Use p* for vararg splices

[odersky]

From the doc page:

The syntax of vararg splices in patterns and function arguments has changed. The new syntax uses a postfix *, analogously to how a vararg parameter is declared.

val arr = Array(1, 2, 3)
val lst = List(0, arr*)                  // vararg splice argument
lst match
   case List(0, 1, xs*) => println(xs)   // binds xs to Seq(2, 3)
   case List(1, _*) =>                   // wildcard pattern

The old syntax for splice arguments will be phased out.

/*!*/ val lst = List(0, arr: _*)         // syntax error
      lst match
         case List(1, 2, xs @ _*)        // ok, equivalent to `xs*`

Syntax

ArgumentPatterns  ::=  ‘(’ [Patterns] ‘)’
                    |  ‘(’ [Patterns ‘,’] Pattern2 ‘*’ ‘)’

ParArgumentExprs  ::=  ‘(’ [‘using’] ExprsInParens ‘)’
                    |  ‘(’ [ExprsInParens ‘,’] PostfixExpr ‘*’ ‘)’

Compatibility considerations

To enable cross compilation between Scala 2 and Scala 3, the compiler will
accept both the old and the new syntax. Under the -source 3.1 setting, an error
will be emitted when the old syntax is encountered.

Fixes #11233

[odersky]

@liufengyun Can you do a technical review for this?

[liufengyun]

LGTM

[liufengyun]

This syntax deviates from Scala 2.

I think it would be good to change it to:

Suggested change

	| ‘(’ [Patterns ‘,’] Pattern2 ‘*’ ‘)’
	| ‘(’ [Patterns ‘,’] id ‘*’ ‘)’

[odersky]

I think that's orthogonal to this PR. I am not sure about that. It's the same as for other ascriptions in patterns. I think they are useful to have.

[liufengyun]

Suggested change

	| ‘(’ [Patterns ‘,’] Pattern2 ‘*’ ‘)’
	| ‘(’ [Patterns ‘,’] id ‘*’ ‘)’