% cat test.c
    main () {
      int x
      x=1
    }
    % gcc test.c  
    test.c:1:1: warning: type specifier missing, defaults to 'int' [-Wimplicit-int]
    main () {
    ^
    test.c:2:8: error: expected ';' at end of declaration
      int x
           ^
           ;
    1 warning and 1 error generated.

   tst.c: In function ‘main’:
   tst.c:3:5: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ before ‘x’
       3 |     x=1
         |     ^
   tst.c:3:5: error: ‘x’ undeclared (first use in this function)
   tst.c:3:5: note: each undeclared identifier is reported only once for each function it appears in
   tst.c:3:8: error: expected ‘;’ before ‘}’ token
       3 |     x=1
         |        ^
         |        ;
       4 | }
         | ~       

I get three errors, all on line 3 rather than 2, and as the first of them says, there are at least four alternatives for resolution besides semicolon.

Full code after adding type to main, including linter message from c.vim:

         1 int main () {                                                                        
         2     int x                                                                            
     E   3     x=1 /\* E: 'x' undeclared (first use in this function)                            
         4 }

.