Comment by mr_00ff00 | Hacker News Reader

Comment by mr_00ff00 | Hacker News Reader

I get the author’s point so this may be a useless nitpick but I am not surprised in Rust that drop is called immediately when a value is assigned to _.

That’s almost like calling delete in c++ and if you changed the C++ code to a class with a print in the destructor, then called “delete” you would get the same behavior.

The “just” underscore in rust I believe is special and basically makes that value inaccessible. So that value really does “destruct” there.

It is surprising for a c++ programmer; the equivalent:

   auto _ = Foo{};

would leave the object alive till the end of scope. This is true for all RAII classes. In C++26 the unnamed placeholder '_' is also special cased to allow redeclaration.

For a C++ programmer, the related "suprising" equivalent is to not name the variable. We had a regex linter in our code to ensure locks were named because of bugs from this.

You mean like `clippy::let_underscore_lock`? I'd like it if there was an annotation for arbitrary types to declare that they behave like a lock so that the lint would pick them up too.

For Rust, yes, we do have that and that'd be great to generalize it. That was a C++ code base using custom lock types that I used a regex linter for. Glad to not be touching it anymore for many reasons.

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