Based on a discussion in the Cabal and Hackage room on Matrix, initiated by @juhp:

In short, the ability to extend a project description to specify one or more *.patch files in respect of a package, for one or more packages, the patch files being applied in the order listed to modify the source code files before they are built; and the extension either happening at the command line or by agumenting Stack's project-level configuration file.

Example of a *.patch file:

• https://github.com/haskell/haskell-language-server/commit/fb17921128bd56ba74872cae9539767e63b9fd79.patch

Example of what @juhp did, using RPM macros:

cabal unpack %{ghcide}
sed -e "s/@GHCIDE@/%{ghcide}/" %{SOURCE1} > cabal.project
( cd %{ghcide}
  %patch -P1 -p2
)

Example of hypothetical command line options:

stack build --patch ghcide:the-first.patch --patch ghcide:second.patch
-  
stack build --patches "ghcide:first.patch,second.patch ..."