Make matches_pattern for braced structs support exhaustiveness with generic structs.

Previously, for field exhaustiveness, we emitted an isolated `Foo { a: _, b: _ }` divorced from the `actual` value, but if `Foo`'s `a` field is generic, we're not able to infer it at all. So instead, put the `match` within a `predicate` matcher and match against the `actual` value to be able to infer the field.

Toward #447

PiperOrigin-RevId: 704716108

Author: marcianx

googletest/src/matchers/matches_pattern.rs

@@ -369,6 +369,66 @@ pub mod internal {
             }
         }
     }
+
+    /// A matcher that ensures that the passed-in function compiles with the
+    /// benefit of inference from the value being tested. The passed-in
+    /// matcher is what is actually invoked for matching.
+    ///
+    /// It forwards all description responsibilities to the passed-in matcher.
+    pub fn compile_assert_and_match<T, M>(
+        must_compile_function: fn(&T),
+        matcher: M,
+    ) -> CompileAssertAndMatch<T, M> {
+        CompileAssertAndMatch { must_compile_function, matcher }
+    }
+
+    #[derive(MatcherBase)]
+    #[doc(hidden)]
+    pub struct CompileAssertAndMatch<T, M> {
+        #[allow(dead_code)]
+        must_compile_function: fn(&T),
+        matcher: M,
+    }
+
+    impl<'a, T: Debug, M> Matcher<&'a T> for CompileAssertAndMatch<T, M>
+    where
+        M: Matcher<&'a T>,
+    {
+        fn matches(&self, actual: &'a T) -> crate::matcher::MatcherResult {
+            self.matcher.matches(actual)
+        }
+
+        fn describe(
+            &self,
+            matcher_result: crate::matcher::MatcherResult,
+        ) -> crate::description::Description {
+            self.matcher.describe(matcher_result)
+        }
+
+        fn explain_match(&self, actual: &'a T) -> crate::description::Description {
+            self.matcher.explain_match(actual)
+        }
+    }
+
+    impl<T: Debug + Copy, M> Matcher<T> for CompileAssertAndMatch<T, M>
+    where
+        M: Matcher<T>,
+    {
+        fn matches(&self, actual: T) -> crate::matcher::MatcherResult {
+            self.matcher.matches(actual)
+        }
+
+        fn describe(
+            &self,
+            matcher_result: crate::matcher::MatcherResult,
+        ) -> crate::description::Description {
+            self.matcher.describe(matcher_result)
+        }
+
+        fn explain_match(&self, actual: T) -> crate::description::Description {
+            self.matcher.explain_match(actual)
+        }
+    }
 }
 
 mod compile_fail_tests {

googletest/src/matchers/mod.rs

@@ -113,7 +113,9 @@ pub mod __internal_unstable_do_not_depend_on_these {
     pub use super::elements_are_matcher::internal::ElementsAre;
     pub use super::field_matcher::internal::field_matcher;
     pub use super::is_matcher::is;
-    pub use super::matches_pattern::internal::{__googletest_macro_matches_pattern, pattern_only};
+    pub use super::matches_pattern::internal::{
+        __googletest_macro_matches_pattern, compile_assert_and_match, pattern_only,
+    };
     pub use super::pointwise_matcher::internal::PointwiseMatcher;
     pub use super::property_matcher::internal::{property_matcher, property_ref_matcher};
     pub use super::result_of_matcher::internal::{result_of, result_of_ref};

googletest/tests/matches_pattern_test.rs

@@ -150,6 +150,20 @@ fn matches_struct_containing_non_copy_field_binding_mode() -> Result<()> {
     verify_that!(actual, matches_pattern!(AStruct { a_string: eq("123") }))
 }
 
+#[test]
+fn matches_generic_struct_exhaustively() -> Result<()> {
+    #[allow(dead_code)]
+    #[derive(Debug)]
+    struct AStruct<T> {
+        a: T,
+        b: u32,
+    }
+    let actual = AStruct { a: 1, b: 3 };
+
+    verify_that!(actual, matches_pattern!(&AStruct { a: _, b: _ }))?;
+    verify_that!(actual, matches_pattern!(AStruct { a: _, b: _ }))
+}
+
 #[test]
 fn matches_struct_with_interleaved_underscore() -> Result<()> {
     #[derive(Debug)]

googletest_macro/src/matches_pattern.rs

@@ -257,6 +257,13 @@ fn parse_braced_pattern_args(
         })
         .collect();
 
+    let matcher = quote! {
+        googletest::matchers::__internal_unstable_do_not_depend_on_these::is(
+            stringify!(#struct_name),
+            all!(#(#field_patterns),* )
+        )
+    };
+
     // Do an exhaustiveness check only if the pattern doesn't end with `..` and has
     // any fields in the pattern. This latter part is required because
     // `matches_pattern!` also uses the brace notation for tuple structs when
@@ -268,36 +275,25 @@ fn parse_braced_pattern_args(
     // matches_pattern!(foo, Struct { bar(): eq(1) })
     // ```
     // and we can't emit an exhaustiveness check based on the `matches_pattern!`.
-    let maybe_assert_exhaustive = if non_exhaustive || field_names.is_empty() {
-        None
+    if non_exhaustive || field_names.is_empty() {
+        Ok(matcher)
     } else {
-        // Note that `struct_name` might be an enum variant (`Enum::Foo`), which is not
-        // a valid type. So we need to infer the type, produce an instance, and match on
-        // it. Fortunately, `[_; 0]` can be trivially initialized to `[]` and can
-        // produce an instance by indexing into it without failing compilation.
-        Some(quote! {
-            fn __matches_pattern_ensure_exhastive_match(i: usize) {
-                let val: [_; 0] = [];
-                let _ = match val[i] {
-                    #struct_name { #(#field_names: _),* } => (),
-                    // The pattern below is unreachable if the type is a struct (as opposed to an
-                    // enum). Since the macro can't know which it is, we always include it and just
-                    // tell the compiler not to complain.
-                    #[allow(unreachable_patterns)]
-                    _ => (),
-                };
-            }
+        Ok(quote! {
+            googletest::matchers::__internal_unstable_do_not_depend_on_these::compile_assert_and_match(
+                |actual| {
+                    // Exhaustively check that all field names are specified.
+                    match actual {
+                        #struct_name { #(#field_names: _),* } => {},
+                        // The pattern below is unreachable if the type is a struct (as opposed to
+                        // an enum). Since the macro can't know which it is, we always include it
+                        // and just tell the compiler not to complain.
+                        #[allow(unreachable_patterns)]
+                        _ => {},
+                    }
+                },
+                #matcher)
         })
-    };
-
-    Ok(quote! {
-        googletest::matchers::__internal_unstable_do_not_depend_on_these::is(
-            stringify!(#struct_name), {
-                #maybe_assert_exhaustive
-                all!( #(#field_patterns),* )
-            }
-        )
-    })
+    }
 }
 
 ////////////////////////////////////////////////////////////////////////////////