Make creusot-contracts buildable with stable rust

[Lysxia]

A problem that @arnaudgolfouse just mentioned to me is that we can't yet build creusot-contracts with a stable rust toolchain.

That seems desirable to make it easy to get started with Creusot on an existing stable Rust project, or even to get started with Creusot at all, since the alternative is to be stuck with nightlies or for users to mess with compile-time configuration to hide creusot-contracts from stable builds.

[jhjourdan]

Could you please list what is blocking here?

[xldenis]

Most likely the proc_macro_hygiene and stmt_expr_attributes features?

[Lysxia]

Compilation fails first because #![feature] itself is not available on stable compilers:

creusot/creusot-contracts/src/lib.rs

Lines 34 to 46 in 9645e8a

	#![feature(
	step_trait,
	allocator_api,
	unboxed_closures,
	tuple_trait,
	strict_provenance,
	panic_internals,
	libstd_sys_internals,
	rt,
	never_type,
	ptr_metadata
	)]
	#![cfg_attr(not(creusot), feature(rustc_attrs))]

If I comment those out to see what is needed on a first pass, most error messages (45 out of 68) come from allocator_api, because we use the unstable Allocator trait:

error[E0658]: use of unstable library feature 'allocator_api'
   --> /home/sam/rust/creusot/creusot-contracts/src/std/vec.rs:215:52
    |
215 | impl<T, A: Allocator> IntoIterator for &mut Vec<T, A> {
    |                                                 ---^
    |                                                 |
    |                                                 help: consider wrapping the inner types in tuple: `(T, A)`
    |
    = note: see issue #32838 <https://github.com/rust-lang/rust/issues/32838> for more information

Interestingly, I don't see any error related to proc_macro_hygiene and stmt_expr_attributes because there are no loop invariants in creusot-contracts.

Of the remaining 23 error messages...

• 7 about tuple_trait:

  error[E0658]: use of unstable library feature 'tuple_trait'
    --> /home/sam/rust/creusot/creusot-contracts/src/std/ops.rs:11:27
     |
  11 | pub trait FnOnceExt<Args: Tuple> {
     |                           ^^^^^
  

• 5 about step_trait (but 2 exist only on my branch with DoubleEndedIterator):

  error[E0658]: use of unstable library feature 'step_trait'
   --> /home/sam/rust/creusot/creusot-contracts/src/std/iter/range.rs:9:42
    |
  9 | impl<Idx: DeepModel<DeepModelTy = Int> + Step> Iterator for Range<Idx> {
    |                                          ^^^^
    |
  

• 5 about "compiler internals" (maybe feature rt or rustc_attrs?)

  error[E0658]: diagnostic items compiler internal support for linting
    --> /home/sam/rust/creusot/creusot-contracts/src/ghost.rs:66:5
     |
  66 |     #[rustc_diagnostic_item = "ghost_box_deref"]
     |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  

• 4 about unboxed_closures

  error[E0658]: the precise format of `Fn`-family traits' type parameters is subject to change
    --> /home/sam/rust/creusot/creusot-contracts/src/std/ops.rs:90:22
     |
  90 | impl<Args: Tuple, F: FnMut<Args>> FnMutExt<Args> for F {
     |                      ^^^^^^^^^^^ help: use parenthetical notation instead: `FnMut(Args) -> ()`
     |
  

• 1 about panic_internals

  error[E0658]: use of unstable library feature 'panic_internals': internal details of the implementation of the `panic!` and related macros
   --> /home/sam/rust/creusot/creusot-contracts/src/std/panicking.rs:3:9
    |
  3 | pub use ::core::panicking::*;
    |         ^^^^^^^^^^^^^^^^^
  

• 1 about never_type

  error[E0658]: the `!` type is experimental
   --> /home/sam/rust/creusot/creusot-contracts/src/invariant.rs:9:20
    |
  9 | impl Invariant for ! {
    |                    ^
  

That doesn't seem too tricky to resolve.

[xldenis]

Oh yea this all seems pretty easy to handle then.