Re-enable rustc overflow checks

[davidsemakula]

Summary

Closes #2115

• [n] y/n | Does it introduce breaking changes?
• [n] y/n | Is it dependent on the specific version of ink or pallet-contracts?

Description

See #2115 for a detailed description of historical issues and limitations of the current solution.

There were 2 primary root causes for the historical issues:

1. Internal issues in rustc related to how library functions from #![no_builtins] annotated crates (most notably compiler intrinsics from the compiler-builtins crate) interacted with LTO (Link Time Optimization) (i.e. LTO would sometimes errorneously optimize out these functions or replace them with recursive calls)
2. No exposure/forwarding of non-default/optional features from the compiler-builtins crate (most notably the mem feature for generating memory-related intrinsics like memcpy) in standard library crates (i.e. alloc, core, std e.t.c), meaning these features couldn't be explicitly enabled with the -Z build-std-features flag when compiling standard library crates using -Z build-std flag.

(1) was generally the more complicated issue, and was ultimately solved by

• Add the no-builtins attribute to functions when no_builtins is applied at the crate level. rust-lang/rust#113716
• Restore #![no_builtins] crates participation in LTO. rust-lang/rust#113923

With the latter explicitly mentioning previously reported issues from ink! as resolved i.e:

• Non existing function imported when compiling for wasm rust-lang/rust#78744
• Compiler builtin calls imported function when compiling with -Z build-std for wasm rust-lang/rust#91158

(2) was resolved by

• library: Forward compiler-builtins "mem" feature rust-lang/rust#77284

Accordingly, with this PR, we now explicitly enable the mem feature of the compiler-builtins crate ( via the compiler-builtins-mem standard library feature).

NOTE: This is functionally equivalent to the previous "work around" for WebAssembly i.e.

• Explicitly link against rlibc to get non buggy version of memcpy ink#1049

As explained by the deprecation notice in rlibc readme

DEPRECATED: This crate used to provide a Rust implementation of some libc functions such as memcpy / memmove / memset, required when developing freestanding applications.
The compiler-builtins crate now provides optimized versions of these functions when enabling the optional mem feature

Lastly, we prefer to enable the mem feature explicitly via the -Zbuild-std-features flag, even if the compiler-builtins crate already attempts to automatically enable the mem feature for certain targets (e.g. targets with a -none suffix) via it's build script, see:

• Add more targets to automatically select mem feature. rust-lang/compiler-builtins#357
• https://github.com/rust-lang/compiler-builtins/blob/ac3a4cd846be6f3c3a587c7ab79a95084ef75868/compiler-builtins/build.rs#L36-L46

Follow ups/ related TODOs

• Add overflowing tests to ink/integration-tests (chore: Add integration test for arithmetic overflow checks ink#2631)
• Update ink! docs (Update overflow safety FAQ ink-docs#487)

Checklist before requesting a review

• My code follows the style guidelines of this project
• I have added an entry to CHANGELOG.md
• I have commented my code, particularly in hard-to-understand areas
• I have added tests that prove my fix is effective or that my feature works
• Any dependent changes have been merged and published in downstream modules

[cmichi]

LGTM! Thank you for researching it!

Could you check if ink-docs requires updating? I have some distant memory about at least an FAQ point.

[davidsemakula]

Could you check if ink-docs requires updating? I have some distant memory about at least an FAQ point.

Yeah, there's an FAQ point in ink! docs that mentions me, will update it.