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Comment by Panzerschrek | Hacker News Reader

Porting Rust enums and pattern matching isn't necessary (C++ already has std::variant).

Lifetime parameters aren't necessary, lifetime contracts may be implemented in a different and much easier way. This may be expressed in form of a function attribute, which may be calculated via constexpr code.

Special operators for borrowing just add more complexity. C++ already achieves same behavior by normal references (which may be mutable and non-mutable).

Introducing immutability by default isn't strictly necessary for achieving safety. C++ developers are already mostly fine writing "const" almost everywhere.

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> Lifetime parameters aren't necessary, lifetime contracts may be implemented in a different and much easier way. This may be expressed in form of a function attribute, which may be calculated via constexpr code.

Wouldn't that just be slightly different syntax for the same idea? If you want to represent relationships like "the pointer in the field Struct::field_name within this argument is linked with the lifetime of the returned pointer" and you want them to be reusable across functions and contexts, isn't the most obvious way to write that a variant of

    struct Struct<'a> {
        char *'a field_name;
    };

    char *'a f<'a>(Struct<'a>& struct);

?

Using tags like 'a may work, but it's not flexible in templated code. A better approach is to allow calculating such tags in compilation time. But in such way you don't even need named tags, you can just use pairs of integers, like reference #n of param #m.

Lifetime parameters are necessary for borrow checking. And you need a special operator for initiating a borrow. You need immutability by default, because mutability everywhere violates exclusivity.

You don't need special operator for initializing borrow. Calling a function like Foo(a) will initialize such borrow, if function parameter is mutable reference. The same is for creating local mutable reference.

Mutability by default is good, but it's not strictly necessary. I repeat, you can just write "const" everywhere manually, except places, where mutation is needed.

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