RFC: --patchast to patch all modules via AST transformations

[timotheecour]

more general and flexible than these:

• --import
  not good coz not flexible: all or nothing, can't apply to just a set of files etc

• --include
  ditto, not flexible

• --trmacros term rewriting macros
  not good coz too magical yet not flexible enough; reminds of perl: cryptic

• --expandMacro:MACRO
  ditto, not flexible

use cases

• can be used as a basis for nimfix v2
  the AST can be rendered alongside nimpretty to re-generate files after a user custom fix; user can always use git revert if he doesn't like the change

• can be used for simple or arbitrarily complex refactorings

• can be used to unhide a symbol wo touching the sources; would keep working even if the file changes unlike with patchFile in nimscript via patchFile("stdlib", "asyncdispatch", "patches/replacement")

usage

nim c --patchast:pathto/module main

# module pathto/module
macro patchModule*(a: NimNode) =
  result = a
  doAssert a.kind == nnkModuleFile
  let mod = a[0]
  if mod.name.s != "ospaths": return
  ...

note

• we could make instead take as input the un-parsed file, ie the filename, and parse ourselves? eg via staticRead
  maybe more flexible in some situations eg when parser introduces a fwd incompatible change and we need to patch a module to support it;
• maybe related to polyfills?
• can that be achieved via patchFile? in any case its useful to have std utilies to deal with patching files and file asts

potential use cases

would be interesting to see if it could be used for these:

• potentially relevant: [superseded] import foo {.all.} nim-lang/Nim#11865 import foo {.privateImport.}
• nimscript: make exec("nim c") honor `-d:nimExe:path nimscript: make exec("nim c") honor -d:nimExe:path nim-lang/RFCs#273
• generating stacktraces
• generating execution traces (execution traces (eg for code coverage, debugging, introspection, profiling) nim-lang/Nim#15827)
• replacement for TRW https://nim-lang.github.io/Nim/manual_experimental.html#term-rewriting-macros

links

• Transform procs at compile time? - Nim forum
• https://forum.nim-lang.org/t/6940#43608