Modify `AttributeTemplate` to support list of alternatives for list and name value attribute styles.
Suggestions now provide more correct suggested code:
```
error[E0805]: malformed `used` attribute input
--> $DIR/used_with_multi_args.rs:3:1
|
LL | #[used(compiler, linker)]
| ^^^^^^------------------^
| |
| expected a single argument here
|
help: try changing it to one of the following valid forms of the attribute
|
LL - #[used(compiler, linker)]
LL + #[used(compiler)]
|
LL - #[used(compiler, linker)]
LL + #[used(linker)]
|
LL - #[used(compiler, linker)]
LL + #[used]
|
```
instead of the prior "masking" of the lack of this feature by suggesting pipe-separated lists:
```
error[E0805]: malformed `used` attribute input
--> $DIR/used_with_multi_args.rs:3:1
|
LL | #[used(compiler, linker)]
| ^^^^^^------------------^
| |
| expected a single argument here
|
help: try changing it to one of the following valid forms of the attribute
|
LL - #[used(compiler, linker)]
LL + #[used(compiler|linker)]
|
LL - #[used(compiler, linker)]
LL + #[used]
|
```
This makes it possible for the `unsafe(...)` syntax to only be
valid at the top level, and the `NestedMetaItem`s will automatically
reject `unsafe(...)`.
