-Wunused:locals does not produce a warning for simple values

[gatisnolv]

Reproduction steps

Scala version: 2.13.8

  object Example extends App {
    def unusedLocal = {
      val a = 1
      2
    }
  }

with the compiler option

"-Wunused:locals"

(and thus -Xlint:unused ...)

Problem

I would expect 'val a' to be reported as 'never used', but there is no warning.

If either line 3 is changed to e.g. val a = 1+1 OR line 4 changed to e.g. '2+1', there is a warning.

[SethTisue]

might be @som-snytt territory?

[som-snytt]

The trivial assignment is eliminated early, which amazes me. "There is no assignment. There is nothing to see here." That's an old Jedi mind trick.

[SethTisue]

So is there any realistic possibility of this progressing?

It's probably merely idle curiosity, but I'm curious what the transform is that causes it to be eliminated?

[jxnu-liguobin]

https://github.com/scala/scala/blob/3faf9a98282a2f06219fd0b05b7fede2ab8fb326/src/compiler/scala/tools/nsc/typechecker/Typers.scala#L6268

I was surprised to find val a: Int = 1; disappeared from here.

[som-snytt]

Probably just inlined.