Parameterize Matcher::matches and Matcher::explain_match.#369
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Parameterize Matcher::matches and Matcher::explain_match.#369
Matcher::matches and Matcher::explain_match.#369Conversation
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Previously, `IsEncodedStringMatcher` converted its actual value input to a `Vec` and constructed a `String` out of that in order to match it. This is because using a string slice caused problems with the borrow checker as described in #323. The fundamental problem is that the type against which the inner matcher is matching is a reference whose lifetime must be named in the trait bound: ``` impl<ActualT: AsRef<[u8]> + Debug, InnerMatcherT> Matcher for IsEncodedStringMatcher<ActualT, InnerMatcherT> where InnerMatcherT: Matcher<ActualT = &'what str>, ^^^^ What goes here??? ``` One option is just to remove the reference: ``` impl<ActualT: AsRef<[u8]> + Debug, InnerMatcherT> Matcher for IsEncodedStringMatcher<ActualT, InnerMatcherT> where InnerMatcherT: Matcher<ActualT = str>, ``` This doesn't work when the inner matcher is `eq`, since one would then be comparing an `str` and a string slice: ``` verify_that!("A string".as_bytes(), is_utf8_string(eq("A string"))) // Compile error: cannot compare str and &str ``` However, this problem does not occur with `StrMatcher`: ``` verify_that!("A string".as_bytes(), is_utf8_string(constains_substring("A string"))) // Passes ``` So this also introduces an easy way to obtain a `StrMatcher` to check string equality: ``` verify_that!("A string".as_bytes(), is_utf8_string(eq_str("A string"))) // Passes ``` This is slightly inconvenient, since one must use a different matcher to compare string equality in this case. But it is well-documented and not too inconvenient to use. So it should be okay. The introduction of `eq_str` also allows fixing a longstanding limitation of the matcher `property!` -- that it could not match on properties returning string slices. This PR therefore also updates the documentation accordingly and adds an appropriate test.
This allows the use of smart pointers for the actual value when invoking `Matcher::matches` and `Matcher::explain_match`. The Protobuf implementation namely uses a [kind of smart pointer](https://github.com/protocolbuffers/protobuf/blob/62d5d9bf67d27d3b0084dabcf67bff2f8238162b/rust/proxied.rs#L113). The lifetime on that smart pointer has no a priori relation to that of the parameter `actual`, causing the borrow checker to complain whenever invoking an inner matcher on the output of `as_view`. This commit addresses the problem by allowing the `ViewProxy` -- rather than a reference to it -- to be passed directly to `Matcher::matches` and `Matcher::explain_match`. It does this by making those two methods generic with respect to the input type. They must only implement `Deref<Target = Self::ActualT>`. The new test `googletest/tests/arena_test.rs` illustrates this with a simplified example. Introducing these generics unfortunately makes `Matcher` no longer object-safe. Since several matchers construct trait objts on `Matcher`, this also introduces an internal-only wrapper trait `ObjectSafeMatcher` which every `Matcher` auto-implements. While this makes some changes to how `Matcher` is defined and implemented, it should have no effect on how it is _used_, other than to fix the aforementioned problem. So it should not affect compatibility for most users of the crate. Fixes #323.
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Note: I've created this PR for now just to check that the changes below fully pass CI.
This allows the use of smart pointers for the actual value when invoking
Matcher::matchesandMatcher::explain_match. The Protobuf implementation namely uses a kind of smart pointer. The lifetime on that smart pointer has no a priori relation to that of the parameteractual, causing the borrow checker to complain whenever invoking an inner matcher on the output ofas_view.This commit addresses the problem by allowing the
ViewProxy-- rather than a reference to it -- to be passed directly toMatcher::matchesandMatcher::explain_match. It does this by making those two methods generic with respect to the input type. They must only implementDeref<Target = Self::ActualT>.The new test
googletest/tests/arena_test.rsillustrates this with a simplified example.Introducing these generics unfortunately makes
Matcherno longer object-safe. Since several matchers construct trait objts onMatcher, this also introduces an internal-only wrapper traitObjectSafeMatcherwhich everyMatcherauto-implements.While this makes some changes to how
Matcheris defined and implemented, it should have no effect on how it is used, other than to fix the aforementioned problem. So it should not affect compatibility for most users of the crate.Fixes #323.