As somebody who spends a lot of time in the text editor, working with code, and on the Linux command line, I see text autocomplete as one of the basic, native functionality. It’s almost like copy-paste, selection, and undo.
Gmail has recently extended the Smart Compose feature with exactly that. No longer it only covers the pre-defined responses like “OK, thanks” and “No problem”. It completes pretty much any words and phrases that you type.
It’s super useful and it makes me wonder why it took so long for somebody to think of this. Surely, the actual implementation is not all that complex (just kidding … it’s not a simple dictionary complete, it’s context aware and smart, so, yeah, it’s pretty complex).