64 lines
2.4 KiB
Plaintext
64 lines
2.4 KiB
Plaintext
Per [RFC 401][rfc401], if you have a function declaration `foo`:
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```
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// For the purposes of this explanation, all of these
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// different kinds of `fn` declarations are equivalent:
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struct S;
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fn foo(x: S) { /* ... */ }
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extern "C" { fn foo(x: S); }
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impl S { fn foo(self) { /* ... */ } }
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```
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the type of `foo` is **not** `fn(S)`, as one might expect.
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Rather, it is a unique, zero-sized marker type written here as `typeof(foo)`.
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However, `typeof(foo)` can be _coerced_ to a function pointer `fn(S)`,
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so you rarely notice this:
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```
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let x: fn(S) = foo; // OK, coerces
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```
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The reason that this matter is that the type `fn(S)` is not specific to
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any particular function: it's a function _pointer_. So calling `x()` results
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in a virtual call, whereas `foo()` is statically dispatched, because the type
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of `foo` tells us precisely what function is being called.
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As noted above, coercions mean that most code doesn't have to be
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concerned with this distinction. However, you can tell the difference
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when using **transmute** to convert a fn item into a fn pointer.
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This is sometimes done as part of an FFI:
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```
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extern "C" fn foo(userdata: Box<i32>) {
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/* ... */
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}
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let f: extern "C" fn(*mut i32) = transmute(foo);
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callback(f);
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```
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Here, transmute is being used to convert the types of the fn arguments.
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This pattern is incorrect because, because the type of `foo` is a function
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**item** (`typeof(foo)`), which is zero-sized, and the target type (`fn()`)
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is a function pointer, which is not zero-sized.
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This pattern should be rewritten. There are a few possible ways to do this:
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- change the original fn declaration to match the expected signature,
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and do the cast in the fn body (the preferred option)
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- cast the fn item of a fn pointer before calling transmute, as shown here:
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```
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let f: extern "C" fn(*mut i32) = transmute(foo as extern "C" fn(_));
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let f: extern "C" fn(*mut i32) = transmute(foo as usize); // works too
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```
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The same applies to transmutes to `*mut fn()`, which were observed in practice.
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Note though that use of this type is generally incorrect.
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The intention is typically to describe a function pointer, but just `fn()`
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alone suffices for that. `*mut fn()` is a pointer to a fn pointer.
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(Since these values are typically just passed to C code, however, this rarely
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makes a difference in practice.)
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[rfc401]: https://github.com/rust-lang/rfcs/blob/master/text/0401-coercions.md
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