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MIR borrowck: AST error message refers to kind of item that can't be borrowed #46629

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MIR borrowck: AST error message refers to kind of item that can't be borrowed#46629
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matthewjasper
Labels
A-NLLArea: Non-lexical lifetimes (NLL)A-borrow-checkerArea: The borrow checkerA-diagnosticsArea: Messages for errors, warnings, and lintsC-enhancementCategory: An issue proposing an enhancement or a PR with one.NLL-diagnosticsWorking towards the "diagnostic parity" goal
Milestone
Rust 2018 RC

Description

@arielb1
arielb1
opened on Dec 10, 2017

e.g. in E0596:

fn main() {
    let x = 1;
    let y = &mut x; //[ast]~ ERROR [E0596]
                    //[mir]~^ ERROR [E0596]
}

AST borrowck knows that x is an "immutable local variable", but MIR semi-inaccurately calls it an immutable item:

error[E0596]: cannot borrow immutable local variable `x` as mutable (Ast)
  --> /home/ariel/Rust/rust-master/src/test/compile-fail/E0596.rs:16:18
   |
15 |     let x = 1;
   |         - consider changing this to `mut x`
16 |     let y = &mut x; //[ast]~ ERROR [E0596]
   |                  ^ cannot borrow mutably

error[E0596]: cannot borrow immutable item `x` as mutable (Mir)
  --> /home/ariel/Rust/rust-master/src/test/compile-fail/E0596.rs:16:13
   |
16 |     let y = &mut x; //[ast]~ ERROR [E0596]
   |             ^^^^^^ cannot borrow as mutable

error: aborting due to 2 previous errors

The AST code for doing that is actually in mem_categorization, and should be ported to MIR borrowck:

rust/src/librustc/middle/mem_categorization.rs

Line 1450 in 02b4d3d

pub fn descriptive_string(&self, tcx: TyCtxt) -> String {

Activity

  1. arielb1
    added
    A-borrow-checkerArea: The borrow checker
    A-diagnosticsArea: Messages for errors, warnings, and lints
    WG-compiler-nll
    on Dec 10, 2017

  2. arielb1 commented on Dec 10, 2017

    @arielb1
    arielb1
    on Dec 10, 2017
    ContributorAuthor

    I think the mc code sometimes generates ugly error messages - it should be called on the lvalue that causes the mutability error, not on the full lvalue - but you can use it as a reference to improve the errors.

  3. arielb1 commented on Dec 10, 2017

    @arielb1
    arielb1
    on Dec 10, 2017
    ContributorAuthor

    Note: if you do this correctly, you should also be able to reroute closure errors into things like E0387 - "cannot assign to data in a captured outer variable in an Fn closure" instead of the standard "cannot borrow immutable item as mutable". See e.g. the borrow-immutable-upvar-mutation test:

    // 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.

#![feature(unboxed_closures)]

// Tests that we can't assign to or mutably borrow upvars from `Fn`
// closures (issue #17780)

fn set(x: &mut usize) { *x = 5; }

fn to_fn<A,F:Fn<A>>(f: F) -> F { f }
fn to_fn_mut<A,F:FnMut<A>>(f: F) -> F { f }

fn main() {
    // By-ref captures
    {
        let mut x = 0;
        let _f = to_fn(|| x = 42); //~ ERROR cannot assign

        let mut y = 0;
        let _g = to_fn(|| set(&mut y)); //~ ERROR cannot borrow

        let mut z = 0;
        let _h = to_fn_mut(|| { set(&mut z); to_fn(|| z = 42); }); //~ ERROR cannot assign
    }

    // By-value captures
    {
        let mut x = 0;
        let _f = to_fn(move || x = 42); //~ ERROR cannot assign

        let mut y = 0;
        let _g = to_fn(move || set(&mut y)); //~ ERROR cannot borrow

        let mut z = 0;
        let _h = to_fn_mut(move || { set(&mut z); to_fn(move || z = 42); }); //~ ERROR cannot assign
    }
}

  4. arielb1 commented on Dec 10, 2017

    @arielb1
    arielb1
    on Dec 10, 2017
    ContributorAuthor

    In that case, the error message we emit is especially terrible, because it is not obvious that the problem is with the closure:

    error[E0594]: cannot assign to immutable item `x` (Mir)
  --> /home/ariel/Rust/rust-master/src/test/compile-fail/borrow-immutable-upvar-mutation.rs:25:27
   |
25 |         let _f = to_fn(|| x = 42); //~ ERROR cannot assign
   |                           ^^^^^^ cannot mutate

error[E0596]: cannot borrow immutable item `y` as mutable (Mir)
  --> /home/ariel/Rust/rust-master/src/test/compile-fail/borrow-immutable-upvar-mutation.rs:28:31
   |
28 |         let _g = to_fn(|| set(&mut y)); //~ ERROR cannot borrow
   |                               ^^^^^^ cannot borrow as mutable

error[E0594]: cannot assign to immutable item `x` (Mir)
  --> /home/ariel/Rust/rust-master/src/test/compile-fail/borrow-immutable-upvar-mutation.rs:37:32
   |
37 |         let _f = to_fn(move || x = 42); //~ ERROR cannot assign
   |                                ^^^^^^ cannot mutate

error[E0596]: cannot borrow immutable item `y` as mutable (Mir)
  --> /home/ariel/Rust/rust-master/src/test/compile-fail/borrow-immutable-upvar-mutation.rs:40:36
   |
40 |         let _g = to_fn(move || set(&mut y)); //~ ERROR cannot borrow
   |                                    ^^^^^^ cannot borrow as mutable

error[E0387]: cannot assign to data in a captured outer variable in an `Fn` closure (Ast)
  --> /home/ariel/Rust/rust-master/src/test/compile-fail/borrow-immutable-upvar-mutation.rs:25:27
   |
25 |         let _f = to_fn(|| x = 42); //~ ERROR cannot assign
   |                           ^^^^^^
   |
help: consider changing this closure to take self by mutable reference
  --> /home/ariel/Rust/rust-master/src/test/compile-fail/borrow-immutable-upvar-mutation.rs:25:24
   |
25 |         let _f = to_fn(|| x = 42); //~ ERROR cannot assign
   |                        ^^^^^^^^^

error[E0387]: cannot borrow data mutably in a captured outer variable in an `Fn` closure (Ast)
  --> /home/ariel/Rust/rust-master/src/test/compile-fail/borrow-immutable-upvar-mutation.rs:28:36
   |
28 |         let _g = to_fn(|| set(&mut y)); //~ ERROR cannot borrow
   |                                    ^
   |
help: consider changing this closure to take self by mutable reference
  --> /home/ariel/Rust/rust-master/src/test/compile-fail/borrow-immutable-upvar-mutation.rs:28:24
   |
28 |         let _g = to_fn(|| set(&mut y)); //~ ERROR cannot borrow
   |                        ^^^^^^^^^^^^^^

error[E0387]: cannot assign to data in a captured outer variable in an `Fn` closure (Ast)
  --> /home/ariel/Rust/rust-master/src/test/compile-fail/borrow-immutable-upvar-mutation.rs:31:55
   |
31 |         let _h = to_fn_mut(|| { set(&mut z); to_fn(|| z = 42); }); //~ ERROR cannot assign
   |                                                       ^^^^^^
   |
help: consider changing this closure to take self by mutable reference
  --> /home/ariel/Rust/rust-master/src/test/compile-fail/borrow-immutable-upvar-mutation.rs:31:52
   |
31 |         let _h = to_fn_mut(|| { set(&mut z); to_fn(|| z = 42); }); //~ ERROR cannot assign
   |                                                    ^^^^^^^^^

error[E0594]: cannot assign to captured outer variable in an `Fn` closure (Ast)
  --> /home/ariel/Rust/rust-master/src/test/compile-fail/borrow-immutable-upvar-mutation.rs:37:32
   |
36 |         let mut x = 0;
   |             ----- help: consider making `mut x` mutable: `mut mut x`
37 |         let _f = to_fn(move || x = 42); //~ ERROR cannot assign
   |                                ^^^^^^
   |
help: consider changing this closure to take self by mutable reference
  --> /home/ariel/Rust/rust-master/src/test/compile-fail/borrow-immutable-upvar-mutation.rs:37:24
   |
37 |         let _f = to_fn(move || x = 42); //~ ERROR cannot assign
   |                        ^^^^^^^^^^^^^^

error[E0596]: cannot borrow captured outer variable in an `Fn` closure as mutable (Ast)
  --> /home/ariel/Rust/rust-master/src/test/compile-fail/borrow-immutable-upvar-mutation.rs:40:41
   |
40 |         let _g = to_fn(move || set(&mut y)); //~ ERROR cannot borrow
   |                                         ^
   |
help: consider changing this closure to take self by mutable reference
  --> /home/ariel/Rust/rust-master/src/test/compile-fail/borrow-immutable-upvar-mutation.rs:40:24
   |
40 |         let _g = to_fn(move || set(&mut y)); //~ ERROR cannot borrow
   |                        ^^^^^^^^^^^^^^^^^^^

error[E0594]: cannot assign to captured outer variable in an `Fn` closure (Ast)
  --> /home/ariel/Rust/rust-master/src/test/compile-fail/borrow-immutable-upvar-mutation.rs:43:65
   |
42 |         let mut z = 0;
   |             ----- help: consider making `mut z` mutable: `mut mut z`
43 |         let _h = to_fn_mut(move || { set(&mut z); to_fn(move || z = 42); }); //~ ERROR cannot assign
   |                                                                 ^^^^^^
   |
help: consider changing this closure to take self by mutable reference
  --> /home/ariel/Rust/rust-master/src/test/compile-fail/borrow-immutable-upvar-mutation.rs:43:57
   |
43 |         let _h = to_fn_mut(move || { set(&mut z); to_fn(move || z = 42); }); //~ ERROR cannot assign
   |                                                         ^^^^^^^^^^^^^^

warning: variable does not need to be mutable
  --> /home/ariel/Rust/rust-master/src/test/compile-fail/borrow-immutable-upvar-mutation.rs:36:13
   |
36 |         let mut x = 0;
   |             ---^^
   |             |
   |             help: remove this `mut`
   |
   = note: #[warn(unused_mut)] on by default

warning: variable does not need to be mutable
  --> /home/ariel/Rust/rust-master/src/test/compile-fail/borrow-immutable-upvar-mutation.rs:39:13
   |
39 |         let mut y = 0;
   |             ---^^
   |             |
   |             help: remove this `mut`

error[E0594]: cannot assign to immutable item `z` (Mir)
  --> /home/ariel/Rust/rust-master/src/test/compile-fail/borrow-immutable-upvar-mutation.rs:31:55
   |
31 |         let _h = to_fn_mut(|| { set(&mut z); to_fn(|| z = 42); }); //~ ERROR cannot assign
   |                                                       ^^^^^^ cannot mutate

error[E0594]: cannot assign to immutable item `z` (Mir)
  --> /home/ariel/Rust/rust-master/src/test/compile-fail/borrow-immutable-upvar-mutation.rs:43:65
   |
43 |         let _h = to_fn_mut(move || { set(&mut z); to_fn(move || z = 42); }); //~ ERROR cannot assign
   |                                                                 ^^^^^^ cannot mutate

error: aborting due to 12 previous errors
  5. pnkfelix
    added
    A-NLLArea: Non-lexical lifetimes (NLL)
    on Jan 10, 2018
  6. XAMPPRocky
    added
    C-enhancementCategory: An issue proposing an enhancement or a PR with one.
    on Mar 26, 2018

  7. nikomatsakis commented on Apr 3, 2018

    @nikomatsakis
    nikomatsakis
    on Apr 3, 2018
    Contributor

    Semi-dup of #47388

  8. nikomatsakis
    added
    NLL-diagnosticsWorking towards the "diagnostic parity" goal
    on Apr 3, 2018

  9. nikomatsakis commented on Jul 3, 2018

    @nikomatsakis
    nikomatsakis
    on Jul 3, 2018
    Contributor

    It seems like we still say things like "immutable item". This doesn't seem great. Tagging for release candidate.

  10. nikomatsakis commented on Jul 3, 2018

    @nikomatsakis
    nikomatsakis
    on Jul 3, 2018
    Contributor
    athena. rustc +nightly E0596.rs -Zborrowck=mir
warning: unused variable: `y`
  --> E0596.rs:16:9
   |
16 |     let y = &mut x; //[ast]~ ERROR [E0596]
   |         ^ help: consider using `_y` instead
   |
   = note: #[warn(unused_variables)] on by default

error[E0596]: cannot borrow immutable item `x` as mutable
  --> E0596.rs:16:13
   |
15 |     let x = 1;
   |         - help: consider changing this to be mutable: `mut x`
16 |     let y = &mut x; //[ast]~ ERROR [E0596]
   |             ^^^^^^ cannot borrow as mutable

error: aborting due to previous error

  11. nikomatsakis commented on Jul 3, 2018

    @nikomatsakis
    nikomatsakis
    on Jul 3, 2018
    Contributor

    Honestly, I think I would prefer to see this phrased rather differently. Something like:

    "cannot borrow x mutably, as it is not declared as mutable"

  12. nikomatsakis
    added this to the Rust 2018 Release Candidate milestone on Jul 3, 2018
  13. matthewjasper
    self-assigned this
    on Jul 9, 2018
  14. bors
    added a commit that references this issue on Jul 21, 2018

    Auto merge of #52405 - matthewjasper:mutability-errors, r=pnkfelix

    942b384
  15. bors
    closed this as completedin #52405on Jul 21, 2018
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