Stability annotation on defaulted generics fails #79499

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1. TimDiekmann

   added

   C-bugCategory: This is a bug.Category: This is a bug.

   on Nov 28, 2020
2. jonas-schievink

   added

   A-stabilityArea: `#[stable]`, `#[unstable]` etc.Area: `#[stable]`, `#[unstable]` etc.

   T-compilerRelevant to the compiler team, which will review and decide on the PR/issue.Relevant to the compiler team, which will review and decide on the PR/issue.

   A-allocatorsArea: Custom and system allocatorsArea: Custom and system allocators

   on Nov 28, 2020
3. varkor commented on Nov 28, 2020

   

   varkor

   on Nov 28, 2020

   Contributor

   More actions

   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) :)

   👀React with 👀4Reacted by Avi Dessauer, Eric Shimizu Karbstein, Moritz Mahringer and Michal Hornický
4. zachs18 commented on Sep 6, 2022

   

   zachs18

   on Sep 6, 2022

   Contributor

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   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.)
5. SaadiSave commented on Mar 29, 2025

   

   SaadiSave

   on Mar 29, 2025

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   If work here has stalled, I'd like to make progress on this issue

   👍React with 👍1Reacted by Avi Dessauer
6. JonathanBrouwer

   added 3 commits that reference this issue on Mar 2, 2026

   Rollup merge of rust-lang#149328 - shua:stralloc, r=Amanieu

   ...

   744b019

   Rollup merge of rust-lang#149328 - shua:stralloc, r=Amanieu

   ...

   83af3a8

   Rollup merge of rust-lang#149328 - shua:stralloc, r=Amanieu

   ...

   de450e6