It’s called Bootstrapping, quoting from Wikipedia:
If one needs a compiler for language X to obtain a compiler for language X (which is written in language X), how did the first compiler get written? Possible methods to solving this chicken or the egg problem include:
- Implementing an interpreter or compiler for language X in language
Y. Niklaus Wirth reported that he wrote the first Pascal compiler in
Fortran. - Another interpreter or compiler for X has already been written in
another language Y; this is how Scheme is often bootstrapped. - Earlier versions of the compiler were written in a subset of X for
which there existed some other compiler; this is how some supersets
of Java, Haskell, and the initial Free Pascal compiler are
bootstrapped. - The compiler for X is cross compiled from another architecture where
there exists a compiler for X; this is how compilers for C are
usually ported to other platforms. Also this is the method used for
Free Pascal after the initial bootstrap. - Writing the compiler in X; then hand-compiling it from source (most
likely in a non-optimized way) and running that on the code to get
an optimized compiler. Donald Knuth used this for his WEB literate
programming system.
And if you are interested, here is Dennis Richie’s first C compiler source.