Consider setting a lower MSRV

[jyn514]

Motivation: I want to use saltwater in bindgen (rust-lang/rust-bindgen#1782), but it has an MSRV of 1.34, which target-lexicon doesn't compile with. It uses the following features not in 1.34:

• #[nonexhaustive]: this can be replaced with a #[doc(hidden)] __Nonexhaustive variant and documentation that the enum is nonexhaustive

error[E0658]: non exhaustive is an experimental feature (see issue #44109)
  --> src/data_model.rs:39:1
   |
39 | #[non_exhaustive]
   | ^^^^^^^^^^^^^^^^^

• Self::variant: this can be replaced with the explicit type name

error: enum variants on type aliases are experimental
  --> src/data_model.rs:15:13
   |
15 |             Self::U8 => 8,
   |             ^^^^^^^^

• extern crate alloc in build.rs: this can be replaced by use std as alloc; as long as the std feature is enabled. I don't need 1.34 support with no_std.

error[E0658]: use of unstable library feature 'alloc': this library is unlikely to be stabilized in its current form or name (see issue #27783)
  --> build.rs:12:1
   |
12 | extern crate alloc;
   | ^^^^^^^^^^^^^^^^^^^

I understand that most bytecodealliance projects aren't considered stable and therefore have no MSRV, but this would help me a lot, I'm willing to make the PRs myself.

[sunfishcode]

The Self and alloc issues seem reasonable to fix. Removing #[nonexhaustive] would be inconvenient; we added it because every time we bump our major version, we cause some of our downstream users to have to update in lock-step with other downstream users. While the proposed alternative here would also work, changing to it would be yet another major-version bump. Does rust-lang/rust-bindgen#1782 (comment) mean that perhaps the minimum Rust version could be bumped instead? nonexhaustive would only need Rust 1.40.

[jyn514]

Hmm this would be unfortunate for me personally since the latest version of rust we have at work is 1.34 and I'd like to eventually use saltwater via bindgen on a work project. If this is hard to do for target-lexicon I might add some more hacks to make sizeof() opt-out for saltwater.

[sunfishcode]

Would it work to change #[nonexhaustive] to #[cfg_attr(feature = rust_1_40, non_exhaustive)], and add rust_1_40 as a default feature in Cargo.toml? And maybe even add a #[cfg(not(feature = rust_1_40))] #[doc(hidden)] __Nonexhaustive too.

[jyn514]

Sure, that works for me. Good idea!