add option to use DST structs for flex arrays #2772

This option uses Rust DST types to model C structures with flexible array members. For example, if you declare: ```c struct record { int len; float values[]; } ``` this means it's a C structure with a well-defined prefix, but a tail of `values` which depends on how much memory we've allocated for it. We can model this in Rust with: ```rust struct record { len: c_int, values: [f32], } ``` which means more or less the same thing - there's a type which has a known prefix, but its suffix is not directly known.

There are some tests which have two incomplete array fields; only the very last one is valid as a flexible array member (in C) or can be a DST field in Rust.

Put the pointer converting methods on the sized prefix types ([T; 0]) since they're the default type and what you're likely to be starting with. Emphasize the ones which work on references since they have safe lifetimes, but also have raw pointer variants, esp for handling uninitialized cases. The flex types have methods which return the sized type along with the length.

Previously it was generating inner `unsafe` blocks for all unsafe functions in conformance with Rust 2024, but now only do it when `--wrap-unsafe-ops` is enabled for consistency with other generated code. Also rename `flex_mut_ref` -> `flex_ref_mut` to make it consistent with `flex_ptr_mut` and general Rust convention.

The conversions between fixed and dynamically sized forms are essentially type-level transforms which should have trivial implementations in terms of generated code, so there's no reason not to make them inline.

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add option to use DST structs for flex arrays #2772

add option to use DST structs for flex arrays #2772

Commits on Apr 1, 2024