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Rollup merge of #86477 - tlyu:e0716-clarification, r=JohnTitor

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E0716: clarify that equivalent code example is erroneous

In E0716, there is a code block that is equivalent to the erroneous
code example. Especially when viewed with `rustc --explain`, it's
not obvious that it is also erroneous, and some users have been
confused when they try to change their code to match the erroneous
equivalent.

`@rustbot` label +A-diagnostics +D-newcomer-roadblock +T-compiler
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Yuki Okushi 2021-07-05 07:13:22 +09:00 • committed by GitHub
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6
compiler/rustc_error_codes/src/error_codes/E0716.md
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@ -14,14 +14,16 @@ Here, the expression `&foo()` is borrowing the expression `foo()`. As `foo()` is
a call to a function, and not the name of a variable, this creates a a call to a function, and not the name of a variable, this creates a
**temporary** -- that temporary stores the return value from `foo()` so that it **temporary** -- that temporary stores the return value from `foo()` so that it
can be borrowed. You could imagine that `let p = bar(&foo());` is equivalent to can be borrowed. You could imagine that `let p = bar(&foo());` is equivalent to
this: the following, which uses an explicit temporary variable.
Erroneous code example:
```compile_fail,E0597 ```compile_fail,E0597
# fn foo() -> i32 { 22 } # fn foo() -> i32 { 22 }
# fn bar(x: &i32) -> &i32 { x } # fn bar(x: &i32) -> &i32 { x }
let p = { let p = {
let tmp = foo(); // the temporary let tmp = foo(); // the temporary
bar(&tmp) bar(&tmp) // error: `tmp` does not live long enough
}; // <-- tmp is freed as we exit this block }; // <-- tmp is freed as we exit this block
let q = p; let q = p;
``` ```