Bump MSRV to 1.59

[claui]

In #309, usage of the asm feature was promoted because it is now part of the stable Rust channel.

This, however, causes build failures when using any Rust version between 1.49 (the documented minimum version) up to 1.58, because the asm feature has only become stable in Rust 1.59.

This PR:

• bumps the documented minimum supported Rust version (MSRV) to 1.59; and

• adds a rust_version entry to Cargo.toml in order to codify the MSRV for cargo to pick it up.
  This leads to a more helpful and more actionable error message, for example:

  error: package parking_lot v0.12.1 ([…]) cannot be built because it requires rustc 1.59 or newer, while the currently active rustc version is 1.58.1

[bjorn3]

In #309, usage of the asm feature was promoted because it is now part of the stable Rust channel.

Even with that PR it is still behind the nightly feature, right?

[claui]

In #309, usage of the asm feature was promoted because it is now part of the stable Rust channel.

Even with that PR it is still behind the nightly feature, right?

It looks like it’s behind the hardware-lock-elision feature.

This command line works with stable Rust 1.58.1:

cargo build --features arc_lock,owning_ref,nightly,serde,send_guard

This one works, too:

cargo build --features arc_lock,owning_ref,nightly,deadlock_detection,serde

But this one fails:

cargo build --features hardware-lock-elision

See error message

   Compiling parking_lot_core v0.9.3 (/home/claudia/Documents/dev/parking_lot/core)
   Compiling lock_api v0.4.7 (/home/claudia/Documents/dev/parking_lot/lock_api)
   Compiling parking_lot v0.12.1 (/home/claudia/Documents/dev/parking_lot)
error[E0658]: use of unstable library feature 'asm': inline assembly is not stable enough for use and is subject to change
  --> src/elision.rs:72:13
   |
72 |             asm!(
   |             ^^^
   |
   = note: see issue #72016 <https://github.com/rust-lang/rust/issues/72016> for more information

error[E0658]: use of unstable library feature 'asm': inline assembly is not stable enough for use and is subject to change
   --> src/elision.rs:102:13
    |
102 |             asm!(
    |             ^^^
    |
    = note: see issue #72016 <https://github.com/rust-lang/rust/issues/72016> for more information

warning: the item `asm` is imported redundantly
  --> src/elision.rs:60:17
   |
9  | use std::arch::asm;
   |     -------------- the item `asm` is already imported here
...
60 |             use core::arch::asm;
   |                 ^^^^^^^^^^^^^^^
   |
   = note: `#[warn(unused_imports)]` on by default

warning: the item `asm` is imported redundantly
  --> src/elision.rs:91:17
   |
9  | use std::arch::asm;
   |     -------------- the item `asm` is already imported here
...
91 |             use core::arch::asm;
   |                 ^^^^^^^^^^^^^^^

error[E0658]: use of unstable library feature 'asm': inline assembly is not stable enough for use and is subject to change
 --> src/elision.rs:9:5
  |
9 | use std::arch::asm;
  |     ^^^^^^^^^^^^^^
  |
  = note: see issue #72016 <https://github.com/rust-lang/rust/issues/72016> for more information

error[E0658]: use of unstable library feature 'asm': inline assembly is not stable enough for use and is subject to change
  --> src/elision.rs:60:17
   |
60 |             use core::arch::asm;
   |                 ^^^^^^^^^^^^^^^
   |
   = note: see issue #72016 <https://github.com/rust-lang/rust/issues/72016> for more information

error[E0658]: use of unstable library feature 'asm': inline assembly is not stable enough for use and is subject to change
  --> src/elision.rs:91:17
   |
91 |             use core::arch::asm;
   |                 ^^^^^^^^^^^^^^^
   |
   = note: see issue #72016 <https://github.com/rust-lang/rust/issues/72016> for more information

For more information about this error, try `rustc --explain E0658`.
warning: `parking_lot` (lib) generated 2 warnings
error: could not compile `parking_lot` due to 5 previous errors; 2 warnings emitted

[bjorn3]

That changed since that PR then. In any case we could say that the MSRV for the hardware-lock-elision feature is 1.59, but keep 1.49 if it is disabled, right?

[claui]

In any case we could say that the MSRV for the hardware-lock-elision feature is 1.59, but keep 1.49 if it is disabled, right?

We could. However, I don’t know a way to encode that in Cargo.toml (in order to get a useful error message when trying to build with Rust < 1.59).

Opened #348 for the alternative.

[Amanieu]

The hardware-lock-elision feature is disabled by default, so you'd have to actually opt-in to use it. I don't think this justifies bumping the MSRV of the entire crate though.

[claui]

@Amanieu I totally agree. (I consider it a major UX issue in cargo that it – surprisingly – propagates the --all-features switch to the entire dependency tree, not just the features of the top-level crate that I meant to build. That’s how I ended up “opting in” to parking_lot’s hardware-lock-elision in the first place, without knowing that the crate even exists.)

Nevertheless, your point still stands. Closing.