Don't allow upcasting to a supertype in the type of the match discriminant. Fixes#23116.
This is a [breaking-change] in that it closes a type hole that previously existed.
r? @pnkfelix
Greetings Rustaceans!
I've just been getting acquainted with Rust through the guide. First let me say that it's already in great shape, chapters are kept a good length to be digestible and paced to move the reader along fluidly, so my compliments to contributors!
Along the way I noticed a few minor copy errors, and also a few areas that I thought more subjectively could stand to be improved. My commits here are divided so that minor edits unlikely to be very contentious could be cherry-picked, and then topically on parts that might generate more discussion.
I also have some comments and questions that aren't directly associated with any changes on this branch yet. I'm not sure how you like to triage this sort of thing but I'll present them below and if it's appropriate they could be moved to separate issues or I might be able to help work some of them out within the scope of this PR. Sorry that these are a lot to take in, pretty much everything below here can be digested independently of the current changes in this PR so you could read the rest later 😄
### Questions and Comments
I'll give stable links to doc revisions as of this writing.
1. The [example using `PartialEq` in the Traits chapter][1] is poor—we have no idea how `PartialEq` works at this point in the text (or at any point, AFAICT), so it isn't clear why it won't work as a trait bound in this situation and `Float` almost magically does, with the aid of existing tailor-made identity functions that seem unlikely to be so conveniently available when we encounter a scenario like this in our real-world code.
This section just seems glossed over, or perhaps content has moved around over time or there's an assumption that implementing equality with `PartialEq` should be covered in the guide eventually so this example will be less foreign. As it stands the text is hard to follow and not very meaningful.
2. I found treatment of the relationship of trait objects to pointers in the *Static and Dynamic Dispatch* chapter unclear. [The "Why Pointers?" section][2] opens with this line:
> The use of language like "fat pointer" implies that a trait object is always a pointer of some form, but why?
But the phrase "fat pointer" hasn't been used anywhere before. This is some of the more complex material in the guide, but this section nevertheless feels displaced, not clearly connecting preceding subject matter. Earlier we've covered the internal representation of trait objects and significance of pointers they contain, but it hasn't been spelled out (other than what `&Foo` syntax suggests) that trait objects are references (and why). That's what the "Why Pointers?" section is aiming to do I gather, but it seems out of place, I think it'd make more sense to cover this before the gory details of their internals.
3. Suggestion: move the *Error Handling* chapter much earlier in the Intermediate section of the guide, or even into the Basics section. I know the Intermediate section isn't intended to be read in order per se, but plenty of people like me are just going to read it straight through anyway 😁 These are pretty fundamental concepts to understand and `Option`, `Result`, and idioms like `unwrap()` and `.ok().expect()` are referenced numerous times throughout the rest of the guide. They feature pretty prominently as early as *Standard Input* and *Guessing Game* chapters in Basics, in fact. I happen to have a good understanding of these already through encountering their analogs in typed functional languages, but if I didn't I believe I really would have appreciated reading *Error Handling* much earlier.
4. In the `rustdoc` chapter, a [comment at the beginning of the first source example][3] refers to a "link" crate attribute being needed. There seems to be no such attribute present in the source. I believe this refers to `crate_type` [according to the reference][4], but it'd be nice if this example were updated/clarified (I think `crate_id` is deprecated/obsolete too).
This brings me to a related comment also: after encountering crate attributes in the reference and also docs on Cargo configuration like `crate-type = ["dylib"]`, I'm uncertain about the relationship/redundancy between these. I'm sure this is the kind of thing where docs are simply struggling to keep pace with rapid changes in Rust and Cargo, just wanted to flag that this distinction ought to be clearly covered in the docs for one or the other at some point, it's presently hard to track down.
5. Minor: link to sample editor configurations in [the introductory chapter][5] is broken, probably the generator automatically translates `.md` links to `.html`. Perhaps it shouldn't do that for absolute URLs.
6. Following from my changes to the enums coverage in [*Compound Data Types*][6] in this PR: sum types are an important topic and I tried to make some improvements, but I think the motivating example of `Character` with `Digit(i32)` and `Other` variants is a pretty weak one, and a better example could greatly improve cohesion with the `Ordering` coverage later in the section and how that ties into pattern matching in the subsequent chapter. I just haven't thought of a better example to suggest yet.
In particular, the text states:
> This may seem rather limiting, but it's a limitation which we can overcome.
This is referring to `Character`, and actually to more than one limitation: the preceding admonition that its variants aren't comparable/don't have ordering, and don't support binary operators like `*` and `+`. Overcoming these limitations actually never gets explained—we next cover how `Ordering` works as an enum itself for plain `i32`s, but never get around to showing how this might be applied to our `Digit` variant type.
Since the coverage of enums already segues into pattern matching and this could be even tighter with a stronger example, it might be nice if our example enum were somehow connected to the final example program for the Basics section too, where `Ordering` reappears. I don't see how it would fit with the current guessing game example, but food for thought.
7. `#[derive]` seems conspicuously missing from the guide. It would probably make sense to introduce after showing simple examples of implementing equality and/or ordering traits by hand, which have been mentioned as possibilities above. Perhaps it's too much to breach this as early as the Basic section though without traits being introduced. `#[derive]` itself and the derivable traits can certainly be saved for Intermediate and referenced as covered later, in any case.
r? @steveklabnik for docs.
[1]: 1576142495/src/doc/trpl/traits.md (our-inverse-example)
[2]: 1576142495/src/doc/trpl/static-and-dynamic-dispatch.md (why-pointers)
[3]: 1576142495/src/doc/trpl/documentation.md (creating-documentation)
[4]: http://doc.rust-lang.org/reference.html#linkage
[5]: 1576142495/src/doc/trpl/hello-world.md
[6]: 1576142495/src/doc/trpl/compound-data-types.md (enums)
possible blanket impls and also triggers internal overflow. Add some
special cases for common uses (&&str, &String) for now; bounds-targeting
deref coercions are probably the right longer term answer.
This commit enables writing a stable implementation of the `Hasher` trait as
well as actually calculating the hash of a vlaue in a stable fashion. The
signature is stabilized as-is.
This commit:
* Introduces `std::convert`, providing an implementation of
RFC 529.
* Deprecates the `AsPath`, `AsOsStr`, and `IntoBytes` traits, all
in favor of the corresponding generic conversion traits.
Consequently, various IO APIs now take `AsRef<Path>` rather than
`AsPath`, and so on. Since the types provided by `std` implement both
traits, this should cause relatively little breakage.
* Deprecates many `from_foo` constructors in favor of `from`.
* Changes `PathBuf::new` to take no argument (creating an empty buffer,
as per convention). The previous behavior is now available as
`PathBuf::from`.
* De-stabilizes `IntoCow`. It's not clear whether we need this separate trait.
Closes#22751Closes#14433
[breaking-change]
This commit marks as `#[stable]` the `Entry` types for the maps provided
by `std`. The main reason these had been left unstable previously was
uncertainty about an eventual trait design, but several plausible
designs have been proposed that all work fine with the current type definitions.
r? @Gankro
Don't allow upcasting to a supertype in the type of the match discriminant. Fixes#23116.
This is a [breaking-change] in that it closes a type hole that previously existed.
r? @pnkfelix
Greetings Rustaceans!
I've just been getting acquainted with Rust through the guide. First let me say that it's already in great shape, chapters are kept a good length to be digestible and paced to move the reader along fluidly, so my compliments to contributors!
Along the way I noticed a few minor copy errors, and also a few areas that I thought more subjectively could stand to be improved. My commits here are divided so that minor edits unlikely to be very contentious could be cherry-picked, and then topically on parts that might generate more discussion.
I also have some comments and questions that aren't directly associated with any changes on this branch yet. I'm not sure how you like to triage this sort of thing but I'll present them below and if it's appropriate they could be moved to separate issues or I might be able to help work some of them out within the scope of this PR. Sorry that these are a lot to take in, pretty much everything below here can be digested independently of the current changes in this PR so you could read the rest later 😄
### Questions and Comments
I'll give stable links to doc revisions as of this writing.
1. The [example using `PartialEq` in the Traits chapter][1] is poor—we have no idea how `PartialEq` works at this point in the text (or at any point, AFAICT), so it isn't clear why it won't work as a trait bound in this situation and `Float` almost magically does, with the aid of existing tailor-made identity functions that seem unlikely to be so conveniently available when we encounter a scenario like this in our real-world code.
This section just seems glossed over, or perhaps content has moved around over time or there's an assumption that implementing equality with `PartialEq` should be covered in the guide eventually so this example will be less foreign. As it stands the text is hard to follow and not very meaningful.
2. I found treatment of the relationship of trait objects to pointers in the *Static and Dynamic Dispatch* chapter unclear. [The "Why Pointers?" section][2] opens with this line:
> The use of language like "fat pointer" implies that a trait object is always a pointer of some form, but why?
But the phrase "fat pointer" hasn't been used anywhere before. This is some of the more complex material in the guide, but this section nevertheless feels displaced, not clearly connecting preceding subject matter. Earlier we've covered the internal representation of trait objects and significance of pointers they contain, but it hasn't been spelled out (other than what `&Foo` syntax suggests) that trait objects are references (and why). That's what the "Why Pointers?" section is aiming to do I gather, but it seems out of place, I think it'd make more sense to cover this before the gory details of their internals.
3. Suggestion: move the *Error Handling* chapter much earlier in the Intermediate section of the guide, or even into the Basics section. I know the Intermediate section isn't intended to be read in order per se, but plenty of people like me are just going to read it straight through anyway 😁 These are pretty fundamental concepts to understand and `Option`, `Result`, and idioms like `unwrap()` and `.ok().expect()` are referenced numerous times throughout the rest of the guide. They feature pretty prominently as early as *Standard Input* and *Guessing Game* chapters in Basics, in fact. I happen to have a good understanding of these already through encountering their analogs in typed functional languages, but if I didn't I believe I really would have appreciated reading *Error Handling* much earlier.
4. In the `rustdoc` chapter, a [comment at the beginning of the first source example][3] refers to a "link" crate attribute being needed. There seems to be no such attribute present in the source. I believe this refers to `crate_type` [according to the reference][4], but it'd be nice if this example were updated/clarified (I think `crate_id` is deprecated/obsolete too).
This brings me to a related comment also: after encountering crate attributes in the reference and also docs on Cargo configuration like `crate-type = ["dylib"]`, I'm uncertain about the relationship/redundancy between these. I'm sure this is the kind of thing where docs are simply struggling to keep pace with rapid changes in Rust and Cargo, just wanted to flag that this distinction ought to be clearly covered in the docs for one or the other at some point, it's presently hard to track down.
5. Minor: link to sample editor configurations in [the introductory chapter][5] is broken, probably the generator automatically translates `.md` links to `.html`. Perhaps it shouldn't do that for absolute URLs.
6. Following from my changes to the enums coverage in [*Compound Data Types*][6] in this PR: sum types are an important topic and I tried to make some improvements, but I think the motivating example of `Character` with `Digit(i32)` and `Other` variants is a pretty weak one, and a better example could greatly improve cohesion with the `Ordering` coverage later in the section and how that ties into pattern matching in the subsequent chapter. I just haven't thought of a better example to suggest yet.
In particular, the text states:
> This may seem rather limiting, but it's a limitation which we can overcome.
This is referring to `Character`, and actually to more than one limitation: the preceding admonition that its variants aren't comparable/don't have ordering, and don't support binary operators like `*` and `+`. Overcoming these limitations actually never gets explained—we next cover how `Ordering` works as an enum itself for plain `i32`s, but never get around to showing how this might be applied to our `Digit` variant type.
Since the coverage of enums already segues into pattern matching and this could be even tighter with a stronger example, it might be nice if our example enum were somehow connected to the final example program for the Basics section too, where `Ordering` reappears. I don't see how it would fit with the current guessing game example, but food for thought.
7. `#[derive]` seems conspicuously missing from the guide. It would probably make sense to introduce after showing simple examples of implementing equality and/or ordering traits by hand, which have been mentioned as possibilities above. Perhaps it's too much to breach this as early as the Basic section though without traits being introduced. `#[derive]` itself and the derivable traits can certainly be saved for Intermediate and referenced as covered later, in any case.
r? @steveklabnik for docs.
[1]: 1576142495/src/doc/trpl/traits.md (our-inverse-example)
[2]: 1576142495/src/doc/trpl/static-and-dynamic-dispatch.md (why-pointers)
[3]: 1576142495/src/doc/trpl/documentation.md (creating-documentation)
[4]: http://doc.rust-lang.org/reference.html#linkage
[5]: 1576142495/src/doc/trpl/hello-world.md
[6]: 1576142495/src/doc/trpl/compound-data-types.md (enums)
Impls on `clean::Type::FixedVector` are now collected in the array
primitive page instead of the slice primitive page.
Also add a primitive docs for arrays to `std`.
impls.
This requires:
1. modifying trait selection a bit so that when we synthesize impls for
fn pointers and closures;
2. adding code to trans so that we can synthesize a `FnMut`/`FnOnce`
impl for a `Fn` closure and so forth.
This commit implements [RFC
909](https://github.com/rust-lang/rfcs/pull/909):
The `std::thread_local` module is now deprecated, and its contents are
available directly in `std::thread` as `LocalKey`, `LocalKeyState`, and
`ScopedKey`.
The macros remain exactly as they were, which means little if any code
should break. Nevertheless, this is technically a:
[breaking-change]
Closes#23547
This small commit stabilizes the `Error` trait as-is, except that `Send`
and `Debug` are added as constraints. The `Send` constraint is because
most uses of `Error` will be for trait objects, and by default we would
like these objects to be transferrable between threads. The `Debug`
constraint is to ensure that e.g. `Box<Error>` is `Debug`, and because
types that implement `Display` should certainly implement `Debug` in any case.
In the near future we expect to add `Any`-like downcasting features to
`Error`, but this is waiting on some additional
mechanisms (`Reflect`). It will be added before 1.0 via default methods.
[breaking-change]
The new `relative_from` method no longer supports the case on unix
where both paths are absolute, which `-C rpath` depended on. This
version fixes the problem by copying the old path_relative_from
function into the rpath module.
Fixes#23140
contains ref-bindings, do not permit any upcasting from the type of
the value being matched. Similarly, do not permit coercion in a `let`.
This is a [breaking-change] in that it closes a type hole that
previously existed, and in that coercion is not performed. You should
be able to work around the latter by converting:
```rust
let ref mut x: T = expr;
```
into
```rust
let x: T = expr;
let ref mut x = x;
```
Restricting coercion not to apply in the case of `let ref` or `let ref mut` is sort
of unexciting to me, but seems the best solution:
1. Mixing coercion and `let ref` or `let ref mut` is a bit odd, because you are taking
the address of a (coerced) temporary, but only sometimes. It's not syntactically evident,
in other words, what's going on. When you're doing a coercion, you're kind of
2. Put another way, I would like to preserve the relationship that
`equality <= subtyping <= coercion <= as-coercion`, where this is
an indication of the number of `(T1,T2)` pairs that are accepted by
the various relations. Trying to mix `let ref mut` and coercion
would create another kind of relation that is like coercion, but
acts differently in the case where a precise match is needed.
3. In any case, this is strictly more conservative than what we had
before and we can undo it in the future if we find a way to make
coercion mix with type equality.
The change to match I feel ok about but similarly unthrilled. There is
some subtle text already concerning whether to use eqtype or subtype
for identifier bindings. The best fix I think would be to always have
match use strict equality but use subtyping on identifier bindings,
but the comment `(*)` explains why that's not working at the moment.
As above, I think we can change this as we clean up the code there.
