I'd also add to this that people often end up with very different ideas of what a unit test is, which confuses things further. I've seen people who write separate tests for each function in their codebase, with the idea that each function is technically a unit that needs to be tested, and that's a sure-fire way to run into tightly-coupled tests.
In my experience, the better approach is to step back and find the longer-living units that are going to remain consistent across the whole codebase. For example, I might have written a `File` class that itself uses a few different classes, methods, and functions in its implementation - a `Stats` class for the mtime, ctime, etc values; a `FileBuilder` class for choosing options when opening the file, etc. If all of that implementation is only used in the `File` class, then I can write my tests only at the `File` level and treat the rest kind of like implementation details.
It may be that it's difficult to test these implementation details just from the `File` level - to me that's usually a sign that my abstraction isn't working very well and I need to fix it. Maybe the difficult-to-test part should actually be a dependency of the class that gets injected in, or maybe I've chosen the wrong abstraction level and I need to rearchitect things to expose the difficult-to-test part more cleanly. But the goal here isn't to create an architecture so that the tests are possible, the goal is to create an architecture that's well-modularised, and these systems are usually easier to test as well.
There's an argument that this isn't a unit test any more - it's an integration test, because it's testing that the different parts of the `File` class's implementation work together properly. My gut feeling is that the distinction between unit and integration is useless, and trying to decide whether this is one or the other is a pointless endeavour. I am testing a unit either way. Whether that unit calls other units internally should be an implementation detail to my tests. Hell, it's an implementation detail whether or not the unit connects to a real database or uses a real filesystem or whatever - as long as I can test the entirety of the unit in a self-contained way, I've got something that I can treat like a unit test.
In my experience, the better approach is to step back and find the longer-living units that are going to remain consistent across the whole codebase. For example, I might have written a `File` class that itself uses a few different classes, methods, and functions in its implementation - a `Stats` class for the mtime, ctime, etc values; a `FileBuilder` class for choosing options when opening the file, etc. If all of that implementation is only used in the `File` class, then I can write my tests only at the `File` level and treat the rest kind of like implementation details.
It may be that it's difficult to test these implementation details just from the `File` level - to me that's usually a sign that my abstraction isn't working very well and I need to fix it. Maybe the difficult-to-test part should actually be a dependency of the class that gets injected in, or maybe I've chosen the wrong abstraction level and I need to rearchitect things to expose the difficult-to-test part more cleanly. But the goal here isn't to create an architecture so that the tests are possible, the goal is to create an architecture that's well-modularised, and these systems are usually easier to test as well.
There's an argument that this isn't a unit test any more - it's an integration test, because it's testing that the different parts of the `File` class's implementation work together properly. My gut feeling is that the distinction between unit and integration is useless, and trying to decide whether this is one or the other is a pointless endeavour. I am testing a unit either way. Whether that unit calls other units internally should be an implementation detail to my tests. Hell, it's an implementation detail whether or not the unit connects to a real database or uses a real filesystem or whatever - as long as I can test the entirety of the unit in a self-contained way, I've got something that I can treat like a unit test.