error[E0308]: mismatched types
  --> $DIR/issue-100285.rs:2:5
   |
LL |   fn foo(n: i32) -> i32 {
   |                     --- expected `i32` because of return type
LL | /     for i in 0..0 {
LL | |
LL | |        if n < 0 {
LL | |         return i;
...  |
LL | |
LL | |     }
   | |_____^ expected `i32`, found `()`
   |
note: the function expects a value to always be returned, but loops might run zero times
  --> $DIR/issue-100285.rs:2:5
   |
LL |     for i in 0..0 {
   |     ^^^^^^^^^^^^^ this might have zero elements to iterate on
...
LL |         return i;
   |         -------- if the loop doesn't execute, this value would never get returned
LL |         } else if n < 10 {
LL |           return 1;
   |           -------- if the loop doesn't execute, this value would never get returned
LL |         } else if n < 20 {
LL |           return 2;
   |           -------- if the loop doesn't execute, this value would never get returned
   = note: if the loop doesn't execute, 3 other values would never get returned
   = help: return a value for the case when the loop has zero elements to iterate on, or consider changing the return type to account for that possibility

error: aborting due to previous error

For more information about this error, try `rustc --explain E0308`.