Such source-level warnings do exist in various forms in various languages, with various levels of fixed analysis done for determining them.

Tying such in with optimizations largely just does not work, given that functions with an unused return value exist, being dead code after inlining, and compilers can emit dead code themselves (e.g. duplicating a piece of code, and then DCEing unused things in one copy; or dead branches of inlined functions); never mind the complete unpredictability of various compiler heuristics now being able to change warning behavior (gcc has some of this type of optimization-dependent warnings, and it annoys the hell out of me)

Copying the compiler's work into your code falls apart the moment you target multiple architectures, as different architectures can often benefit from quite-different implementations.

And there's the whole thing that most compiler optimization stages often do not translate well or at all to the source language (e.g. LLVMs poison semantics do not exist in C, nor any language afaik; goto spam!; and there are optimizations that can be applied to safe code that cannot be translated back to safe code without entirely undoing the optimization (e.g. replacing known-unused variables or array elements with undefined ones))