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Rollup merge of #67934 - GuillaumeGomez:clean-up-e0178, r=Dylan-DPC

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Clean up E0178 explanation

r? @Dylan-DPC
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Yuki Okushi 2020-01-07 13:46:12 +09:00 • committed by GitHub
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src/librustc_error_codes/error_codes/E0178.md
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@ -1,16 +1,27 @@
In types, the `+` type operator has low precedence, so it is often necessary
to use parentheses.
The `+` type operator was used in an ambiguous context.
For example:
Erroneous code example:
```compile_fail,E0178
trait Foo {}
struct Bar<'a> {
w: &'a Foo + Copy, // error, use &'a (Foo + Copy)
x: &'a Foo + 'a, // error, use &'a (Foo + 'a)
y: &'a mut Foo + 'a, // error, use &'a mut (Foo + 'a)
z: fn() -> Foo + 'a, // error, use fn() -> (Foo + 'a)
x: &'a Foo + 'a, // error!
y: &'a mut Foo + 'a, // error!
z: fn() -> Foo + 'a, // error!
}
```
In types, the `+` type operator has low precedence, so it is often necessary
to use parentheses:
```
trait Foo {}
struct Bar<'a> {
x: &'a (Foo + 'a), // ok!
y: &'a mut (Foo + 'a), // ok!
z: fn() -> (Foo + 'a), // ok!
}
```