Improve error messages for leaking of `this` as constructor arguments

[liufengyun]

Minimized example

Given the following program

class A:
  // should report one error here
  val b = new B(this) // error
  val m = 10
  val n = 20

class B(a: A):
  val x = a.m
  val y = a.n

Output

Compile with -Ysafe-init, the compiler generates the following warnings:

-- Warning: tests/init/neg/leak-constr.scala:8:12 ------------------------------
8 |  val x = a.m
  |          ^^^
  |Access field on a value with an unknown initialization status. Calling trace:
  |-> class A:	[ leak-constr.scala:1 ]
  |   ^
  |-> val b = new B(this) // error	[ leak-constr.scala:3 ]
  |           ^^^^^^^^^^^
  |-> class B(a: A):	[ leak-constr.scala:7 ]
  |   ^
  |-> val x = a.m	[ leak-constr.scala:8 ]
  |           ^^^
-- Warning: tests/init/neg/leak-constr.scala:9:12 ------------------------------
9 |  val y = a.n
  |          ^^^
  |Access field on a value with an unknown initialization status. Calling trace:
  |-> class A:	[ leak-constr.scala:1 ]
  |   ^
  |-> val b = new B(this) // error	[ leak-constr.scala:3 ]
  |           ^^^^^^^^^^^
  |-> class B(a: A):	[ leak-constr.scala:7 ]
  |   ^
  |-> val y = a.n	[ leak-constr.scala:9 ]
  |           ^^^
2 warnings found

Expectation

The warnings are legit, however, there are two problems:

• The leaking cause too many errors to be useful
• The error does not point to the leaking site and is a little difficult to understand

Ideally, we should only report one error at the leaking site if the leaking is unsafe.