Disallow building with overflow-checks off

[leighmcculloch]

What

• Add a build-script probe in build.rs that detects whether overflow-checks are enabled by attempting an overflowing u8 addition and catching whether it panics
• Set a custom cfg flag (soroban_sdk_internal_overflow_checks_enabled) when checks are on
• Add a compile_error! in lib.rs that fires when the flag is absent, with a link to the docs

Why

Cargo's default release profile disables overflow-checks. Without this guard developers can silently build contracts with wrapping arithmetic, leading to unexpected behavior. The tooling for Soroban contracts that generates boilerplate for new contracts sets overflow-checks = true for the release profile, and docs encourage the same pattern, but this makes it a compile-time error so that anyone doesn't accidentally turn that off.

This is a workaround until cfg(overflow_checks) is stabilized (rust-lang/rust#111466).

Known limitations

It may not detect overflow-checks being disabled in cross-compilation setups where the host and target profiles diverge or where the build-override profile is explicitly set to overflow-checks = true when the main profile is set to false. These are advanced configurations, and the check is primarily present to prevent developers using the default release profile that has checks off.

[Copilot]

Pull request overview

This PR introduces a compile-time guard to prevent building Soroban contracts when integer overflow checks are disabled, using a build-time probe and an internal cfg flag.

Changes:

• Adds a build-script probe in soroban-sdk/build.rs that detects whether overflow-checks are enabled by attempting an overflowing u8 addition and setting cfg(soroban_sdk_internal_overflow_checks_enabled) when a panic is observed.
• Introduces a compile_error! in soroban-sdk/src/lib.rs that triggers when the internal overflow-checks cfg flag is absent, with a pointer to the documentation on correctly configuring the release profile.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
soroban-sdk/build.rs	Adds a runtime probe and corresponding rustc-cfg/rustc-check-cfg output to encode overflow-checks status into a cfg flag at compile time.
soroban-sdk/src/lib.rs	Adds a compile_error! guarded by the new cfg flag to hard-fail builds when overflow checks are disabled, with a link to the recommended release profile configuration.