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Original file line number | Diff line number | Diff line change |
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- Start Date: 2014-10-07 | ||
- RFC PR: [rust-lang/rfcs#240](https://github.com/rust-lang/rfcs/pull/240) | ||
- Rust Issue: [rust-lang/rust#17863](https://github.com/rust-lang/rust/issues/17863) | ||
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# Summary | ||
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This is a *conventions RFC* for settling the location of `unsafe` APIs relative | ||
to the types they work with, as well as the use of `raw` submodules. | ||
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The brief summary is: | ||
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* Unsafe APIs should be made into methods or static functions in the same cases | ||
that safe APIs would be. | ||
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* `raw` submodules should be used only to *define* explicit low-level | ||
representations. | ||
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# Motivation | ||
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Many data structures provide unsafe APIs either for avoiding checks or working | ||
directly with their (otherwise private) representation. For example, `string` | ||
provides: | ||
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* An `as_mut_vec` method on `String` that provides a `Vec<u8>` view of the | ||
string. This method makes it easy to work with the byte-based representation | ||
of the string, but thereby also allows violation of the utf8 guarantee. | ||
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* A `raw` submodule with a number of free functions, like `from_parts`, that | ||
constructs a `String` instances from a raw-pointer-based representation, a | ||
`from_utf8` variant that does not actually check for utf8 validity, and so | ||
on. The unifying theme is that all of these functions avoid checking some key | ||
invariant. | ||
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The problem is that currently, there is no clear/consistent guideline about | ||
which of these APIs should live as methods/static functions associated with a | ||
type, and which should live in a `raw` submodule. Both forms appear throughout | ||
the standard library. | ||
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# Detailed design | ||
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The proposed convention is: | ||
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* When an unsafe function/method is clearly "about" a certain type (as a way of | ||
constructing, destructuring, or modifying values of that type), it should be a | ||
method or static function on that type. This is the same as the convention for | ||
placement of safe functions/methods. So functions like | ||
`string::raw::from_parts` would become static functions on `String`. | ||
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* `raw` submodules should only be used to *define* low-level | ||
types/representations (and methods/functions on them). Methods for converting | ||
to/from such low-level types should be available directly on the high-level | ||
types. Examples: `core::raw`, `sync::raw`. | ||
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The benefits are: | ||
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* *Ergonomics*. You can gain easy access to unsafe APIs merely by having a value | ||
of the type (or, for static functions, importing the type). | ||
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* *Consistency and simplicity*. The rules for placement of unsafe APIs are the | ||
same as those for safe APIs. | ||
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The perspective here is that marking APIs `unsafe` is enough to deter their use | ||
in ordinary situations; they don't need to be further distinguished by placement | ||
into a separate module. | ||
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There are also some naming conventions to go along with unsafe static functions | ||
and methods: | ||
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* When an unsafe function/method is an unchecked variant of an otherwise safe | ||
API, it should be marked using an `_unchecked` suffix. | ||
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For example, the `String` module should provide both `from_utf8` and | ||
`from_utf8_unchecked` constructors, where the latter does not actually check | ||
the utf8 encoding. The `string::raw::slice_bytes` and | ||
`string::raw::slice_unchecked` functions should be merged into a single | ||
`slice_unchecked` method on strings that checks neither bounds nor utf8 | ||
boundaries. | ||
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* When an unsafe function/method produces or consumes a low-level representation | ||
of a data structure, the API should use `raw` in its name. Specifically, | ||
`from_raw_parts` is the typical name used for constructing a value from e.g. a | ||
pointer-based representation. | ||
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* Otherwise, *consider* using a name that suggests *why* the API is unsafe. In | ||
some cases, like `String::as_mut_vec`, other stronger conventions apply, and the | ||
`unsafe` qualifier on the signature (together with API documentation) is | ||
enough. | ||
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The unsafe methods and static functions for a given type should be placed in | ||
their own `impl` block, at the end of the module defining the type; this will | ||
ensure that they are grouped together in rustdoc. (Thanks @kballard for the | ||
suggestion.) | ||
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# Drawbacks | ||
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One potential drawback of these conventions is that the documentation for a | ||
module will be cluttered with rarely-used `unsafe` APIs, whereas the `raw` | ||
submodule approach neatly groups these APIs. But rustdoc could easily be | ||
changed to either hide or separate out `unsafe` APIs by default, and in the | ||
meantime the `impl` block grouping should help. | ||
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More specifically, the convention of placing unsafe constructors in `raw` makes | ||
them very easy to find. But the usual `from_` convention, together with the | ||
naming conventions suggested above, should make it fairly easy to discover such | ||
constructors even when they're supplied directly as static functions. | ||
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More generally, these conventions give `unsafe` APIs more equal status with safe | ||
APIs. Whether this is a *drawback* depends on your philosophy about the status | ||
of unsafe programming. But on a technical level, the key point is that the APIs | ||
are marked `unsafe`, so users still have to opt-in to using them. *Ed note: from | ||
my perspective, low-level/unsafe programming is important to support, and there | ||
is no reason to penalize its ergonomics given that it's opt-in anyway.* | ||
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# Alternatives | ||
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There are a few alternatives: | ||
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* Rather than providing unsafe APIs directly as methods/static functions, they | ||
could be grouped into a single extension trait. For example, the `String` type | ||
could be accompanied by a `StringRaw` extension trait providing APIs for | ||
working with raw string representations. This would allow a clear grouping of | ||
unsafe APIs, while still providing them as methods/static functions and | ||
allowing them to easily be imported with e.g. `use std::string::StringRaw`. | ||
On the other hand, it still further penalizes the raw APIs (beyond marking | ||
them `unsafe`), and given that rustdoc could easily provide API grouping, it's | ||
unclear exactly what the benefit is. | ||
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* ([Suggested by @kballard](https://github.com/rust-lang/rfcs/pull/240#issuecomment-55635468)): | ||
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> Use `raw` for functions that construct a value of the type without checking | ||
> for one or more invariants. | ||
The advantage is that it's easy to find such invariant-ignoring functions. The | ||
disadvantage is that their ergonomics is worsened, since they much be | ||
separately imported or referenced through a lengthy path: | ||
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```rust | ||
// Compare the ergonomics: | ||
string::raw::slice_unchecked(some_string, start, end) | ||
some_string.slice_unchecked(start, end) | ||
``` | ||
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* Another suggestion by @kballard is to keep the basic structure of `raw` | ||
submodules, but use associated types to improve the ergonomics. Details (and | ||
discussions of pros/cons) are in | ||
[this comment](https://github.com/rust-lang/rfcs/pull/240/files#r17572875). | ||
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* Use `raw` submodules to group together *all* manipulation of low-level | ||
representations. No module in `std` currently does this; existing modules | ||
provide some free functions in `raw`, and some unsafe methods, without a clear | ||
driving principle. The ergonomics of moving *everything* into free functions | ||
in a `raw` submodule are quite poor. | ||
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# Unresolved questions | ||
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The `core::raw` module provides structs with public representations equivalent | ||
to several built-in and library types (boxes, closures, slices, etc.). It's not | ||
clear whether the name of this module, or the location of its contents, should | ||
change as a result of this RFC. The module is a special case, because not all of | ||
the types it deals with even have corresponding modules/type declarations -- so | ||
it probably suffices to leave decisions about it to the API stabilization | ||
process. |
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