The pinning APIs of Rust language guarantee memory location stability for self-referential and asynchronous constructs, as long as used according to the pinning API contract. Rust ensures violations of such contract are impossible in regular safe code, but not in unsafe code where unsafe pinning APIs can be used. Library authors can encapsulate arbitrary unsafe code within regular library functions. These can be freely called in higher-level code without explicit warnings. Therefore, it is crucial to analyze library functions to rule out pinning API contract violations. Unfortunately, such testing relies on manual analysis by library authors, which is ineffective. Our goal is to develop a methodology that, given a library, attempts to construct programs that intentionally breach the pinning API contract by chaining library function calls, thereby verifying their soundness. We introduce RPIL, a novel intermediate representation that models functions' critical behaviors pertaining to pinning APIs. We implement PinChecker, a synthesis-driven violation detection tool guided by RPIL, which automatically synthesizes bug-revealing programs. Our experiments on 13 popular Rust libraries from crates.io found 2 confirmed bugs.