Confusing about liveness-based constraints for lifetimes in `nll` rfc

help
1

Here gives a control flow graph:

// let mut foo: i32;
// let mut bar: i32;
// let mut p: &i32;

A
[ p = &foo     ]
[ if condition ] ----\ (true)
       |             |
       |     B       v
       |     [ print(*p)     ]
       |     [ ...           ]
       |     [ p = &bar      ]
       |     [ ...           ]
       |     [ goto C        ]
       |             |
       +-------------/
       |
C      v
[ print(*p)    ]
[ return       ]

Here and Here all say that 'p doesn't include B/1 but include B/3.

So what's the difference between ... and ...???
Is the meaning of the first ... different from meaing of the second ...?

2

The meaning is the same (... is just a placeholder), but the liveness of p differs for the two lines.

#![allow(unused)]
fn main() {
let mut foo: T = ...;
let mut bar: T = ...;
let mut p: &'p T = &foo;
// `p` is live here: its value may be used on the next line.
if condition {
   // `p` is live here: its value will be used on the next line.
   print(*p);
   // `p` is DEAD here: its value will not be used.
   p = &bar;
   // `p` is live here: its value will be used later.
}
// `p` is live here: its value may be used on the next line.
print(*p);
// `p` is DEAD here: its value will not be used.
}

So the difference is, p is dead at B/1 but live at B/3.

1 Like
3

a placeholder for what?
why isn't it like this?

A
[ p = &foo     ]
[ if condition ] ----\ (true)
       |             |
       |     B       v
       |     [ ...           ]
       |     [ print(*p)     ]
       |     [ ...           ]
       |     [ p = &bar      ]
       |     [ ...           ]
       |     [ goto C        ]
       |             |
       +-------------/
       |
C      v
[ print(*p)    ]
[ return       ]

what's the reason of inserting the placeholder?

4

So that one can unambiguously refer to the region of code between p's use and reassignment, for example.

1 Like
5

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