Thanks to a large number of generous backers, a crowdfunded "edit in place" mode was added to Processing for 3.4. When a vector layer is active and editable, this mode allows users to execute suitable algorithms directly on the features from the layer, modifying their attributes and geometries in-place. All edits are added to the layer’s “edit buffer”, so you can easily undo and redo the changes before saving them back to the data source.
For those power users who perform frequent data edits, this functionality is also exposed via QGIS 3.0’s “Locator bar” (that “type to locate” bar which sits in the bottom-left corner). Typing ‘ef’ (“edit features”) followed by the name of the corresponding operation allows you to perform the edits directly via the keyboard. E.g.