add WellKnownTraits chapter to the book #428
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| # Well known traits | |||
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| Not all traits can be encoded in Rust's type system: special traits | |||
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I think a better phrasing would be: "Not all trait logic can be encoded in Rust types: ..."
| As an example, consider the following two aspects of `Sized` logic: | ||
| 1) In order to prove that a struct implements `Sized`, we need to prove | ||
| that the last field of that struct is `Sized`. | ||
| 2) Structs need all of their fields, except, maybe, the last one to be `Sized`. |
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While these two points are valid, the flow is a bit weird. I would say something like
As an example, consider the logic for
Sizedin regards to structs: A struct can have at most one!Sizedfield, and it must be the last. And the last field isn'tSized, then neither is the struct itself.
| Auto traits are another kind of well known traits. | ||
| The idea is that the type implements an auto trait if all data owned by that type implements it, | ||
| with an ability to opt-out via special syntax: | ||
| ```rust,ignore | ||
| impl !AutoTrait for Foo {} | ||
| ``` | ||
| Common examples of auto trais are `Send` and `Sync`. Since this semantic is not expressable with | ||
| "regular" impls, it needs special support in chalk too. |
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Mmm, this is...not exactly correct. At least, not by the definition we've been using for well known traits. Instead I would say something like
Auto traits, while not exactly well known traits, do also have special logic. The idea is that the type implements an auto trait if all data owned by that type implements it, with an ability to specifically opt-out or opt-in. Additionally, auto traits are coinductive. Some common examples of auto traits are
SendandSync.
| | well-formedness | ✅ | 🗿 | ✅ | 🗿 | ✅ | 🗿 | 🗿 | 🗿 | 🗿 | | ||
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| legend: | ||
| 🗿 - not applicable |
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I think maybe just leave this empty. A bit less clutter.
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What about ⚬? Makes it clear we didn't forget, but is unobtrusive.
| ❌ - not implemented | ||
| ❌ in the column type means that type is not yet in chalk | ||
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| The list of types not yet in chalk is not full, but auto traits/`WellKnownTrait`s |
| Not all traits can be encoded in Rust's type system: special traits | ||
| like `Sized`, `Drop` or `Unsize` need additional compiler support in order to | ||
| function properly. To address this, chalk introduces a notion of `WellKnownTrait`s: | ||
| a subset of rustc's trait lang items that need special handling in trait system logic. |
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I think the key point here is something like:
For most traits, the question of whether some type T implements the trait is determined by looking solely at the impls that exist for the trait. But there are some well-known traits where we have "built-in" impls that are never expressly written in the compiler, they are built-in to the language itself. In some cases, these impls also encode complex conditions that an ordinary impl cannot express. To address this, chalk has a notion of aWellKnownTrait -- basically, a trait which is built-in to the language and where we will generate custom logic.
| | well-formedness | ✅ | 🗿 | ✅ | 🗿 | ✅ | 🗿 | 🗿 | 🗿 | 🗿 | | ||
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| legend: | ||
| 🗿 - not applicable |
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What about ⚬? Makes it clear we didn't forget, but is unobtrusive.
| | structs | 🗿 | 🗿 | ✅ | ✅ | 🗿 | 🗿 | 🗿 | 🗿 | ✅ | | ||
| | scalar types | 📚 | 📚 | ✅ | 🗿 | 🗿 | 🗿 | 🗿 | 🗿 | ❌ | | ||
| | trait objects | 🗿 | 🗿 | 🗿 | ✅ | 🗿 | 🗿 | 🗿 | 🗿 | 🗿 | | ||
| | functions | ✅ | ✅ | ✅ | 🗿 | 🗿 | ❌ | 🗿 | 🗿 | ❌ | |
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Does this refer to "function pointers" or FnDef types?
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Hmm, it refers to TyData::Function which is presume is FnDef
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No, that's function pointer, and should be renamed.
| # Current state | ||
| | Type | Copy | Clone | Sized | Unsize | Drop | Fn | Unpin | Generator | auto traits | | ||
| | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | | ||
| | tuple types | ✅ | ✅ | ✅ | ✅ | 🗿 | 🗿 | 🗿 | 🗿 | ❌ | |
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I wonder if these can link to the relevant enum variant, where applicable
I don't know what to use for n/a so I chose 🗿, if you have a better idea I'll change it
closes #416
please be(!) harsh, English is not my strongest suit