Deserialization in serde panics in rustc 1.48+ with some enum inner types

[nicodemus26]

This error presents as: attempted to leave type `nalgebra::ArrayStorage<...>` uninitialized, which is invalid

This line of code panics:

nalgebra/src/base/array_storage.rs

Line 380 in c0f4ee6

let mut out: Self::Value = unsafe { mem::uninitialized() };

This panic is mentioned in the 1.48 detailed notes as

mem::uninitialized will now panic if any inner types inside a struct or enum disallow zero-initialization.

The tracking issue for this is here: rust-lang/rust#66151

The way visit_seq is implemented looks like it should correctly initialize the whole matrix or drop the partially initialized matrix and return an Err. Can this function use a compiler version switch to use MaybeUninit::uninit().assume_init() mentioned in the std::mem::uninitialized deprecation notes?

[nicodemus26]

This minimal example triggers the failure:

src/main.rs

extern crate nalgebra;
extern crate serde;
extern crate serde_json;

use nalgebra::Vector2;
use serde_derive::{Serialize, Deserialize};
use serde_json::from_str;

#[derive(Serialize, Deserialize, Debug, PartialEq, Copy, Clone)]
#[serde(tag = "letter", content = "value")]
enum Stuff {
    A(f64),
    B(f64),
}

fn main() {
    let json = r#"[{"letter":"A", "value":123.4}, {"letter":"B", "value":567.8}]"#;
    
    // Boom:
    // thread 'main' panicked at 'attempted to leave type `nalgebra::ArrayStorage<Stuff, nalgebra::U2, nalgebra::U1>` uninitialized, which is invalid', /home/nicodemus/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/mem/mod.rs:658:9
    let parsed: Result<Vector2<Stuff>, _> = from_str(json);
    
    println!("parsed: {:?}", parsed);
}

Cargo.toml

[package]
name = "nalgebra_issue_812"
version = "0.1.0"
authors = ["Nicodemus Paradiso <nicodemus26@gmail.com>"]
edition = "2018"

[dependencies]
serde = "1.0.118"
serde_json = "1.0.60"
serde_derive = "1.0.118"

[dependencies.nalgebra]
version = "0.23.1"
features = [
    "std", "serde-serialize"
]

[nicodemus26]

With 1.47:

$ rustup override set 1.47                                                                  
info: using existing install for '1.47-x86_64-unknown-linux-gnu'
info: override toolchain set to '1.47-x86_64-unknown-linux-gnu'

  1.47-x86_64-unknown-linux-gnu unchanged - rustc 1.47.0 (18bf6b4f0 2020-10-07)

$ cargo run
...
   Compiling nalgebra_issue_812 v0.1.0
    Finished dev [unoptimized + debuginfo] target(s) in 39.54s
     Running `target/debug/nalgebra_issue_812`
parsed: Ok(Matrix { data: [A(123.4), B(567.8)] })

[sebcrozet]

Thank you for noticing this. I will release the version 0.23.2 with a fix.