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Modify RFC #803 (type ascription) to make type ascription expressions lvalues #987

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Merged
nikomatsakis merged 3 commits into from
Jun 10, 2015
Merged

Modify RFC #803 (type ascription) to make type ascription expressions lvalues #987

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906439d
Modify RFC #803 (type ascription) to make type ascription expressions…
Mar 18, 2015
18fe78e
Update the text about lvalues
Mar 23, 2015
d69cf92
Clarify text about lvalues
Jun 10, 2015
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Modify RFC #803 (type ascription) to make type ascription expressions… 
… lvalues.
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Nick Cameron committed Mar 23, 2015
commit 906439df42ad86d8bb3ae6ffa27d32dd1823cd06
14 changes: 4 additions & 10 deletions text/0803-type-ascription.md
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Expand Up @@ -172,10 +172,10 @@ lvalue position), then we don't have the soundness problem, but we do get the
unexpected result that `&(x: T)` is not in fact a reference to `x`, but a
reference to a temporary copy of `x`.

The proposed solution is that type ascription expressions are rvalues, but
taking a reference of such an expression is forbidden. I.e., type asciption is
forbidden in the following contexts (where `<expr>` is a type ascription
expression):
The proposed solution is that type ascription expressions are lvalues, where
the type ascription expression is in reference context, then we require the
ascribed type to exactly match the type of the expression, i.e., neither
subtyping nor coercion is allowed. These contexts are:

```
&[mut] <expr>
Expand All @@ -184,12 +184,6 @@ match <expr> { .. ref [mut] x .. => { .. } .. }
<expr>.foo() // due to autoref
```

Like other rvalues, type ascription would not be allowed as the lhs of assignment.

Note that, if type asciption is required in such a context, an lvalue can be
forced by using `{}`, e.g., write `&mut { foo: T }`, rather than `&mut (foo: T)`.


# Drawbacks

More syntax, another feature in the language.
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