Issue warnings if braces are missing

[odersky]

Issue warnings if brace structure does not correspond to indentation structure.

There are three cases.

Case 1: In a brace-delimited region, if there would be an <outdent> between
one line and the next, a warning is now issued that states:

Line is indented too far to the left or a `}' is missing

This is very helpful for finding missing closing braces. It prevents errors like:

if (x < 0) {
  println(1)
  println(2)

println("done")

Case 2: If significant indentation is turned off (i.e. under -noindent) and we are at the start
of an indented sub-part (i.e. an <indent> would have been inserted under significant indentation),
and the indented part ends in a newline, check that the next statement starts at an indentation width less than the sub-part. This prevents errors like

if (x < 0)
  println(1)
  println(2)

println("done")

Case 3: Under -noindent, warn if code that follows an empty template is
indented more deeply than the template header. Inserting braces
automatically would change meaning in this case.

Example:

  trait A
    case class B() extends A  // error: Line is indented too far to the right
    case object C extends A   // error: Line is indented too far to the right

[liufengyun]

LGTM

I'm thinking if we could strengthen the rules for infix operators. Currently, the following program runs with no problem:

@main def Test = {
  val a = 6
  var b = 0
  if (a > 5) then
      b = a
- 5

  assert(b == 1)
}

If would be good to check that indentation of infix operator should be bigger than that of currentRegion.outer?

[liufengyun]

The check does not handle code at top-level:

trait A
    case class B() extends A
    case object C extends A

[odersky]

OK, I'll add a fix to the follow-up PR.

[odersky]

If would be good to check that indentation of infix operator should be bigger than that of currentRegion.outer?

Yes, maybe. But I am not sure the compiler has to do it. Currently the compiler only enforces those indentation rules that make sure that braces around well-indented code are optional. To do more,
we'd have to formulate the rules and come up with an algorithm to enforce them. This might need several cycles to get right, or maybe there won't be an agreement what is the correct formulation of these rules..