zclsyntax: track whether parser is in recovery mode

The parser has some recovery heuristics but they will not always achieve
the best result. To prevent unsuccessful recovery from causing a cascade
of confusing follow-on errors, we'll track in the parser whether recovery
has been attempted and then the specific sub-parsers can use this to
skip certain "unexpected token type" errors in recovery situations,
assuming instead that an earlier error will cover the problem and that
we just want to bail out quickly.

zcl/zclsyntax/parser.go

@@ -9,6 +9,12 @@ import (
 
 type parser struct {
 	*peeker
+
+	// set to true if any recovery is attempted. The parser can use this
+	// to attempt to reduce error noise by suppressing "bad token" errors
+	// in recovery mode, assuming that the recovery heuristics have failed
+	// in this case and left the peeker in a wrong place.
+	recovery bool
 }
 
 func (p *parser) ParseBody(end TokenType) (*Body, zcl.Diagnostics) {
@@ -194,6 +200,7 @@ Token:
 // unpredictable results on input with bad bracketer nesting.
 func (p *parser) recover(end TokenType) {
 	start := p.oppositeBracket(end)
+	p.recovery = true
 
 	nest := 0
 	for {

zcl/zclsyntax/public.go

@@ -11,7 +11,7 @@ import (
 func ParseConfig(src []byte, filename string, start zcl.Pos) (*Body, zcl.Diagnostics) {
 	tokens := LexConfig(src, filename, start)
 	peeker := newPeeker(tokens, false)
-	parser := &parser{peeker}
+	parser := &parser{peeker: peeker}
 	return parser.ParseBody(TokenEOF)
 }