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Attempted documentation of coercions

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Trying to summarize here only the cases that will make sense at the
level of the rust book
This commit is contained in:
Martin Pool 2015-11-27 17:35:12 -08:00
parent f34e6ff084
commit d6952121e0
1 changed files with 19 additions and 3 deletions
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22
src/doc/book/casting-between-types.md
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@ -5,6 +5,22 @@ different types between each other. The first, `as`, is for safe casts.
In contrast, `transmute` allows for arbitrary casting, and is one of the
most dangerous features of Rust!
# Coercion
Coercion between types is implicit and has no explicit syntax. Coercion occurs
in `let`, `const`, and `static` statements; in function call arguments; in
field values in struct initialization; and in a function result.
The main cases of coercion are:
* `&mut T` to `&T`
* `*mut T` to `*const T`
* `&T` to `*const T`
* `&mut T` to `*mut T`
# `as`
The `as` keyword does safe casting:
@ -31,10 +47,10 @@ For example:
```rust
let a = "hello";
let b = a as String
let b = a as String;
```
Coercions always occur implicitly so this form is only for clarity.
All coercions will be made implicitly when necessary and unambiguous.
## Numeric casts
@ -58,7 +74,7 @@ Perhaps surprisingly, it is safe to cast pointers to and from integers, and
to cast between pointers to different types subject to some constraints. It
is only unsafe to dereference the pointer.
* `e` has type `*T`, `U` is a pointer to `*U_0`, and either `U_0: Sized` or
* `e` has type `*T`, `U` has type `*U_0`, and either `U_0: Sized` or
unsize_kind(`T`) = unsize_kind(`U_0`); a *ptr-ptr-cast*
* `e` has type `*T` and `U` is a numeric type, while `T: Sized`; *ptr-addr-cast*
* `e` is an integer and `U` is `*U_0`, while `U_0: Sized`; *addr-ptr-cast*