allow constants to refer to mutable/external memory, but reject such constants as patterns

[RalfJung]

Reference update for rust-lang/rust#140942.

[ehuss]

I'm trying to figure out, which reference rule rejects the following?

static mut S: i32 = 1;
const C: &mut i32 = unsafe { &mut S }; // ERROR: constructing invalid value

What I'm getting at, is that it seems like despite removing items.const.final-value-immutable, it seems like there is still some restriction somewhere. What I'm thinking:

• final value of &mut — never ok?
• final value of & of a static mut — now ok
• final value of & of something with interior mutability — sometimes ok?

Is there maybe still some "final value must not have mutable references" restriction?

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Is there some rule in the reference that would explain the following?

const C: &AtomicU32 = &AtomicU32::new(1);  // ERROR: constants cannot refer to interior mutable data

It seems like a pretty strong blanket statement that "constants cannot refer to interior mutable data", but the AtomicU32 example from rust-lang/rust#140942 seems like you can refer to interior mutable data. What exactly is the difference between a reference to a static versus a reference to a value created in the initializer?

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Similarly, I'm trying to understand E0764. Is the description of E0764 out of date?

const C: &mut i32 = &mut 1; // ERROR: mutable references are not allowed in the final value of constants

Questions:

• Is there a rule in the reference that explains this error?
• "Mutable references (&mut) can only be used in constant functions, not statics or constants." seems to be wrong now, as you can use mutable references in statics or constants (with some limitations), right?
• "Remember: you cannot use a function call inside a constant or static"... I...don't understand this at all. The example below it clearly shows using a function call inside a constant. I can't even guess what this is trying to say.