I recently switched to cabal HEAD (was prior 3b8ee83) and for installations of new libs I get:

Installing library in C:\Users\abiehl\AppData\Roaming\cabal\store\ghc-8.0.2\incoming\new-16045\Users\abiehl\AppData\Roaming\cabal\store\ghc-8.0.2\regex-compat-_-0.95.1.4-a13ab0d5ccd9fb2c4a580fc0674bee71629b38f1\lib
copyFile: does not exist (...)

I suspected a too-long path but the actual path is only ~190 characters long. So maybe it actually doesn't create parent directories? I am not sure what the culprit here is exactly.

The respective copy command is issued here: 3b8ee83#diff-d998ae67b007b4623e9c789feb2c47abR929.

Also the path seems quite redundant! Why not put the library under C:\Users\abiehl\AppData\Roaming\cabal\store\ghc-8.0.2\incoming\new-16045 instead of C:\Users\abiehl\AppData\Roaming\cabal\store\ghc-8.0.2\incoming\new-16045\Users\abiehl\AppData\Roaming\cabal\store\ghc-8.0.2\regex-compat-_-0.95.1.4-a13ab0d5ccd9fb2c4a580fc0674bee71629b38f1\lib. The new-* directory is unique already! By dropping the redundant part we can save some precious bytes for paths.