False positive in ptr_arg when function pushes to &mut Vec

[jorgecarleitao]

Summary

When a function has the signature &mut Vec<T> and pushes to it, it cannot be converted to &mut [T]. However, in some cases, clippy recommends doing so.

Lint Name

ptr_arg

Reproducer

It is not easy to trigger this, so I will use the example I found it:

git clone [email protected]:jorgecarleitao/arrow2.git
cargo clippy --features full

I saw this happen:

warning: writing `&mut Vec` instead of `&mut [_]` involves a new object where a slice will do
   --> src/io/ipc/write/serialize.rs:209:12
    |
209 |     nodes: &mut Vec<ipc::FieldNode>,
    |            ^^^^^^^^^^^^^^^^^^^^^^^^ help: change this to: `&mut [ipc::FieldNode]`
    |
    = note: `#[warn(clippy::ptr_arg)]` on by default
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#ptr_arg

warning: writing `&mut Vec` instead of `&mut [_]` involves a new object where a slice will do
   --> src/io/ipc/write/serialize.rs:298:12
    |
298 |     nodes: &mut Vec<ipc::FieldNode>,
    |            ^^^^^^^^^^^^^^^^^^^^^^^^ help: change this to: `&mut [ipc::FieldNode]`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#ptr_arg

warning: writing `&mut Vec` instead of `&mut [_]` involves a new object where a slice will do
   --> src/io/ipc/write/serialize.rs:341:12
    |
341 |     nodes: &mut Vec<ipc::FieldNode>,
    |            ^^^^^^^^^^^^^^^^^^^^^^^^ help: change this to: `&mut [ipc::FieldNode]`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#ptr_arg

warning: writing `&mut Vec` instead of `&mut [_]` involves a new object where a slice will do
   --> src/io/ipc/write/serialize.rs:642:14
    |
642 |     buffers: &mut Vec<ipc::Buffer>,
    |              ^^^^^^^^^^^^^^^^^^^^^ help: change this to: `&mut [ipc::Buffer]`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#ptr_arg

warning: `arrow2` (lib) generated 4 warnings

However, If any of them are changed as suggested, the code does not compile.

The image below quickly illustrates the issue: buffer.push is not implemented for &mut [ipc::Buffer], but clippy recommends replacing it:

Version

rustc 1.61.0-nightly (45e2c2881 2022-02-20)
binary: rustc
commit-hash: 45e2c2881d11324d610815bfff097e25c412199e
commit-date: 2022-02-20
host: x86_64-unknown-linux-gnu
release: 1.61.0-nightly
LLVM version: 14.0.0

Additional Labels

No response

[jorgecarleitao]

@Jarcho, has this been fixed and released? This is now showing in rust stable 1.60.

[Jarcho]

I think it just missed the beta branch in February, so it should be in 1.61.