C# auto-completion should not show type names after readonly and ref in namespace scope.

[acaly]

Summary

Transferred from #66948.

When you type public readonly str in a namespace, public readonly string is selected as the first candidate over the desired public readonly struct, although C# does not allow field declaration in namespace scope.

Background and Motivation

I think the current behavior is probably because the compiler identifies readonly and ref as a "prefix of type name". However, it does not check whether such type names are allowed in the current scope.

Although this only affects a small number of cases, it sometimes becomes a problem because to get the correct public readonly struct, one needs to type in at least 4 letters of the word, or to press down arrow.

Proposed Feature

The auto-completion after keyword readonly and ref should check whether it's in namespace scope. If so, don't show type names as candidates.

There might be other cases to consider in addition to the readonly and ref.

Alternative Designs

No alternative design proposed.

[CyrusNajmabadi]

Hey @acaly , feel free to open a PR here to fix this if you're interested.