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Date: Wed, 19 Aug 2026 13:23:49 +0200
Message-Id: <DKSVRS7UAL9U.3RUTW74RVSEHQ@kernel.org>
Subject: Re: [PATCH 1/6] rust: alloc: add Vec::push_init
Cc: "Lorenzo Stoakes" <ljs@kernel.org>, "Vlastimil Babka"
 <vbabka@kernel.org>, "Liam R. Howlett" <liam@infradead.org>, "Uladzislau
 Rezki" <urezki@gmail.com>, "Miguel Ojeda" <ojeda@kernel.org>, "Boqun Feng"
 <boqun@kernel.org>, "Gary Guo" <gary@garyguo.net>,
 =?utf-8?q?Bj=C3=B6rn_Roy_Baron?= <bjorn3_gh@protonmail.com>, "Benno Lossin"
 <lossin@kernel.org>, "Andreas Hindborg" <a.hindborg@kernel.org>, "Alice
 Ryhl" <aliceryhl@google.com>, "Trevor Gross" <tmgross@umich.edu>, "Daniel
 Almeida" <daniel.almeida@collabora.com>, "Tamir Duberstein"
 <tamird@kernel.org>, "Alexandre Courbot" <acourbot@nvidia.com>,
 =?utf-8?q?Onur_=C3=96zkan?= <work@onurozkan.dev>, "David Airlie"
 <airlied@gmail.com>, "Simona Vetter" <simona@ffwll.ch>, "John Hubbard"
 <jhubbard@nvidia.com>, "Alistair Popple" <apopple@nvidia.com>, "Timur Tabi"
 <ttabi@nvidia.com>, <rust-for-linux@vger.kernel.org>,
 <linux-kernel@vger.kernel.org>, <nova-gpu@lists.linux.dev>,
 <dri-devel@lists.freedesktop.org>
To: "Eliot Courtney" <ecourtney@nvidia.com>
From: "Danilo Krummrich" <dakr@kernel.org>
References: <20260817-b4-nvkv-v1-0-b84db5e84b67@nvidia.com>
 <20260817-b4-nvkv-v1-1-b84db5e84b67@nvidia.com>
In-Reply-To: <20260817-b4-nvkv-v1-1-b84db5e84b67@nvidia.com>

On Mon Aug 17, 2026 at 2:56 PM CEST, Eliot Courtney wrote:
> +    pub fn push_init<E>(&mut self, init: impl Init<T, E>, flags: Flags) =
-> Result<(), E>
> +    where
> +        E: From<AllocError>,

This signature rejects impl Init<T, Infallible>, which is the reason why we=
 have
e.g. Box::init() and Box::try_init() with different fallible signatures.

So, if we follow InPlaceInit, it'd be

	pub fn push_init<E>(&mut self, init: impl Init<T, E>, flags: Flags) -> Res=
ult<(), Error>
	where
	    Error: From<E>;

and

	pub fn try_push_init<E>(&mut self, init: impl Init<T, E>, flags: Flags) ->=
 Result<(), E>
	where
	    E: From<AllocError>;

In theory we could also simplify it to

	pub fn push_init(&mut self, init: impl Init<T>, flags: Flags) -> Result<()=
, AllocError>

and

	pub fn try_push_init<E>(&mut self, init: impl Init<T, E>, flags: Flags) ->=
 Result<(), E>
	where
	    E: From<AllocError>,

However, InPlaceInit actually achieves more with the init() and try_init()
distinction:

  What init() accepts, but try_init() does not accept:
    - impl Init<T, Infallible>
    - impl Init<T, E> where Error: From<E> but NOT E: From<AllocError>

  What try_init() accepts, but init() does not accept:
    - impl Init<T, E> where E: From<AllocError> but NOT Error: From<E>

So, with the simplification we'd technically lose out on the

	impl Init<T, E> where Error: From<E> but NOT E: From<AllocError>

case.

In any case, init() and try_init() seem a bit mixed up on their purpose
regarding fallibility and error type strategy, but in order to really cover=
 all
cases I think it is necessary.