What is 6th unary operator?

[khiri-co]

In reference 7.2.11 Unary operator expressions

Rust defines six symbolic unary operators.

But in document there is only 5 ops; '-', '*', '!', 'box', '&'.
What is the 6th unary op?

By the way, in Rust '+' is not an unary op. So this expression is invalid; 'let a = +(1+2);'.
Considering the symmetry of "+" & '-', I think '+' may be an unary operator.
Of course, '+' unary op has no effect.

[nagisa]

.. is unary operator in some contexts.

In general reference is somewhat out-of-date, so there might actually be any number of operators not mentioned in reference and introduced recently.

[japaric]

TL;DR Probably the deprecated ~ operator, that was used to box things. ~"foo" created a Box<str>, where as ~[1, 2] created a Box<[T]>.

According to the compiler internals, we have 4 unary operators (in the AST):

(Operator name : expression)

• UnUniq: ~"string" (which is deprecated syntax)
• UnDeref: *boxed_thing
• UnNot: !bits
• UnNeg: -num

From the POV of the compiler, the feature gated box thing is an expression. And references &foo/&mut bar are also expressions.

You may think of & as the "address of" operator, and box as the "box" operator. But that's not quite right, because when you use them in patterns like let &e = &(2, 3), they don't perform any operation at all.

[japaric]

(Oh and the reference is probably out of date in several places, we haven't put effort in updating it yet)