Fix `EcoVec`'s `Send` and `Sync` bounds

[CosmicHorrorDev]

Last PR I can do for the day (got a lot of IRL stuff today), but it's another soundness fix

This fixes EcoVec's Send and Sync bounds to match the standard library's Arc

Arc-like constructs need both Send and Sync on the inner type to allow for Send or Sync. Without this it's trivial to violate the inner type's bound

E.g. a type can be Send + !Sync, but that would allow you to create two Arcs on the same thread and then pass one of the Arcs to another thread. From there you have a reference to the type from multiple threads even though it's !Sync. A simple example program that makes miri angry

use std::cell::Cell;

use ecow::eco_vec;

fn main() {
    let send_not_sync = Cell::new("foo");
    let one = eco_vec![send_not_sync];
    let another = one.clone();

    std::thread::spawn(move || loop {
        another[0].set("bar");
        another[0].set("foo");
    });

    // 😱
    loop {
        println!("{}", one[0].get());
    }
}

$ cargo +nightly miri run
// -- 8< -- build output -- 8< --
error: Undefined Behavior: Data race detected between (1) Read on thread `main` and (2) Write on thread `<unnamed>` at alloc1739+0x18. (2) just happened here
   --> /.../rustlib/src/rust/library/core/src/cell.rs:413:31
    |
413 |         mem::replace(unsafe { &mut *self.value.get() }, val)
    |                               ^^^^^^^^^^^^^^^^^^^^^^ Data race detected between (1) Read on thread `main` and (2) Write on thread `<unnamed>` at alloc1739+0x18. (2) just happened here
    |
help: and (1) occurred earlier here
   --> src/main.rs:17:24
    |
17  |         println!("{}", one[0].get());
    |                        ^^^^^^^^^^^^
    = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
    = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
    = note: BACKTRACE (of the first span):
    = note: inside `std::cell::Cell::<&str>::replace` at /.../rustlib/src/rust/library/core/src/cell.rs:413:31: 413:53
    = note: inside `std::cell::Cell::<&str>::set` at /.../rustlib/src/rust/library/core/src/cell.rs:363:19: 363:36
note: inside closure
   --> src/main.rs:11:9
    |
11  |         another[0].set("bar");
    |         ^^^^^^^^^^^^^^^^^^^^^

note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace

error: aborting due to previous error

---

For even more fun you can change one of the strings that get swapped to something really big and then you can get fun runtime issues.

$ # --release optimizes stuff away
$ # --show-all prevents arbitrary bytes from messing with my terminal session
$ cargo run | bat --show-all
// -- 8< -- searching for "home" -- 8< --
9962   │ foo#/rustc/fc594f15669680fa70d255faec3ca3fb507c3405/library/std/src/io/mod.rsfailed·to·write·whole·bufferfor
       │ matter·errorfatal·runtime·error:·␊
9963   │ thread·result·panicked·on·dropassertion·failed:·self.is_allocated()/.../ecow/s
       │ rc/vec.rsassertion·failed:·self.is_unique()assertion·failed:·target·>·
self.capacity()␀␀␀␀␀␀␀␀␀attempt·to·add
       │ ·with·overflow␀␀␀␀attempt·to·subtract·with·overflow␀␀␀␀␀%                                                   26;46m␀␀␀␀␀␀␀␀␀␀attempt·to·multiply·with·overflowreference
       │ ·count·overflow␀␀␀␀␀␀␀,O\xFC\xFFhO\xFC\xFFjO\xFC\xFFlO\xFC\xFFnO\xFC\xFFthere·is·no·such·thing·as·a·relaxed·
       │ fence␀␀␀/rustc/fc594f15669680fa70d255faec3ca3fb507c3405/library/core/src/sync/atomic.rs␀>P\xFC\xFFSP\xFC\xFF
       │ hP\xFC\xFF}P\xFC\xFF\x92P\xFC\xFFfailed·to·spawn·thread/rustc/fc594f15669680fa70d255faec3ca3fb507c3405/libra
       │ ry/std/src/thread/mod.rsthread·name·may·not·contain·interior·null·bytes␀␀\xA2e\xFC\xFF\xDAe\xFC\xFFie\xFC\xF
       │ F\x89e\xFC\xFF\xE2g\xFC\xFF\u{1a}h\xFC\xFF\xA9g\xFC\xFF\xC9g\xFC\xFFinternal·error:·entered·unreachable·code
       │ /rustc/fc594f15669680fa70d255faec3ca3fb507c3405/library/std/src/io/error/repr_bitpacked.rs␀␀\xFEx\xFC\xFF\u{
       │ 12}y\xFC\xFF·y\xFC\xFF3y\xFC\xFF/.../Pro␊

(Being totally honest I'm just having fun seeing what wacky stuff can happen with Rust's UB. Sometimes I miss how cursed C programming can be)

[laurmaedje]

Thanks for the fix and thanks for spotting this! I copied this from Arc, but was apparently a bit too trigger-happy with my multi-cursor delete of the ?Sized bounds and stability attributes. But these kinds of things are exactly why I posted on reddit and waited with publishing to crates.io.

[laurmaedje]

Also, miri is absolutely awesome!

[CosmicHorrorDev]

Also, miri is absolutely awesome!

Can't be more true!