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rustc: Tweak the borrow on closure invocations
This alters the borrow checker's requirements on invoking closures from
requiring an immutable borrow to requiring a unique immutable borrow. This means
that it is illegal to invoke a closure through a `&` pointer because there is no
guarantee that is not aliased. This does not mean that a closure is required to
be in a mutable location, but rather a location which can be proven to be
unique (often through a mutable pointer).
For example, the following code is unsound and is no longer allowed:
type Fn<'a> = ||:'a;
fn call(f: |Fn|) {
f(|| {
f(|| {})
});
}
fn main() {
call(|a| {
a();
});
}
There is no replacement for this pattern. For all closures which are stored in
structures, it was previously allowed to invoke the closure through `&self` but
it now requires invocation through `&mut self`.
The standard library has a good number of violations of this new rule, but the
fixes will be separated into multiple breaking change commits.
Closes rust-lang#12224
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64 changes: 64 additions & 0 deletions
64
src/test/compile-fail/borrowck-call-is-borrow-issue-12224.rs
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,64 @@ | ||
| // Copyright 2014 The Rust Project Developers. See the COPYRIGHT | ||
| // file at the top-level directory of this distribution and at | ||
| // http://rust-lang.org/COPYRIGHT. | ||
| // | ||
| // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or | ||
| // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license | ||
| // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your | ||
| // option. This file may not be copied, modified, or distributed | ||
| // except according to those terms. | ||
|
|
||
| // Ensure that invoking a closure counts as a unique immutable borrow | ||
|
|
||
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|
||
| type Fn<'a> = ||:'a; | ||
|
|
||
| struct Test<'a> { | ||
| f: ||: 'a | ||
| } | ||
|
|
||
| fn call(f: |Fn|) { | ||
| f(|| { | ||
| //~^ ERROR: closure requires unique access to `f` but it is already borrowed | ||
| f(|| {}) | ||
| }); | ||
| } | ||
|
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||
| fn test1() { | ||
| call(|a| { | ||
| a(); | ||
| }); | ||
| } | ||
|
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||
| fn test2(f: &||) { | ||
| (*f)(); //~ ERROR: closure invocation in a `&` reference | ||
| } | ||
|
|
||
| fn test3(f: &mut ||) { | ||
| (*f)(); | ||
| } | ||
|
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||
| fn test4(f: &Test) { | ||
| (f.f)() //~ ERROR: closure invocation in a `&` reference | ||
| } | ||
|
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||
| fn test5(f: &mut Test) { | ||
| (f.f)() | ||
| } | ||
|
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||
| fn test6() { | ||
| let f = || {}; | ||
| (|| { | ||
| f(); | ||
| })(); | ||
| } | ||
|
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||
| fn test7() { | ||
| fn foo(_: |g: |int|, b: int|) {} | ||
| let f = |g: |int|, b: int| {}; | ||
| f(|a| { //~ ERROR: cannot borrow `f` as immutable because previous closure | ||
| foo(f); //~ ERROR: cannot move out of captured outer variable | ||
| }, 3); | ||
| } | ||
|
|
||
| fn main() {} |