mem::uninitialized is on it’s way to being deprecated, so I wouldn’t count on it. Instead you could use mem::MaybeUninit, which has no overhead, to deal with uninitialized data.
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mem::MaybeUninit is not const friendly yet too. It doesn’t really matter which we’d use, potentially it should be const friendly
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I intentionally excluded this feature from the RFC - as I understand it, allowing access to uninintialized constants when initializing other constants is one of the main bad things about C++'s static initializers. I hoped that by not allowing global constructors to initialize statics (forcing use of a mutable static with Option or similar instead), we could avoid that problem.
What's your use case for uninitialized constants in particular?
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when initializing other constants is one of the main bad things about C++'s static initializers.
There is nothing bad about it, it is just you cannot depend on initialization order of statics.
I don't understand why you want to avoid user intentionally using uninitialized on global statics. How it is different from using it on non-global variables? It is not different, when using unsafe user takes responsibility to use unitialized variable properly when relying on global constructor functions.
What’s your use case for uninitialized constants in particular?
Initialize global variable that lacks const fn initializer (ideally I'd like to avoid mut for global statics that need one time initialization)
static X: Option<T> = None; not sufficient? Who cares about mut? This is already unsafe code, and that static definitely can't be inlined.
Option is not necessary for one time initialization, just adding few bytes to the size of executable
Also currently Option<T> would require mut static in order to perform initialization.
Basically we’d want to replace lazy_static kind of initialization
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lazy_static also requires a Option<T>-like structure under the covers, so it’s not adding overhead to use a Option<T> with main-entry initialization rather than first-use. In fact, I’d expect a macro to wrap main-entry initialization in an API like lazy_static’s, to allow it to be used safely (unlike static mut, which is next to impossible to get right).
And if you’re really determined to use an uninitialized state to start with, I’m fairly certain that MaybeUninit::uninitialized doesn’t have any actual barriers to being const other than deciding what that means.
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@mcy Writing to a non-mut static is UB, just like writing to anything that has a non-mut reference pointing at it. The only exception in either case is UnsafeCell.
I’m totally aware that impl Freeze statics wind up on a readonly page; my point was that I think there is somewhat unnecessary fear of static mut in the context of library ctors, which are already very hard to do safely.
I’m not talking about readonly pages, I’m talking about miscompilation due to constant propagation et. al. assuming you never write to that non-mut static.
Did you mean to include a mut in your post?
Yeah, I think we’re talking past each other here. I also may have written that post before my weekend coffee- definitely meant a mut static. I blame getting mixed up from C++ having the opposite convention.
It is const. (And it's called MaybeUninit::uninit but whatever. 
)
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There is another use-case I realized that I have: enumerating test cases. I maintain a library for data-driven tests and currently the way it enumerates tests is by using a nightly-only feature of providing my own test runner (https://github.com/rust-lang/rust/blob/master/src/doc/unstable-book/src/language-features/custom-test-frameworks.md) and using #[test_case] on my tests to let the compiler to enumerate all of them and pass to the function I control (I also use a nasty hack to allow accepting my own descriptor instead of test::TestDescAndFn).
Seems like global constructors could be an alternative to that? So instead of relying on syntactic transformation compiler performs (which is explained here https://blog.jrenner.net/rust/testing/2018/07/19/test-in-2018.html), it would be a global list of "test cases" (or even multiple lists, for different test types, and then test runner should know how to handle lists of different types?).
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I think enumerating test cases (or anything of the form "global list") would be better served by Idea: global static variables extendable at compile-time. This sort of thing does not require global constructors.
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I saw in that thread that you've basically written a library for that purpose. If it can be done in a library (and it's done really well, with no real ugliness at usage time), why should it be a core language feature? Having a declaration which looks like a regular array but is missing some of its elements sounds confusing enough to me to warrant the extra thought imposed by depending on a library (especially considering how solutions to some other important problems, like date/time handling or random number generation, weren't deemed "core" or "stable" enough to make it into even the stdlib, let alone the core language).
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The main reason I would argue for it would be to allow proper support an all platforms, not just those with intrinsics already available to implement the functionality. In particular, I believe only the compiler has the ability to coalesce values together in wasm-unknown-unknown - it's just doesn't support link sections as a target.
If it was possible to implement it on all platforms in a library, then I would agree with you. But if the alternatives end up being a library supported only on Windows, OSX and Linux, and a std implementation with support for many for platforms, then I'd go for the latter.
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Are you talking about https://crates.io/crates/linkme ? It uses similar linker magic to https://crates.io/crates/ctor and https://crates.io/crates/inventory and suffers from the same issues as these two (in my experiments, all three just silently stop working in some cases).
There is like really not much one can do without compiler support here, I don't think so 
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I'm writing a plugin in Rust for a C application that expects plugins to do all initialization via static constructors. While the described alternatives solve most problems in pure-Rust projects, they don't help with this kind of FFI.
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