Suggest using a temporary variable to fix borrowck errors #83174

camelid · 2021-03-15T22:20:14Z

In Rust, nesting method calls with both require &mut access to self
produces a borrow-check error:

error[E0499]: cannot borrow `*self` as mutable more than once at a time
 --> src/lib.rs:7:14
  |
7 |     self.foo(self.bar());
  |     ---------^^^^^^^^^^-
  |     |    |   |
  |     |    |   second mutable borrow occurs here
  |     |    first borrow later used by call
  |     first mutable borrow occurs here

That's because Rust has a left-to-right evaluation order, and the method
receiver is passed first. Thus, the argument to the method cannot then
mutate self.

There's an easy solution to this error: just extract a local variable
for the inner argument:

let tmp = self.bar();
self.foo(tmp);

However, the error doesn't give any suggestion of how to solve the
problem. As a result, new users may assume that it's impossible to
express their code correctly and get stuck.

This commit adds a (non-structured) suggestion to extract a local
variable for the inner argument to solve the error. The suggestion uses
heuristics that eliminate most false positives, though there are a few
false negatives (cases where the suggestion should be emitted but is
not). Those other cases can be implemented in a future change.

rust-highfive · 2021-03-15T22:20:16Z

r? @oli-obk

(rust-highfive has picked a reviewer for you, use r? to override)

compiler/rustc_mir/src/borrow_check/diagnostics/conflict_errors.rs

src/test/ui/borrowck/borrowck-assign-comp-idx.stderr

bors · 2021-05-02T17:34:42Z

☔ The latest upstream changes (presumably #84822) made this pull request unmergeable. Please resolve the merge conflicts.

camelid · 2021-06-12T23:45:44Z

Starting with a rebase so this doesn't get too stale.

camelid · 2021-10-20T05:00:55Z

Status update: I've figured out, and implemented, a way to find the locations of
the outer and inner calls.

camelid · 2021-10-20T19:25:39Z

FYI, I just noticed this (pre-existing) test has a typo in it, but it probably shouldn't matter now that MIR borrowck is the default, right?

https://github.com/rust-lang/rust/blob/e6875793dd149345fca224f84d71814a0daa1be7/src/test/ui/borrowck/two-phase-sneaky.rs#L1

camelid

@oli-obk I've made some progress on this. When you get a chance, could you look at the code and the errors? There are a few issues where the wrong span is used, but the false positive rate is much lower 🎉

I left comments on all the test outputs that have issues. I would appreciate suggestions on how to improve the accuracy :)

src/test/ui/borrowck/suggest-local-var-double-mut.stderr

src/test/ui/borrowck/suggest-local-var-imm-and-mut.stderr

camelid · 2021-10-20T19:23:43Z

src/test/ui/borrowck/two-phase-nonrecv-autoref.nll.stderr

+help: ...and then using that local as the argument to this call
+  --> $DIR/two-phase-nonrecv-autoref.rs:51:9
+   |
+LL |         f(f(10));
+   |         ^


This span's not great, but not terrible either.

I like it more than the one that goes to the end of the call as spans won't overlap this way

This regressed with #83174 (comment); the suggestion is no longer reported. I think let's just leave it for now.

src/test/ui/borrowck/two-phase-sneaky.stderr

src/test/ui/codemap_tests/one_line.stderr

src/test/ui/coercion/coerce-overloaded-autoderef-fail.stderr

src/test/ui/issues/issue-18566.stderr

src/test/ui/issues/issue-61623.stderr

bors · 2021-12-10T19:49:42Z

📌 Commit 381b275 has been approved by oli-obk

bors · 2021-12-10T19:49:43Z

🌲 The tree is currently closed for pull requests below priority 100. This pull request will be tested once the tree is reopened.

camelid · 2021-12-10T19:54:58Z

@oli-obk thank you so much for helping me with this PR over the past 9 months! ❤️

Suggest using a temporary variable to fix borrowck errors Fixes rust-lang#77834. In Rust, nesting method calls with both require `&mut` access to `self` produces a borrow-check error: error[E0499]: cannot borrow `*self` as mutable more than once at a time --> src/lib.rs:7:14 | 7 | self.foo(self.bar()); | ---------^^^^^^^^^^- | | | | | | | second mutable borrow occurs here | | first borrow later used by call | first mutable borrow occurs here That's because Rust has a left-to-right evaluation order, and the method receiver is passed first. Thus, the argument to the method cannot then mutate `self`. There's an easy solution to this error: just extract a local variable for the inner argument: let tmp = self.bar(); self.foo(tmp); However, the error doesn't give any suggestion of how to solve the problem. As a result, new users may assume that it's impossible to express their code correctly and get stuck. This commit adds a (non-structured) suggestion to extract a local variable for the inner argument to solve the error. The suggestion uses heuristics that eliminate most false positives, though there are a few false negatives (cases where the suggestion should be emitted but is not). Those other cases can be implemented in a future change.

matthiaskrgr · 2021-12-10T21:39:16Z

@bors r- rollup-
failed in a rollup:
#91756 (comment)
Can you please double check that there is no test failure? Thanks!

In Rust, nesting method calls with both require `&mut` access to `self` produces a borrow-check error: error[E0499]: cannot borrow `*self` as mutable more than once at a time --> src/lib.rs:7:14 | 7 | self.foo(self.bar()); | ---------^^^^^^^^^^- | | | | | | | second mutable borrow occurs here | | first borrow later used by call | first mutable borrow occurs here That's because Rust has a left-to-right evaluation order, and the method receiver is passed first. Thus, the argument to the method cannot then mutate `self`. There's an easy solution to this error: just extract a local variable for the inner argument: let tmp = self.bar(); self.foo(tmp); However, the error doesn't give any suggestion of how to solve the problem. As a result, new users may assume that it's impossible to express their code correctly and get stuck. This commit adds a (non-structured) suggestion to extract a local variable for the inner argument to solve the error. The suggestion uses heuristics that eliminate most false positives, though there are a few false negatives (cases where the suggestion should be emitted but is not). Those other cases can be implemented in a future change.

camelid · 2021-12-10T22:35:36Z

src/test/ui/borrowck/suggest-local-var-imm-and-mut.nll.stderr

+help: try adding a local storing this argument...
+  --> $DIR/suggest-local-var-imm-and-mut.rs:12:22
+   |
+LL |             self.foo(self.bar());
+   |                      ^^^^^^^^^^
+help: ...and then using that local as the argument to this call
+  --> $DIR/suggest-local-var-imm-and-mut.rs:12:13
+   |
+LL |             self.foo(self.bar());
+   |             ^^^^^^^^^^^^^^^^^^^^


It's intriguing that the suggestion doesn't show up in normal mode but does (correctly) show up in nll mode.

camelid · 2021-12-10T22:36:11Z

@bors r=oli-obk

bors · 2021-12-10T22:36:13Z

📌 Commit e273152 has been approved by oli-obk

Suggest using a temporary variable to fix borrowck errors Fixes rust-lang#77834. In Rust, nesting method calls with both require `&mut` access to `self` produces a borrow-check error: error[E0499]: cannot borrow `*self` as mutable more than once at a time --> src/lib.rs:7:14 | 7 | self.foo(self.bar()); | ---------^^^^^^^^^^- | | | | | | | second mutable borrow occurs here | | first borrow later used by call | first mutable borrow occurs here That's because Rust has a left-to-right evaluation order, and the method receiver is passed first. Thus, the argument to the method cannot then mutate `self`. There's an easy solution to this error: just extract a local variable for the inner argument: let tmp = self.bar(); self.foo(tmp); However, the error doesn't give any suggestion of how to solve the problem. As a result, new users may assume that it's impossible to express their code correctly and get stuck. This commit adds a (non-structured) suggestion to extract a local variable for the inner argument to solve the error. The suggestion uses heuristics that eliminate most false positives, though there are a few false negatives (cases where the suggestion should be emitted but is not). Those other cases can be implemented in a future change.

…askrgr Rollup of 6 pull requests Successful merges: - rust-lang#83174 (Suggest using a temporary variable to fix borrowck errors) - rust-lang#89734 (Point at capture points for non-`'static` reference crossing a `yield` point) - rust-lang#90270 (Make `Borrow` and `BorrowMut` impls `const`) - rust-lang#90741 (Const `Option::cloned`) - rust-lang#91548 (Add spin_loop hint for RISC-V architecture) - rust-lang#91721 (Minor improvements to `future::join!`'s implementation) Failed merges: r? `@ghost` `@rustbot` modify labels: rollup

rust-highfive assigned oli-obk Mar 15, 2021

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camelid commented Mar 15, 2021

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camelid commented Oct 20, 2021

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Rollup of 22 pull requests #91756

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Rollup of 6 pull requests #91775

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Rollup of 6 pull requests #91790

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Rollup of 6 pull requests #91799

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