What is the difference between struct{a int;b int} and struct{b int;a int}?

Question

What is the difference between these two structs other than that they aren't considered equivalent?

package main
func main() {}
func f1(s struct{a int;b int}) { f2(s) }
func f2(s struct{b int;a int}) {}

$ go run a.go 
# command-line-arguments
./a.go:3: cannot use s (type struct { a int; b int }) as type struct { b int; a int } in argument to f2

Note: this does compile:

package main
func main() {}
func f1(s struct{a int;b int}) { f2(s) }
func f2(s struct{a int;b int}) {}

Answer 1

"The order of structs' fields is important on the low level" How?

This will impact the reflection, like func (v Value) Field(i int) Value:

Field returns the i'th field of the struct v

The first field 'a' in the first structure wouldn't be the same first in the second structure.
That also will influence serialization with marshaler methods (encoding package).