July 12
https://github.com/dlang/dmd/pull/8493

It's a small change, but it illustrates an important concept - separating code into functions that calculate things (which can be made pure) and functions that modify state.

Doing this more generally will significantly reduce the cognitive complexity of the compiler.
July 16
On Thursday, 12 July 2018 at 23:58:23 UTC, Walter Bright wrote:
> https://github.com/dlang/dmd/pull/8493
>
> It's a small change, but it illustrates an important concept - separating code into functions that calculate things (which can be made pure) and functions that modify state.
>
> Doing this more generally will significantly reduce the cognitive complexity of the compiler.

It obvious that the compilation-process is in essence a pure function mapping source_code to machine_code.

Therefore this is theoretically applicable to most of the compiler.