I'm using GitLab (which calls them merge requests rather than pull requests). From a couple of the comments below it seems like GitLab behaviour may be different from GitHub's.
I'm trying to get my head around how using pull requests for code reviews would work where you're developing on feature branches with a shared remote repository (rather than separate remote repositories for each developer).
Most workflows I've seen talk about pull requests from a feature branch to a shared develop branch.
The problem I see with this is when you're not sure what order the features will be released in, or even if some experimental features will be released at all. So you want to keep the feature branches "pure", without cross-contamination from other feature branches, until they're released.
To avoid mixing the changes from one feature branch with another you would never be able to merge develop into a feature branch, or rebase the feature branch on develop until the other features have been released. Pull requests would end up really messy, comparing the changes in a feature branch with a develop branch which would include changes merged from other feature branches.
The only solution I can think of would be to create a pull request between the head of a feature branch and the commit from the previous pull request on that feature branch. However, I don't think most repository hosting services like GitLab, BitBucket or GitHub support pull requests from an earlier commit to a later one on the same branch.
I feel like I'm missing something obvious here because this must be a common problem. How do other people resolve this issue of using pull requests for code reviews while keeping code in feature branches separate?