Do not spawn redundant hints

[popzxc]

Closes #5206

This is a second part of the fix (first was #5997).

This PR adds a new method to the CompletionContext: no_completion_required. If this method returns true, it essentially means that user is unlikely to expect any hints from the IDE at this cursor position.

Currently, checks for the following cases were added:

• Previous item is fn: user creates a new function, names of existing functions won't be helpful. Exception for this case is impl Foo for Bar -- we must suggest trait function names.
• User entered for _ i<|>: it's obviously going to be in keyword, any hints here will be confusing.

More checks may be added there later, but currently I've only figured two cases.

[SomeoneToIgnore]

I totally agree with the idea: we need a better way to filter out the completions if the position is not appropriate.

I'm not entirely convinced that the CompletionContext is a good place to have those cheks, but the PR is rather small and, what's more important, adds the tests for cases with no completions, so I like the idea to merge it.
I'll try to share my vision later today in zulip

Thank you.

bors r+

[bors]

Build succeeded:

• Rust (macos-latest)
• Rust (ubuntu-latest)
• Rust (windows-latest)
• TypeScript (ubuntu-latest)
• TypeScript (windows-latest)

[SomeoneToIgnore]

Here's the link, for the reference: https://rust-lang.zulipchat.com/#narrow/stream/185405-t-compiler.2Fwg-rls-2.2E0/topic/Completion.20refactoring/near/213647413

[matklad]

Previous item is fn: user creates a new function, names of existing functions won't be helpful.

fwiw, long-term we should plug "create from usage" into this.

ie, in

fn main() {
   no_such_fn_yet(92)
}

fn n

we should complete no_such_fn_yet(_: i32)

[popzxc]

Yeah, that makes sense. As I wrote in zulip, I only see this as a temporary solution until completions management will become more picky about kinds of suggestions that will be presented to user.