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Add section on lvals, rvals and temps.

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Graydon Hoare 2012-10-05 14:19:50 -07:00
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doc/rust.md
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@ -1468,6 +1468,25 @@ structure of expressions. Blocks themselves are expressions, so the nesting
sequence of block, statement, expression, and block can repeatedly nest to an
arbitrary depth.
#### Lvalues, rvalues and temporaries
Expressions are divided into two main categories: _lvalues_ and _rvalues_.
Likewise within each expression, sub-expressions may occur in _lvalue context_ or _rvalue context_.
The evaluation of an expression depends both on its own category and the context it occurs within.
Path, field and index expressions are lvalues.
All other expressions are rvalues.
The left operand of an assignment expression and the operand of the borrow operator are lvalue contexts.
All other expression contexts are rvalue contexts.
When an lvalue is evaluated in an _lvalue context_, it denotes a memory location;
when evaluated in an _rvalue context_, it denotes the value held _in_ that memory location.
When an rvalue is used in lvalue context, a temporary un-named lvalue is created and used instead.
A temporary's lifetime equals the largest lifetime of any borrowed pointer that points to it.
### Literal expressions
A _literal expression_ consists of one of the [literal](#literals)