Merge #5089 #5161 #5184 #5185 #5186

5089: Disable auto-complete on comments r=matklad a=BGluth

Resolves #4907 by disabling any auto-completion on comments.

As flodiebold [pointed out](https://github.com/rust-analyzer/rust-analyzer/issues/4907#issuecomment-648439979), in the future we may want to support some form of auto-completion within doc comments, but for now it was suggested to just disable auto-completion on them entirely.

The implementation involves adding a new field `is_comment` to `CompletionContext` and checking if the immediate token we auto-completed on is a comment. I couldn't see a case where we need to check any of the ancestors, but let me know if this is not sufficient. I also wasn't sure if it was necessary to add a new field to this struct, but I decided it's probably the best option if we want to potentially do auto-completion on doc comments in the future.

Finally, the three tests I added should I think ideally not filter results by `CompletionKind::Keyword`, but if I want to get unfiltered results, I need access to a non-public function [get_all_completion_items](https://github.com/rust-analyzer/rust-analyzer/blob/9a4d02faf9c47f401b8756c3f7fcab2198f5f9cd/crates/ra_ide/src/completion/test_utils.rs#L32-L39) which I don't know if I should make public just for this.



5161: SSR: Add initial support for placeholder constraints r=matklad a=davidlattimore



5184: Always install required nightly extension if current one is not nightly r=matklad a=Veetaha

This is weird, but having switched back to stable by uninstalling the extension appears that vscode doesn't destroy the `PersistentState` and thus changing to `nightly` channel doesn't work because the last check for nightly extension was less than 1 hour ago. The simple solution is to skip this check if we know that the current extension version is not nightly.

5185: Force showing extension activation error pop-up notification r=matklad a=Veetaha

Fixes https://github.com/rust-analyzer/rust-analyzer/issues/5091

5186: fix: correct pd/ppd/tfn/tmod completion doc r=matklad a=fannheyward

https://github.com/rust-analyzer/rust-analyzer/blob/a33eefa3b26000b3018e6bb873f18dbe15ab4ab7/crates/ra_ide/src/completion/complete_snippet.rs#L23-L24

Co-authored-by: BGluth <[email protected]>
Co-authored-by: David Lattimore <[email protected]>
Co-authored-by: Veetaha <[email protected]>
Co-authored-by: Heyward Fann <[email protected]>