More descriptive error messages for invalid lvalues

For '(a) = b': 'syntax error' -> 'expected statement'
For 'a, (b) = c': 'syntax error' -> 'expected identifier or field'
For 'a, b() = c': 'syntax error' -> 'expected call or indexing'

spec/parser_spec.lua

@@ -395,7 +395,7 @@ describe("parser", function()
          assert.same({line = 1, column = 2, end_column = 2, msg = "expected '=' near <eof>"}, get_error("a"))
          assert.same({line = 1, column = 5, end_column = 5, msg = "expected expression near <eof>"}, get_error("a = "))
          assert.same({line = 1, column = 5, end_column = 5, msg = "expected statement near '='"}, get_error("a() = b"))
-         assert.same({line = 1, column = 5, end_column = 5, msg = "syntax error near '='"}, get_error("(a) = b"))
+         assert.same({line = 1, column = 1, end_column = 1, msg = "expected statement near '('"}, get_error("(a) = b"))
          assert.same({line = 1, column = 1, end_column = 1, msg = "expected statement near '1'"}, get_error("1 = b"))
       end)
 
@@ -412,8 +412,8 @@ describe("parser", function()
          assert.same({line = 1, column = 4, end_column = 4, msg = "expected identifier or field near '='"}, get_error("a, = b"))
          assert.same({line = 1, column = 8, end_column = 8, msg = "expected expression near <eof>"}, get_error("a, b = "))
          assert.same({line = 1, column = 10, end_column = 10, msg = "expected expression near <eof>"}, get_error("a, b = c,"))
-         assert.same({line = 1, column = 8, end_column = 8, msg = "syntax error near '='"}, get_error("a, b() = c"))
-         assert.same({line = 1, column = 8, end_column = 8, msg = "syntax error near '='"}, get_error("a, (b) = c"))
+         assert.same({line = 1, column = 8, end_column = 8, msg = "expected call or indexing near '='"}, get_error("a, b() = c"))
+         assert.same({line = 1, column = 4, end_column = 4, msg = "expected identifier or field near '('"}, get_error("a, (b) = c"))
       end)
    end)
 

src/luacheck/parser.lua

@@ -68,7 +68,6 @@ local function token_name(token)
 end
 
 local function parse_error(state, msg)
-   msg = msg or "syntax error"
    local token_repr, end_column
 
    if state.token == "eof" then
@@ -626,18 +625,19 @@ local function parse_expression_statement(state, loc)
 
    repeat
       local first_token = state.token
+      local first_loc = lhs and location(state) or loc
       local expected = lhs and "identifier or field" or "statement"
       local primary_expression, is_prefix = parse_primary_expression(state, expected)
 
       if is_prefix and first_token == "(" then
          -- (expr) is invalid.
-         parse_error(state)
+         lexer.syntax_error(first_loc, first_loc.column, "expected " .. expected .. " near '('")
       end
 
       if primary_expression.tag == "Call" or primary_expression.tag == "Invoke" then
          if lhs then
             -- This is an assingment, and a call is not a valid lvalue.
-            parse_error(state)
+            parse_error(state, "expected call or indexing")
          else
             -- It is a call.
             primary_expression.location = loc