That's not a particularly hard problem, all tools which try to solve it (for example, IntelliJ), solve it easily. For example, here's the syntax tree that rust-analyzer builds for this example:

    fn foo() { }

    fn

    fn bar() { }

Syntax tree (whitespace nodes elided for clarity):

    SOURCE_FILE@[0; 31)
      FN_DEF@[0; 12)
        FN_KW@[0; 2)
        NAME@[3; 6)
          IDENT@[3; 6) "foo"
        ...
      FN_DEF@[14; 16)
        FN_KW@[14; 16)
        err: `expected a name`
      FN_DEF@[18; 30)
        FN_KW@[18; 20)
        NAME@[21; 24)
          IDENT@[21; 24) "bar"
        ...

The problem is, a lot of tools are build with the assumption that the code is correct&complete, and don't try to tackle this problem at all.