hcl/hclsyntax: Produce better error message for invalid apostrophe

hcl/hclsyntax/parser_test.go

@@ -2109,6 +2109,44 @@ block "valid" {}
 				},
 			},
 		},
+		{
+			`a = 'str'`,
+			2, // Invalid character and expression
+			&Body{
+				Attributes: Attributes{
+					"a": {
+						Name: "a",
+						Expr: &LiteralValueExpr{
+							SrcRange: hcl.Range{
+								Start: hcl.Pos{Line: 1, Column: 5, Byte: 4},
+								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},
+						},
+						SrcRange: hcl.Range{
+							Start: hcl.Pos{Line: 1, Column: 1, Byte: 0},
+							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: 10, Byte: 9},
+				},
+				EndRange: hcl.Range{
+					Start: hcl.Pos{Line: 1, Column: 10, Byte: 9},
+					End:   hcl.Pos{Line: 1, Column: 10, Byte: 9},
+				},
+			},
+		},
 		{
 			"a = sort(data.first.ref.attr)[count.index]\n",
 			0,

hcl/hclsyntax/token.go

@@ -184,6 +184,7 @@ func checkInvalidTokens(tokens Tokens) hcl.Diagnostics {
 	toldBitwise := 0
 	toldExponent := 0
 	toldBacktick := 0
+	toldApostrophe := 0
 	toldSemicolon := 0
 	toldTabs := 0
 	toldBadUTF8 := 0
@@ -238,6 +239,19 @@ func checkInvalidTokens(tokens Tokens) hcl.Diagnostics {
 			if toldBacktick <= 2 {
 				toldBacktick++
 			}
+		case TokenApostrophe:
+			if (toldApostrophe % 2) == 0 {
+				newDiag := &hcl.Diagnostic{
+					Severity: hcl.DiagError,
+					Summary:  "Invalid character",
+					Detail:   "The ' character is not valid. Use double quotes (\") to enclose strings.",
+					Subject:  &tok.Range,
+				}
+				diags = append(diags, newDiag)
+			}
+			if toldApostrophe <= 2 {
+				toldApostrophe++
+			}
 		case TokenSemicolon:
 			if toldSemicolon < 1 {
 				diags = append(diags, &hcl.Diagnostic{