Improve error reporting for comma-separated args

hcl/hclsyntax/parser.go

@@ -236,10 +236,18 @@ func (p *parser) finishParsingBodyAttribute(ident Token, singleLine bool) (Node,
 			end := p.Peek()
 			if end.Type != TokenNewline && end.Type != TokenEOF {
 				if !p.recovery {
+					summary := "Missing newline after argument"
+					detail := "An argument definition must end with a newline."
+
+					if end.Type == TokenComma {
+						summary = "Unexpected comma after argument"
+						detail = "Argument definitions must be separated by newlines, not commas. " + detail
+					}
+
 					diags = append(diags, &hcl.Diagnostic{
 						Severity: hcl.DiagError,
-						Summary:  "Missing newline after argument",
-						Detail:   "An argument definition must end with a newline.",
+						Summary:  summary,
+						Detail:   detail,
 						Subject:  &end.Range,
 						Context:  hcl.RangeBetween(ident.Range, end.Range).Ptr(),
 					})

hcl/hclsyntax/parser_test.go

@@ -2030,6 +2030,47 @@ block "valid" {}
 				},
 			},
 		},
+		{
+			"a = 1,",
+			1,
+			&Body{
+				Attributes: Attributes{
+					"a": {
+						Name: "a",
+						Expr: &LiteralValueExpr{
+							Val: cty.NumberIntVal(1),
+
+							SrcRange: hcl.Range{
+								Start: hcl.Pos{Line: 1, Column: 5, Byte: 4},
+								End:   hcl.Pos{Line: 1, Column: 6, Byte: 5},
+							},
+						},
+
+						SrcRange: hcl.Range{
+							Start: hcl.Pos{Line: 1, Column: 1, Byte: 0},
+							End:   hcl.Pos{Line: 1, Column: 6, Byte: 5},
+						},
+						NameRange: hcl.Range{
+							Start: hcl.Pos{Line: 1, Column: 1, Byte: 0},
+							End:   hcl.Pos{Line: 1, Column: 2, Byte: 1},
+						},
+						EqualsRange: hcl.Range{
+							Start: hcl.Pos{Line: 1, Column: 3, Byte: 2},
+							End:   hcl.Pos{Line: 1, Column: 4, Byte: 3},
+						},
+					},
+				},
+				Blocks: Blocks{},
+				SrcRange: hcl.Range{
+					Start: hcl.Pos{Line: 1, Column: 1, Byte: 0},
+					End:   hcl.Pos{Line: 1, Column: 7, Byte: 6},
+				},
+				EndRange: hcl.Range{
+					Start: hcl.Pos{Line: 1, Column: 7, Byte: 6},
+					End:   hcl.Pos{Line: 1, Column: 7, Byte: 6},
+				},
+			},
+		},
 		{
 			"a = sort(data.first.ref.attr)[count.index]\n",
 			0,