two-phase borrows are causing #[feature(nll)] to reject a mutable borrow incorrectly #48070
Description
In this example, the NLL checker incorrectly judges foo to be borrowed more than once:
#![feature(nll)]
struct Foo {
x: u32
}
impl Foo {
fn twiddle(&mut self) -> &mut Self { self }
fn twaddle(&mut self) -> &mut Self { self }
fn emit(&mut self) {
self.x += 1;
}
}
fn main() {
let mut foo = Foo { x: 0 };
match 22 {
22 => &mut foo,
44 => foo.twiddle(),
_ => foo.twaddle(),
}.emit();
}I get this error:
error[E0499]: cannot borrow `foo` as mutable more than once at a time
--> src/main.rs:17:5
|
17 | / match 22 {
18 | | 22 => &mut foo,
19 | | 44 => foo.twiddle(),
| | --- first mutable borrow occurs here
20 | | _ => foo.twaddle(),
21 | | }.emit();
| |_____^ second mutable borrow occurs here
This works without NLL. This was found while trying to bootstrap rustc with NLL enabled, where it manifests as:
error[E0499]: cannot borrow `diag_builder` as mutable more than once at a time
--> src/librustc/lint/levels.rs:304:13
|
304 | / match forbid_src {
305 | | LintSource::Default => &mut diag_builder,
306 | | LintSource::Node(_, forbid_source_span) => {
307 | | diag_builder.span_label(forbid_source_span,
... |
311 | | diag_builder.note("`forbid` lint level was set on command line")
| | ------------ first mutable borrow occurs here
312 | | }
313 | | }.emit();
| |_____________^ second mutable borrow occurs here