reusing build cache, any interest?

[ipatch]

Output of brew config

na

Output of brew doctor

na

Description of issue

i run homebrew in a non standard prefix on several different boxes (computers). my primary computer is a 4th gen i7 2015 macbook running arch linux. i have homebrew setup in $HOME/homebrew for my standard user. often when i run brew upgrade the process can take up to several hours depending on what is being upgraded, for the fact that i have to rebuild the offending package / formula from source. i think this could be alleviated if the upgrade command could reuse the build cache ie. for a cmake project one often sets up a build directory, and keeps directory around separate from the source code directory then just rebuilds the project using the existing build dir after upgrading the source directory. i do not use gnu autotools as frequent as cmake, so i'm not sure if the same process applies for it as well, but it'd seem worth taking a look into. i know this would require some significant changes to the way homebrew/brew operates, but often i use brew install --keep-tmp when troubleshooting formula that either do not build or install correctly, and it'd be nice to have a flag such as brew install --use-tmp or something of that grammar to reuse the build cache to avoid extremely long build times. also i believe this would significantly help projects that build artifacts using CI. but that just my 2 cents.

[gromgit]

one often sets up a build directory, and keeps directory around separate from the source code directory then just rebuilds the project using the existing build dir after upgrading the source directory

That runs counter to the general best practice of removing all build artefacts after a source and/or dependencies upgrade. Dependency upgrades in particular can obsolete pre-built objects even if the main code doesn't change, and major source version bumps may touch so many files that a complete rebuild would be the only sane thing to do.