cmd/yacc: adjust expansion of $n to be more useful in errors
When the compiler echoes back an expression, it shows the
generated yacc expression. Change the generated code to
use a slice so that $3 shows up as yyDollar[3] in such messages.
Consider changing testdata/expr/expr.y to say:
$$.Sub(float64($1), $3)
(The float64 conversion is incorrect.)
Before:
expr.y:70[expr.go:486]: cannot convert exprS[exprpt - 2].num (type *big.Rat) to type float64
After:
expr.y:70[expr.go:492]: cannot convert exprDollar[1].num (type *big.Rat) to type float64
Change-Id: I74e494069df588e62299d1fccb282f3658d8f8f4
Reviewed-on: https://go-review.googlesource.com/4630 Reviewed-by: Rob Pike <r@golang.org>