Add support for _Return_value

[secure-sw-dev-bot]

This issue was copied from checkedc/checkedc-clang#205

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Checked C allows return bounds expression for functions to use the keyword _Return_value to denote the return value of a function. It can be used in bounds expressions and bounds can be declared for it.

The proposed work items are:

• Extend the IR with a way to represent Return_value and Current_expr_value The like approach is to create a new kind of expression similar to PositionalParameterExpr, that is used only in bounds expressions.
• Extend the syntax to allow them to be parsed where an identifier can be parsed. Construct the new IR expression to represent it.
• _Return_value should only be allowed in the context of parsing a return bounds expression.
• Current_expr_value should only be allowed in the context of parsing a bounds expression (excluding a return bounds expression).

Note that I need the IR representation for _Current_expr_value soon (in the next few days), so if anyone picks this up soon, they should coordinate the IR work me.

[secure-sw-dev-bot]

Comment from @dtarditi:

I am replacing _Current_expr_value with expression temporaries. This leads to a cleaner, more efficient semantics. With _Current_expr_value, we had to adjust uses of it when inferring bounds expressions. The adjustment had to to offset the change between a subexpression value and the parent expression value. With expression temporaries, we no longer have to make this adjustment. An expression temporary is a temporary variable inserted by the compiler.

The point at which assignment of a value to a temporary variable happens can be controlled precisely This is unlike C semantics for nested assignments to source-level variables. which informally says that "the assignments have to happen at some point before the evaluation of the expression is complete" (more precisely, happen in an order that respects sequence points).

[secure-sw-dev-bot]

Comment from @sulekhark:

This issue got automatically closed by mistake: GitHub closed issue number N in the checkedc-clang repository when a 3C PR that has a comment "Fixes issue N" (where N refers to the issue number in CCI's repository), was merged to master branch in the checkedc-clang repository. Reopening it.