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| -rw-r--r-- | src/librustc/infer/region_inference/README.md | 108 |
1 files changed, 59 insertions, 49 deletions
diff --git a/src/librustc/infer/region_inference/README.md b/src/librustc/infer/region_inference/README.md index 4f24b37682e..f5c254a4594 100644 --- a/src/librustc/infer/region_inference/README.md +++ b/src/librustc/infer/region_inference/README.md @@ -121,17 +121,19 @@ every expression, block, and pattern (patterns are considered to "execute" by testing the value they are applied to and creating any relevant bindings). So, for example: - fn foo(x: isize, y: isize) { // -+ - // +------------+ // | - // | +-----+ // | - // | +-+ +-+ +-+ // | - // | | | | | | | // | - // v v v v v v v // | - let z = x + y; // | - ... // | - } // -+ - - fn bar() { ... } +```rust +fn foo(x: isize, y: isize) { // -+ +// +------------+ // | +// | +-----+ // | +// | +-+ +-+ +-+ // | +// | | | | | | | // | +// v v v v v v v // | + let z = x + y; // | + ... // | +} // -+ + +fn bar() { ... } +``` In this example, there is a region for the fn body block as a whole, and then a subregion for the declaration of the local variable. @@ -160,7 +162,9 @@ this, we get a lot of spurious errors around nested calls, in particular when combined with `&mut` functions. For example, a call like this one - self.foo(self.bar()) +```rust +self.foo(self.bar()) +``` where both `foo` and `bar` are `&mut self` functions will always yield an error. @@ -168,20 +172,22 @@ an error. Here is a more involved example (which is safe) so we can see what's going on: - struct Foo { f: usize, g: usize } - ... - fn add(p: &mut usize, v: usize) { - *p += v; - } - ... - fn inc(p: &mut usize) -> usize { - *p += 1; *p - } - fn weird() { - let mut x: Box<Foo> = box Foo { ... }; - 'a: add(&mut (*x).f, - 'b: inc(&mut (*x).f)) // (..) - } +```rust +struct Foo { f: usize, g: usize } +// ... +fn add(p: &mut usize, v: usize) { + *p += v; +} +// ... +fn inc(p: &mut usize) -> usize { + *p += 1; *p +} +fn weird() { + let mut x: Box<Foo> = box Foo { ... }; + 'a: add(&mut (*x).f, + 'b: inc(&mut (*x).f)) // (..) +} +``` The important part is the line marked `(..)` which contains a call to `add()`. The first argument is a mutable borrow of the field `f`. The @@ -197,16 +203,18 @@ can see that this error is unnecessary. Let's examine the lifetimes involved with `'a` in detail. We'll break apart all the steps involved in a call expression: - 'a: { - 'a_arg1: let a_temp1: ... = add; - 'a_arg2: let a_temp2: &'a mut usize = &'a mut (*x).f; - 'a_arg3: let a_temp3: usize = { - let b_temp1: ... = inc; - let b_temp2: &'b = &'b mut (*x).f; - 'b_call: b_temp1(b_temp2) - }; - 'a_call: a_temp1(a_temp2, a_temp3) // (**) - } +```rust +'a: { + 'a_arg1: let a_temp1: ... = add; + 'a_arg2: let a_temp2: &'a mut usize = &'a mut (*x).f; + 'a_arg3: let a_temp3: usize = { + let b_temp1: ... = inc; + let b_temp2: &'b = &'b mut (*x).f; + 'b_call: b_temp1(b_temp2) + }; + 'a_call: a_temp1(a_temp2, a_temp3) // (**) +} +``` Here we see that the lifetime `'a` includes a number of substatements. In particular, there is this lifetime I've called `'a_call` that @@ -225,19 +233,21 @@ it will not be *dereferenced* during the evaluation of the second argument, it can still be *invalidated* by that evaluation. Consider this similar but unsound example: - struct Foo { f: usize, g: usize } - ... - fn add(p: &mut usize, v: usize) { - *p += v; - } - ... - fn consume(x: Box<Foo>) -> usize { - x.f + x.g - } - fn weird() { - let mut x: Box<Foo> = box Foo { ... }; - 'a: add(&mut (*x).f, consume(x)) // (..) - } +```rust +struct Foo { f: usize, g: usize } +// ... +fn add(p: &mut usize, v: usize) { + *p += v; +} +// ... +fn consume(x: Box<Foo>) -> usize { + x.f + x.g +} +fn weird() { + let mut x: Box<Foo> = box Foo { ... }; + 'a: add(&mut (*x).f, consume(x)) // (..) +} +``` In this case, the second argument to `add` actually consumes `x`, thus invalidating the first argument. |