> Cachegrind’s output showed that the trivial methods for the simple BytePos and CharPos types in the parser are (a) extremely hot and (b) not being inlined.

To me, this points to a deficiency in the rustc compiler. All trivial methods (trivial in that they will always be cheaper than a function call) should be automatically treated by the compiler as if they had been marked #[inline]. Currently a rust programmer has to annotate every single trivial getter and setter with that attribute, which is both tiresome and easy to forget.