Remove most uses of uncheckedNN

[SolalPirelli]

Removed all of the ones that are locally easy to remove, i.e., no complex invariants across methods or fields.

Remaining:

scala3 % grep --include=\*.scala -Fr 'uncheckedNN' compiler
compiler/src/dotty/tools/dotc/typer/Implicits.scala:      if isImport then if ctx.importInfo.uncheckedNN.isWildcardImport then WildImport else NamedImport else Definition
compiler/src/dotty/tools/dotc/typer/Implicits.scala:        val outer = outerImplicits.uncheckedNN
compiler/src/dotty/tools/dotc/typer/Typer.scala:          ctx.isImportContext && namedImportRef(ctx.importInfo.uncheckedNN).exists
compiler/src/dotty/tools/dotc/typer/Typer.scala:          val curImport = ctx.importInfo.uncheckedNN

These are all more complex invariants.

Plus the def:

compiler/src/dotty/tools/package.scala:     *  occurrences of uncheckedNN.
compiler/src/dotty/tools/package.scala:    transparent inline def uncheckedNN: T = x.asInstanceOf[T]

How much have you relied on LLM-based tools in this contribution?

Not at all

How was the solution tested?

Covered by existing tests (this is a refactoring)

[SolalPirelli]

(scroll to the last file in this diff view to see the definition of this -- it's as you'd expect, hopefully)

[SolalPirelli]

no point in uninitialized for explicitly nullable stuff

[sjrd]

This does not produce good code because it performs a type test on : T rather than a single null test. The best code is obtained with

inline def initialize[T](getter: T | Null, inline setter: T => Unit, inline value: => T): T =
  if getter != null then
    getter
  else
    val res = value
    setter(res)
    res

(note also the signature. setter must be inline)

[SolalPirelli]

OK, thanks. I wonder if that's something we could improve in the compiler.

[sjrd]

Ugh. This is just wrong :(
(not that it was better before your PR)

[SolalPirelli]

I know 😭