Stability annotation on defaulted generics fails

[TimDiekmann]

I'm adding <A: AllocRef = Global> to string types. Strings don't have a generic parameter, so the defaulted generic parameter is the first parameter:

pub struct String<#[unstable(feature = "allocator_api", issue = "32838")] A: AllocRef = Global> {}

However, the compiler complains, that this annotation is "useless":

error: This stability annotation is useless
   --> library/alloc/src/string.rs:280:75
    |
280 | pub struct String<#[unstable(feature = "allocator_api", issue = "32838")] A: AllocRef = Global> {
    |                                                                           ^

Was this simply not added by #77118 or is this a false positive in general?

cc @varkor @Avi-D-coder

[varkor]

The error is occurring here, for reference:

rust/compiler/rustc_passes/src/stability.rs

Lines 156 to 161 in 1d27267

	// Error if prohibited, or can't inherit anything from a container.
	if kind == AnnotationKind::Prohibited
	|| (kind == AnnotationKind::Container && stab.level.is_stable() && is_deprecated)
	{
	self.tcx.sess.span_err(item_sp, "This stability annotation is useless");
	}

Off the top of my head, I'm not sure which part of the condition is satisfied. There's no reason I see to expect this not to work, so it could well have been a case overlooked in #77118.

@Avi-D-coder: would you be interested in taking a look to see what's going wrong? I should be able to give more immediate response than last time (it'd just take me a little while to find time to debug, myself) :)

[zachs18]

If I remove derive(PartialOrd, Eq, Ord) from struct String (i.e. manually implement them, like PartialEq already was), then I don't get the This stability annotation is useless error anymore, so I think the error might be coming from a derive macro instead of the struct declaration itself.

---

Specifically, for this code (outside of stdlib, because IDK how to cargo expand inside stdlib)

#![feature(allocator_api)]
#![feature(std_internals)]

use std::alloc::{Allocator, Global};

#[derive(PartialOrd, Eq, Ord)]
pub struct String<#[unstable(feature = "allocator_api", issue = "32838")] A: Allocator = Global> {
    vec: Vec<u8, A>,
}
impl<A: Allocator, B: Allocator> PartialEq<String<B>> for String<A> {
    fn eq(&self, other: &String<B>) -> bool { todo!() }
}

cargo +nightly expand gives

// error "stability attributes may not be used outside of the standard library" ignored
// .. etc
impl<#[unstable(feature = "allocator_api", issue = "32838")] A: Allocator>
    ::core::marker::StructuralEq for String<A>
{
}
// .. etc (None of the other impls have the stability annotation)

and I think this impl is what's causing the error; it just looks like its the struct definition because the Eq derive macro copies the span from the struct definition.

---

If this is correct, maybe this issue should be renamed to something like "Eq derive macro copies stability annotation on generics into impl StructuralEq, causing compilation error"?

(Either way, if String (/CString/etc) is made allocator-aware, it would be better to manually implement (Partial){Eq,Ord} anyway, so as not to have an <A: Allocator + (Partial){Eq,Ord}> bound.)