Effect of cargo profiles on code in std

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How do profile settings effect behavior of code compiled in the standard library?

For example the overflow-checks profile option?

I was debugging something today which I incorrectly thought was an overflow from my code calling Duration::add. It turns out this wasn't the bug, but it got me thinking... How do profile settings effect overflow behavior of code defined in the standard library?

My understanding before was: if I compiled with overflow-checks = true all integer overflows (even in std) would cause a panic.

However, I also thought the standard library was pre-built by rustup, so my own compilation flags shouldn't have any effect on std.

By inspecting the source I realized there's no way Duration::add could overflow as it uses checked_add with expect internally.

Anyone know of a function in the standard library which has different behavior depending on overflow compilation flags?

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Not all code is compiled to native machine code. Generics and inlineable functions are compiled as MIR.

Overflow checks are not a regular feature flag. They are compiled to special intrinsic, which the compiler is able to change later when the code is imported from libstd.

debug_assert! in libstd also got special instructions recently, which can be turned on or off after compilation.

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