Use annotation to enable -explain locally, similar to @nowarn("verbose")

[lrytz]

Adding an annotation is a lot more convenient than changing the build to add a compiler flag.

Scala 3 supports @nowarn("verbose") to display applicable message filters ("Matching filters..." section):

scala> @nowarn("v") def f = try 1
1 warning found
-- [E002] Syntax Warning: ------------------------------------------------------
1 |@nowarn("v") def f = try 1
  |                     ^^^^^
  |                   A try without catch or finally is equivalent to putting
  |                   its body in a block; no exceptions are handled.
  |Matching filters for @nowarn or -Wconf:
  |  - id=E2
  |  - name=EmptyCatchAndFinallyBlock
  |
  | longer explanation available when compiling with `-explain`

It would be practical if -explain could be enabled in the same way, not only for warnings but also for errors.

Reusing @nowarn("v") would be trivial, but maybe a different annotation is desired?

https://github.com/scala/scala3/compare/main...lrytz:scala3:nowarnExplain?expand=1

scala> def f: Int = ""
-- [E007] Type Mismatch Error: -------------------------------------------------
1 |def f: Int = ""
  |             ^^
  |             Found:    ("" : String)
  |             Required: Int
  |
  | longer explanation available when compiling with `-explain`
1 error found

scala> @annotation.nowarn("v") def f: Int = ""
-- [E007] Type Mismatch Error: -------------------------------------------------
1 |@annotation.nowarn("v") def f: Int = ""
  |                                     ^^
  |                                     Found:    ("" : String)
  |                                     Required: Int
  |-----------------------------------------------------------------------------
  | Explanation (enabled by `-explain`)
  |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
  |
  | Tree: ""
  | I tried to show that
  |   ("" : String)
  | conforms to
  |   Int
  | but none of the attempts shown below succeeded:
  |
  |   ==> ("" : String)  <:  Int
  |     ==> String  <:  Int  = false
  |
  | The tests were made under the empty constraint
   -----------------------------------------------------------------------------
1 error found

[lrytz]

Alternative, suggested by @dwijnand: make the compiler understand //> using options -explain in individual source files.

[som-snytt]

I needed this feature today. (For a test where I don't want to explain everything, but I want to show certain explanations.)