Confusing error msg for lifetime mismatch (possibly a bug in lifetime inference as well)

[daniel-vainsencher]

The problem is demonstrated by compiling [1]. I can work around the problem by using [2](first half of the diff is relevant to this problem, second part not so much), or by patching ndarrays to give parameters of binary operators distinct lifetime parameters.

A few problems here:

1. The basic error message is not very clear:
   src/lib.rs:30:28: 30:38 error: cannot infer an appropriate lifetime for borrow expression due to conflicting requirements [E0495]
   src/lib.rs:30 let delta = data - &self.mean;
   <arrow pointing at &self.mean>
   Lifetime of what expression cannot be inferred? What are the conflicting requirements? as someone with an only approximate mental model of lifetimes (i.e., a typical rust user) this is not helping me a lot to solve the problem nor to learn about lifetimes.
2. The compiler suggests something that doesn't work. I don't expect suggestions to always work, but maybe its worth pointing this case out so that the suggestion can be improved or made conditional.
   src/lib.rs:27:5: 38:6 help: consider using an explicit lifetime parameter as shown: fn next_value<'a>(&'a mut self, data: &'a V, weight: f64)
   src/lib.rs:27 pub fn next_value(&mut self, data : &V, weight : f64) {
3. @bluss suggested that rustc should be able to infer for the self and data borrows the intersection of their lifetimes, hence this should not be happening at all. I have no idea.
4. The work around in [2] suggested by @bluss leaves me even more confused. I can change the lifetime of data by ... (what is that, actually, reborrowing it?), and then the borrow of self is ok? But if that's a good way to go about things, maybe the compiler should suggest this method sometimes.

Anyway, if any part of the documentation (error: no extended information for [E0495]) can enlighten me, pointers accepted gladly.

[1] daniel-vainsencher/online_weighted_stats@8f9cf03
[2] daniel-vainsencher/online_weighted_stats@0bfc04d

[bluss]

I think this is a representative minimization. Playground link.

use std::ops::Add;

struct Foo;

impl<'a> Add<&'a Foo> for &'a Foo {
    type Output = ();
    fn add(self, rhs: &'a Foo) {}
}

fn try_to_add(input: &Foo) {
    let local = Foo;
    &*input + &local; // ok
    input + &local; // error: `local` does not live long enough
}

fn main() {
}

[daniel-vainsencher]

I get completely different error messages at your playground link, btw,
though still somewhat confusing: it is not clear why the block suffix does
not suffice, seems to cover the relevant part of the anonymous lifetime #1.

On Tue, Mar 1, 2016 at 11:34 PM, bluss [email protected] wrote:

I think this is a representative minimization. Playground link.
https://play.rust-lang.org/?gist=59e6b8c113e4e7eda1fe&version=stable

use std::ops::Add;
struct Foo;
impl<'a> Add<&'a Foo> for &'a Foo {
type Output = ();
fn add(self, rhs: &'a Foo) {}
}
fn try_to_add(input: &Foo) {
let local = Foo;
&*input + &local; // ok
input + &local; // error: local does not live long enough
}
fn main() {
}

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Reply to this email directly or view it on GitHub
#1523 (comment).

Daniel Vainsencher

[nagisa]

Wrong repository?
On Mar 2, 2016 7:28 AM, "Daniel Vainsencher" [email protected]
wrote:

I get completely different error messages at your playground link, btw,
though still somewhat confusing: it is not clear why the block suffix does
not suffice, seems to cover the relevant part of the anonymous lifetime #1.

On Tue, Mar 1, 2016 at 11:34 PM, bluss [email protected] wrote:

I think this is a representative minimization. Playground link.
https://play.rust-lang.org/?gist=59e6b8c113e4e7eda1fe&version=stable

use std::ops::Add;
struct Foo;
impl<'a> Add<&'a Foo> for &'a Foo {
type Output = ();
fn add(self, rhs: &'a Foo) {}
}
fn try_to_add(input: &Foo) {
let local = Foo;
&*input + &local; // ok
input + &local; // error: local does not live long enough
}
fn main() {
}

—
Reply to this email directly or view it on GitHub
#1523 (comment).

Daniel Vainsencher

—
Reply to this email directly or view it on GitHub
#1523 (comment).

[bluss]

Oops, this should be in rust-lang/rust ! I didn't notice the difference. Thanks nagisa.

[ticki]

@daniel-vainsencher, could you open a issue in rust-lang/rust?

[daniel-vainsencher]

Opened in the right repo, copied over relevant information.