hcl/hclsyntax: Allow newline between { and "for" keyword

We were previously enabling newline-sensitivity too soon, before checking
for the "for" keyword, and thus seeing the newline token instead of the
for token when peeking ahead.

Now we peek for the for token first and turn on newline-sensitivity only
if it isn't present. This fixes another bug in turn where we would
previously have parsed the for expression itself in newline-sensitive
mode, which we no longer do because we delegate to the for expression
parser sooner.

hcl/hclsyntax/expression_test.go

@@ -499,6 +499,14 @@ upper(
 			0,
 		},
 
+		{
+			"{\n  for k, v in {hello: \"world\"}:\nk => v\n}",
+			nil,
+			cty.ObjectVal(map[string]cty.Value{
+				"hello": cty.StringVal("world"),
+			}),
+			0,
+		},
 		{
 			`{for k, v in {hello: "world"}: k => v if k == "hello"}`,
 			nil,

hcl/hclsyntax/parser.go

@@ -1090,13 +1090,13 @@ func (p *parser) parseObjectCons() (Expression, hcl.Diagnostics) {
 		panic("parseObjectCons called without peeker pointing to open brace")
 	}
 
-	p.PushIncludeNewlines(true)
-	defer p.PopIncludeNewlines()
-
 	if forKeyword.TokenMatches(p.Peek()) {
 		return p.finishParsingForExpr(open)
 	}
 
+	p.PushIncludeNewlines(true)
+	defer p.PopIncludeNewlines()
+
 	var close Token
 
 	var diags hcl.Diagnostics