cmd/go: unnecessary parens in error message

[rsc]

$ go build
toolchain.go:131:24: syntax error: cannot use assignment (gotoolchain) = ("path") as value

The code only says if gotoolchain = "path" {, a typo of = for ==.

I am not sure what the parens mean in this error message.
Every time I see them (apparently I do this with some frequency)
it is a bit jarring, almost like they are single-element tuples,
but then I remember Go has no tuples, at least not parenthesized ones like Python.
It would be better not to show them:

toolchain.go:131:24: syntax error: cannot use assignment gotoolchain = "path" as value

Or maybe:

toolchain.go:131:24: syntax error: cannot use assignment as value: gotoolchain = "path"

/cc @griesemer

[griesemer]

Trivial repro: https://go.dev/play/p/nFjETHH2pFk?v=gotip

[griesemer]

There's explicit code in the parser to emphasize the LHS and RHS for this kind of error:

// A common syntax error is to write '=' instead of '==',
// which turns an expression into an assignment. Provide
// a more explicit error message in that case to prevent
// further confusion.
var str string
if as, ok := s.(*AssignStmt); ok && as.Op == 0 {
	// Emphasize Lhs and Rhs of assignment with parentheses to highlight '='.
	// Do it always - it's not worth going through the trouble of doing it
	// only for "complex" left and right sides.
	str = "assignment (" + String(as.Lhs) + ") = (" + String(as.Rhs) + ")"
} else {
	str = String(s)
}

I added this in response to #23385.

This is trivially changed (or even removed), but we should be conscious of the issue. Without the special casing, the error for

package p

func _(x, y int) {
	if x = y {}
}

will be:

syntax error: cannot use x = y as value

This is problematic when the LHS and RHS are more complex as in x || y = z where we want to emphasize x || y as the LHS. But perhaps the parentheses logic should just apply of a side is complex.

[gopherbot]

Change https://go.dev/cl/500975 mentions this issue: cmd/compile/internal/syntax: better error message when using = instead of ==