rust/src/librustc_error_codes/error_codes/E0669.md

Cannot convert inline assembly operand to a single LLVM value.

This error usually happens when trying to pass in a value to an input inline assembly operand that is actually a pair of values. In particular, this can happen when trying to pass in a slice, for instance a &str. In Rust, these values are represented internally as a pair of values, the pointer and its length. When passed as an input operand, this pair of values can not be coerced into a register and thus we must fail with an error.