Allow unreachable_code in wasm error helper

[leighmcculloch]

What

Add #[allow(unreachable_code)] attribute to the error_from_error_val function for wasm targets to suppress a compiler warning introduced in Rust 1.92.0.

Why

I started seeing the following errors when building contracts using the soroban-sdk:

warning: unreachable expression
    --> soroban-sdk/src/env.rs:1770:6
     |
1769 |         self.env_impl.error_from_error_val(e)
     |         ------------------------------------- any code following this expression is unreachable
1770 |     }
     |      ^ unreachable expression
     |
note: this expression has type `Infallible`, which is uninhabited
    --> soroban-sdk/src/env.rs:1769:9
     |
1769 |         self.env_impl.error_from_error_val(e)
     |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
     = note: `#[warn(unreachable_code)]` (part of `#[warn(unused)]`) on by default

It appears Rust 1.92.0 changed how unreachable_code warnings are detected.

The error_from_error_val function returns Infallible, an uninhabited type. This is intentional behaviour. The function is designed to never return so the warning is suppressed.

I think this is probably the result of rust-lang/rust#142390 which landed in 1.92.0, but that change is quite deep and so I'm not certain of that.

In any case, I don't think what the code is doing here is incorrect, just a new warning is highlighting that there's an unreachable condition here, which is expected, so we should allow the condition.

[Copilot]

Pull request overview

This PR adds a #[allow(unreachable_code)] attribute to suppress a new compiler warning introduced in Rust 1.92.0 for the error_from_error_val function when targeting wasm.

Changes:

• Adds #[allow(unreachable_code)] attribute to the wasm-targeted error_from_error_val function to suppress a warning about unreachable closing brace

[leighmcculloch]

The docs ci failure is unrelated to this change and a compiler bug:

• [ICE]: rustdoc panics with "no resolutions for a doc link" when resolving intra-doc link in #[deprecated] note via glob re-export rust-lang/rust#151411

[graydon]

I concur with your assessment (though I also concur with "this is a little murky and I could be wrong")