To be clear, Mercurial does not have a staging area, but it does have allow selective commits (and selective uncommits) via prompt-based or interactive UI selection of hunks. Disagreeing with the need for a staging area is not the same as saying selective commits are unnecessary (I use Mercurial more than git and I rarely commit everything in my working directory in a single commit - I like small commits).
When there's an expectation or requirement that each commit builds (and even passes tests), how can you do partial commits? Do you work exclusively on projects without such requirements? Do you rely solely on CI to ensure that your commit compiles? Do you not use CI and not care if a commit is broken... you'll squash a fix in later, or not even squash it and leave a broken commit in the repo?
Each commit should build and pass tests, yes. When I say "partial commits", I don't mean that the commits are arbitrary - each commit should be as small as possible to implement a specific fix/feature. I've also heard it described as the smallest unit that you may want to revert.
For example, if you are working on something, but it requires adding an API to some module, then the first commit 1 is to add the new API (+ tests), and the second commit is the new code that uses that API.
Unfortunately many developers I have worked with would just combine these (and more) into a single commit (because they are part of the same work task). However this makes review, bisect, blame and revert harder (if you need to revert commit 2, you don't want to also revert the API you added if that was tested and bug-free).
Why would partial commits necessarily break anything?
In fact, often partial commits are necessary for builds.
As an example (and to be fair, this was a transitional project), I once worked on a project where the local dev directly acquired packages from different parts of the application, but the actual CI was broken up into different pipelines which required some parts to be built first, its outputs packaged and added to the registry, and downstream parts to be built after.
Committing everything at once would literally break the CI.
If the dev's working tree isn't exactly what they checked in, how do they build or test the commit? Do they YOLO a partial commit and wait for it to be accepted or rejected by the CI? Isn't that a problem to be solved by improving the CI pipeline?
