Add a new formatter module that provides a macro to conditionally print different things for Debug and non-Debug values using inherent-method specialization as described in dtolnay/case-studies#14.

PiperOrigin-RevId: 690016541

Author: Googler

googletest/src/fmt.rs

@@ -0,0 +1,67 @@
+// Copyright 2024 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+/// Functions for use only by the procedural macros in this module.
+///
+/// **For internal use only. API stablility is not guaranteed!**
+#[doc(hidden)]
+pub mod internal {
+    use std::fmt::{Debug, Write};
+
+    /// Wrapper to allow for inherent-method specialization based on whether a
+    /// type implements `Debug`.
+    pub struct FormatWrapper<'a, T: ?Sized>(pub &'a T);
+
+    /// Default implementation to render values that implement `Debug`.
+    ///
+    /// Used for autoref specialization to conditionally
+    /// render only values that implement `Debug`. See also
+    /// [`FormatNonDebugFallback`].
+    impl<'a, T: Debug + ?Sized> FormatWrapper<'a, T> {
+        #[track_caller]
+        pub fn __googletest_write_expr_value(&self, output: &mut dyn Write, expr_label: &str) {
+            write!(output, "\n  {} = {:?},", expr_label, self.0)
+                .expect("Formatting to String should never fail");
+        }
+    }
+
+    /// Fallback implementation for rendering values for non-`Debug` types..
+    ///
+    /// Used for inherent-method specialization to conditionally render only
+    /// values that implement `Debug`. See also the specialized inherent impl on
+    /// [`FormatWrapper`] above.
+    pub trait FormatNonDebugFallback {
+        fn __googletest_write_expr_value(&self, output: &mut dyn Write, expr_label: &str);
+    }
+
+    impl<'a, T: ?Sized> FormatNonDebugFallback for FormatWrapper<'a, T> {
+        #[track_caller]
+        fn __googletest_write_expr_value(&self, output: &mut dyn Write, expr_label: &str) {
+            write!(output, "\n  {} does not implement Debug,", expr_label)
+                .expect("Formatting to String should never fail");
+        }
+    }
+
+    #[macro_export]
+    macro_rules! __googletest__write_expr_value(
+        ($output:expr, $expr_str:expr, $value:expr $(,)?) => {
+            {
+                use $crate::fmt::internal::FormatNonDebugFallback as _;
+                $crate::fmt::internal::FormatWrapper(&$value)
+                    .__googletest_write_expr_value(&mut $output, $expr_str)
+            }
+        }
+    );
+    pub use __googletest__write_expr_value;
+}

googletest/src/lib.rs

@@ -23,6 +23,8 @@ extern crate quickcheck;
 pub mod assertions;
 pub mod description;
 pub mod fixtures;
+#[macro_use]
+pub mod fmt;
 pub mod internal;
 pub mod matcher;
 pub mod matcher_support;

googletest/tests/fmt_test.rs

@@ -0,0 +1,58 @@
+// Copyright 2024 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+mod write_expr_value {
+    use googletest::prelude::*;
+
+    // Converts the formatting call to a `String` for testing.
+    macro_rules! write_expr_value {
+        ($expr_str:expr, $expr: expr $(,)?) => {{
+            let mut s = String::new();
+            ::googletest::fmt::internal::__googletest__write_expr_value!(s, $expr_str, $expr);
+            s
+        }};
+    }
+
+    #[test]
+    fn test_with_debug_value_references() -> Result<()> {
+        #[derive(Debug)]
+        struct Foo;
+        let mut val = Foo;
+
+        verify_that!(write_expr_value!("val", val), eq("\n  val = Foo,"))?;
+        verify_that!(write_expr_value!("val", &val), eq("\n  val = Foo,"))?;
+        verify_that!(write_expr_value!("val", &&val), eq("\n  val = Foo,"))?;
+        verify_that!(write_expr_value!("val", &mut val), eq("\n  val = Foo,"))?;
+        verify_that!(write_expr_value!("val", &mut &mut val), eq("\n  val = Foo,"))?;
+
+        Ok(())
+    }
+
+    #[test]
+    fn test_with_non_debug_value_references() -> Result<()> {
+        struct Foo;
+        let mut val = Foo;
+
+        verify_that!(write_expr_value!("val", val), eq("\n  val does not implement Debug,"))?;
+        verify_that!(write_expr_value!("val", &val), eq("\n  val does not implement Debug,"))?;
+        verify_that!(write_expr_value!("val", &&val), eq("\n  val does not implement Debug,"))?;
+        verify_that!(write_expr_value!("val", &mut val), eq("\n  val does not implement Debug,"))?;
+        verify_that!(
+            write_expr_value!("val", &mut &mut val),
+            eq("\n  val does not implement Debug,")
+        )?;
+
+        Ok(())
+    }
+}