Unable to inline tailrec methods

[allanrenucci]

import annotation.tailrec
class Test {
  @inline @tailrec def rec(x: Int): Int = {
    if (x > 0) rec(x - 1)
    else 0
  }

  def bar = rec(100)
}

fails to compile with Dotty

-- Error: tests/allan/Test.scala:25:18 -----------------------------------------
25 |    if (x > 0) rec(x - 1)
   |               ^^^^^^^^^^
   |               Maximal number of successive inlines (32) exceeded,
   |               Maybe this is caused by a recursive inline method?
   |               You can use -Xmax:inlines to change the limit.
   | This location is in code that was inlined at tests/allan/Test.scala:25

Scalac does not have this limitation because inlining happens after the tailrec optimisation.

I believe this change of behavior might break a lot of code, and we should turn the error into a warning.

[sjrd]

There's a lot of code like that in Scala.js. It's very useful because the tail-recursion is first turned into a loop by scalac, and then that loop is inlined at call site by the optimizer.

[smarter]

I think we should just not interpret @inline as if it was equivalent to inline in Dotty. The latter has its own semantics that users need to be aware of when writing code.

[Blaisorblade]

New plan from weekly meeting: cross-compiling code should use a new @forceInline annotation, added to dotty-library, ignored by Scala 2 but supported by Dotty meaning inline (code that wants inlining cross-compiler could use @inline @forceInline).