433a13b473
Suggest using a temporary variable to fix borrowck errors Fixes #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. |
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