedVcdOutput {
+    writer: Option<RcWriter>,
+    expected_path: PathBuf,
+    expected_contents: Result<String, (Option<PathBuf>, std::io::Error)>,
+    location: &'static Location<'static>,
+}
+
+impl CheckedVcdOutput {
+    #[must_use]
+    #[track_caller]
+    pub fn new<T: BundleType>(sim: &mut Simulation<T>, expected_path: PathBuf) -> Self {
+        let writer = RcWriter::default();
+        sim.add_trace_writer(VcdWriterDecls::new(writer.clone()));
+        Self {
+            writer: Some(writer),
+            expected_contents: std::fs::read_to_string(&expected_path).map_err(|e| {
+                eprintln!(
+                    "error: failed to read expected VCD from: {}",
+                    expected_path.display(),
+                );
+                (std::env::current_dir().ok(), e)
+            }),
+            expected_path,
+            location: Location::caller(),
+        }
+    }
+    #[must_use]
+    #[track_caller]
+    #[doc(hidden)]
+    pub fn __checked_vcd_output_macro_helper<T: BundleType>(
+        sim: &mut Simulation<T>,
+        cargo_manifest_dir: &'static str,
+        path: &'static str,
+    ) -> Self {
+        Self::new(sim, Path::new(cargo_manifest_dir).join(path))
+    }
+    pub fn with_vcd_output<R>(&self, f: impl FnOnce(&str) -> R) -> R {
+        let Some(writer) = &self.writer else {
+            unreachable!();
+        };
+        writer.clone().borrow(|output| {
+            let Ok(output) = str::from_utf8(output) else {
+                unreachable!("VcdWriter writes valid UTF-8");
+            };
+            f(output)
+        })
+    }
+    #[track_caller]
+    pub fn finish(mut self) {
+        let Ok(()) = self.finish_impl(|msg| panic!("{msg}"));
+    }
+    fn finish_impl<E>(
+        &mut self,
+        error: impl FnOnce(std::fmt::Arguments<'_>) -> E,
+    ) -> Result<(), E> {
+        let Self {
+            writer: Some(writer),
+            expected_path,
+            expected_contents,
+            location,
+        } = self
+        else {
+            // already finished
+            return Ok(());
+        };
+        let Ok(vcd) = String::from_utf8(writer.take()) else {
+            unreachable!("VcdWriter writes valid UTF-8");
+        };
+        let expected_path_d = expected_path.display();
+        if expected_contents
+            .as_ref()
+            .is_ok_and(|expected_contents| *expected_contents == vcd)
+        {
+            // avoid written output from being split from threads interleaving writes to stdout
+            let _stdout = std::io::stderr().lock();
+            // use println to get output captured by tests
+            println!("\n{location}: generated VCD matches the expected VCD in {expected_path_d}");
+            return Ok(());
+        }
+        // avoid written output from being split from threads interleaving writes to stderr
+        let _stderr = std::io::stderr().lock();
+        let error = |msg: std::fmt::Arguments<'_>| {
+            // print msg at both beginning and end so it's easier to find when the vcd is huge
+            Err(error(format_args!(
+                "\n{msg}####### VCD:\n{vcd}\n#######\n{msg}"
+            )))
+        };
+        let error = |msg: std::fmt::Arguments<'_>| match &*expected_contents {
+            Ok(_) => error(format_args!(
+                "{location}: generated VCD doesn't match the expected VCD in {expected_path_d}\n\
+                {msg}",
+            )),
+            Err((Some(current_dir), e)) => error(format_args!(
+                "{location}: generated VCD doesn't match the expected VCD in {expected_path_d}\n\
+                error: failed to read: {e}\n\
+                current dir: {current_dir}\n\
+                {msg}",
+                current_dir = current_dir.display(),
+            )),
+            Err((None, e)) => error(format_args!(
+                "{location}: generated VCD doesn't match the expected VCD in {expected_path_d}\n\
+                error: failed to read: {e}\n\
+                {msg}",
+            )),
+        };
+        const OVERWRITE_VAR_NAME: &str = "OVERWRITE_EXPECTED_VCD";
+        const OVERWRITE_VAR_VALUE: &str = "overwrite";
+        match std::env::var_os(OVERWRITE_VAR_NAME) {
+            Some(v) if v == OVERWRITE_VAR_VALUE => match std::fs::write(&expected_path, &vcd) {
+                Ok(()) => error(format_args!(
+                    "warning: since `{OVERWRITE_VAR_NAME}={OVERWRITE_VAR_VALUE}` is set -- writing the generated VCD to {expected_path_d}\n"
+                )),
+                Err(e) => error(format_args!(
+                    "error: since `{OVERWRITE_VAR_NAME}={OVERWRITE_VAR_VALUE}` is set -- tried to write the generated VCD to {expected_path_d}\n\
+                    error: failed to write: {e}"
+                )),
+            },
+            _ => error(format_args!(
+                "note: rerun the test with the environment variable `{OVERWRITE_VAR_NAME}={OVERWRITE_VAR_VALUE}`\n\
+                to update the expected output to match the generated output.\n"
+            )),
+        }
+    }
+}
+
+impl Drop for CheckedVcdOutput {
+    #[track_caller]
+    fn drop(&mut self) {
+        let _ = self.finish_impl(|msg| {
+            if std::thread::panicking() {
+                eprintln!("{msg}"); // use eprintln to get output captured by tests
+            } else {
+                panic!("{msg}");
+            }
+        });
+    }
+}
+
+#[macro_export]
+/// Use in tests to check that [`Simulation`] generates the expected VCD traces, by comparing to a `.vcd` file containing the expected traces.
+///
+/// Use like so:
+/// ```
+/// # use fayalite::prelude::*;
+/// #
+/// # #[hdl_module]
+/// # fn my_module() {
+/// #     #[hdl]
+/// #     let a: UInt<8> = m.input();
+/// #     #[hdl]
+/// #     let b: UInt<8> = m.output();
+/// #     connect(b, 0u8);
+/// #     #[hdl]
+/// #     if a.cmp_eq(100u8) {
+/// #         connect(b, 42u8);
+/// #     }
+/// # }
+/// // inside your #[test] fn my_test():
+///
+/// // get the module to simulate:
+/// let m = my_module();
+/// // create a simulation of the module:
+/// let mut sim = Simulation::new(m);
+/// // set up the expected VCD traces, the given .vcd path is relative to env!("CARGO_MANIFEST_DIR")
+/// let _checked_vcd_output = checked_vcd_output!(
+///     &mut sim,
+///     "tests/expected/my_test.vcd",
+/// );
+/// // now run the simulation like normal:
+/// sim.write(sim.io().a, 0u8);
+/// assert_eq!(sim.read(sim.io().b).as_int(), 0);
+/// sim.advance_time(SimDuration::from_micros(1));
+/// sim.write(sim.io().a, 100u8);
+/// assert_eq!(sim.read(sim.io().b).as_int(), 42);
+/// ```
+macro_rules! checked_vcd_output {
+    ($sim:expr, $path_relative_to_manifest_dir:expr $(,)?) => {
+        $crate::testing::CheckedVcdOutput::__checked_vcd_output_macro_helper(
+            $sim,
+            $crate::__std::env!("CARGO_MANIFEST_DIR"),
+            $crate::__std::concat!($path_relative_to_manifest_dir),
+        )
+    };
+}
+
+pub use checked_vcd_output;

crates/fayalite/tests/expected/my_test.md

@@ -0,0 +1,6 @@
+<!--
+SPDX-License-Identifier: LGPL-3.0-or-later
+See Notices.txt for copyright information
+-->
+
+`my_test.vcd` is used in the doctest of `fayalite::testing::checked_vcd_output`

crates/fayalite/tests/expected/my_test.vcd

@@ -0,0 +1,13 @@
+$timescale 1 ps $end
+$scope module my_module $end
+$var wire 8 gAF7X a $end
+$var wire 8 QS=a/ b $end
+$upscope $end
+$enddefinitions $end
+$dumpvars
+b0 gAF7X
+b0 QS=a/
+$end
+#1000000
+b1100100 gAF7X
+b101010 QS=a/

scripts/check-copyright.sh

@@ -47,7 +47,7 @@ function main()
         */LICENSE.md|*/Notices.txt)
             # copyright file
             ;;
-        /crates/fayalite/tests/ui/*.stderr|/crates/fayalite/tests/sim/expected/*.vcd|/crates/fayalite/tests/sim/expected/*.txt)
+        /crates/fayalite/tests/ui/*.stderr|/crates/fayalite/tests/expected/*.vcd|/crates/fayalite/tests/sim/expected/*.vcd|/crates/fayalite/tests/sim/expected/*.txt)
             # file that can't contain copyright header
             ;;
         /.forgejo/workflows/*.yml|*/.gitignore|*.toml|*/Makefile|*/_CoqProject)