This commit shreds all remnants of libextra from the compiler and standard
distribution. Two modules, c_vec/tempfile, were moved into libstd after some
cleanup, and the other modules were moved to separate crates as seen fit.
Closes#8784Closes#12413Closes#12576
Previously, the cfg attribute `cfg(not(a, b))` was translated to `(!a && !b)`,
but this isn't very useful because that can already be expressed as
`cfg(not(a), not(b))`. This commit changes the translation to `!(a && b)` which
is more symmetrical of the rest of the `cfg` attribute.
Put another way, I would expect `cfg(clause)` to be the opposite of
`cfg(not(clause))`, but this is not currently the case with multiple element
clauses.
Fix a test that was missed in the chan/port renaming (PR #12815). This was missed because it is skipped on linux and windows, and the mac bots were moving at the time the PR landed.
# Summary
This patch introduces the `_` token into the type grammar, with the meaning "infer this type".
With this change, the following two lines become equivalent:
```
let x = foo();
let x: _ = foo();
```
But due to its composability, it enables partial type hints like this:
```
let x: Bar<_> = baz();
```
Using it on the item level is explicitly forbidden, as the Rust language does not enable global type inference by design.
This implements the feature requested in https://github.com/mozilla/rust/issues/9508.
# Things requiring clarification
- The change to enable it is very small, but I have only limited understanding of the related code, so the approach here might be wrong.
- In particular, while this patch works, it does so in a way not originally intended according to the code comments.
- This probably needs more tests, or rather feedback for which tests are still missing.
- I'm unsure how this interacts with lifetime parameters, and whether it is correct in regard to them.
- Partial type hints on the right side of `as` like `&foo as *_` work in both a normal function contexts and in constexprs like `static foo: *int = &'static 123 as *_`. The question is whether this should be allowed in general.
# Todo for this PR
- The manual and tutorial still needs updating.
# Bugs I'm unsure how to fix
- Requesting inference for the top level of the right hand side of a `as` fails to infer correctly, even if all possible hints are given:
```
.../type_hole_1.rs:35:18: 35:22 error: the type of this value must be known in this context
.../type_hole_1.rs:35 let a: int = 1u32 as _;
^~~~
```
lint: add lint for use of a `~[T]`.
This is useless at the moment (since pretty much every crate uses
`~[]`), but should help avoid regressions once completely removed from a
crate.
## read+write modifier '+'
This small sugar was left out in the original implementation (#5359).
When an output operand with the '+' modifier is encountered, we store the index of that operand alongside the expression to create and append an input operand later. The following lines are equivalent:
```
asm!("" : "+m"(expr));
asm!("" : "=m"(expr) : "0"(expr));
```
## misplaced options and clobbers give a warning
It's really annoying when a small typo might change behavior without any warning.
```
asm!("mov $1, $0" : "=r"(x) : "r"(8u) : "cc" , "volatile");
//~^ WARNING expected a clobber, but found an option
```
## liveness
Fixed incorrect order of propagation.
Sometimes it caused spurious warnings in code: `warning: value assigned to `i` is never read, #[warn(dead_assignment)] on by default`
~~Note: Rebased on top of another PR. (uses other changes)~~
* [x] Implement read+write
* [x] Warn about misplaced options
* [x] Fix liveness (`dead_assignment` lint)
* [x] Add all tests
Whenever a failure happens, if a program is run with
`RUST_LOG=std::rt::backtrace` a backtrace will be printed to the task's stderr
handle. Stack traces are uncondtionally printed on double-failure and
rtabort!().
This ended up having a nontrivial implementation, and here's some highlights of
it:
* We're bundling libbacktrace for everything but OSX and Windows
* We use libgcc_s and its libunwind apis to get a backtrace of instruction
pointers
* On OSX we use dladdr() to go from an instruction pointer to a symbol
* On unix that isn't OSX, we use libbacktrace to get symbols
* Windows, as usual, has an entirely separate implementation
Lots more fun details and comments can be found in the source itself.
Closes#10128
Whenever a failure happens, if a program is run with
`RUST_LOG=std::rt::backtrace` a backtrace will be printed to the task's stderr
handle. Stack traces are uncondtionally printed on double-failure and
rtabort!().
This ended up having a nontrivial implementation, and here's some highlights of
it:
* We're bundling libbacktrace for everything but OSX and Windows
* We use libgcc_s and its libunwind apis to get a backtrace of instruction
pointers
* On OSX we use dladdr() to go from an instruction pointer to a symbol
* On unix that isn't OSX, we use libbacktrace to get symbols
* Windows, as usual, has an entirely separate implementation
Lots more fun details and comments can be found in the source itself.
Closes#10128
If a TTY fails to get initialized, it still needs to have uv_close invoked on
it. This fixes the problem by constructing the TtyWatcher struct before the call
to uv_tty_init. The struct has a destructor on it which will close the handle
properly.
Closes#12666
If a TTY fails to get initialized, it still needs to have uv_close invoked on
it. This fixes the problem by constructing the TtyWatcher struct before the call
to uv_tty_init. The struct has a destructor on it which will close the handle
properly.
Closes#12666
The `~str` type is not long for this world as it will be superseded by the
soon-to-come DST changes for the language. The new type will be
`~Str`, and matching over the allocation will no longer be supported.
Matching on `&str` will continue to work, in both a pre and post DST world.
Closes#12803 (std: Relax an assertion in oneshot selection) r=brson
Closes#12818 (green: Fix a scheduler assertion on yielding) r=brson
Closes#12819 (doc: discuss try! in std::io) r=alexcrichton
Closes#12820 (Use generic impls for `Hash`) r=alexcrichton
Closes#12826 (Remove remaining nolink usages) r=alexcrichton
Closes#12835 (Emacs: always jump the cursor if needed on indent) r=brson
Closes#12838 (Json method cleanup) r=alexcrichton
Closes#12843 (rustdoc: whitelist the headers that get a § on hover) r=alexcrichton
Closes#12844 (docs: add two unlisted libraries to the index page) r=pnkfelix
Closes#12846 (Added a test that checks that unary structs can be mutably borrowed) r=sfackler
Closes#12847 (mk: Fix warnings about duplicated rules) r=nmatsakis
This is needed to make progress on #10296 as the default bounds will no longer
include Send. I believe that this was the originally intended syntax for procs,
and it just hasn't been necessary up until now.
This is needed to make progress on #10296 as the default bounds will no longer
include Send. I believe that this was the originally intended syntax for procs,
and it just hasn't been necessary up until now.
This should be called far less than it is because it does expensive OS
interactions and seeding of the internal RNG, `task_rng` amortises this
cost. The main problem is the name is so short and suggestive.
The direct equivalent is `StdRng::new`, which does precisely the same
thing.
The deprecation will make migrating away from the function easier.
It is often convenient to have forms of weak linkage or other various types of
linkage. Sadly, just using these flavors of linkage are not compatible with
Rust's typesystem and how it considers some pointers to be non-null.
As a compromise, this commit adds support for weak linkage to external symbols,
but it requires that this is only placed on extern statics of type `*T`.
Codegen-wise, we get translations like:
```rust
// rust code
extern {
#[linkage = "extern_weak"]
static foo: *i32;
}
// generated IR
@foo = extern_weak global i32
@_some_internal_symbol = internal global *i32 @foo
```
All references to the rust value of `foo` then reference `_some_internal_symbol`
instead of the symbol `_foo` itself. This allows us to guarantee that the
address of `foo` will never be null while the value may sometimes be null.
An example was implemented in `std::rt::thread` to determine if
`__pthread_get_minstack()` is available at runtime, and a test is checked in to
use it for a static value as well. Function pointers a little odd because you
still need to transmute the pointer value to a function pointer, but it's
thankfully better than not having this capability at all.
Thanks to @bnoordhuis for the original patch, most of this work is still his!
It is often convenient to have forms of weak linkage or other various types of
linkage. Sadly, just using these flavors of linkage are not compatible with
Rust's typesystem and how it considers some pointers to be non-null.
As a compromise, this commit adds support for weak linkage to external symbols,
but it requires that this is only placed on extern statics of type `*T`.
Codegen-wise, we get translations like:
// rust code
extern {
#[linkage = "extern_weak"]
static foo: *i32;
}
// generated IR
@foo = extern_weak global i32
@_some_internal_symbol = internal global *i32 @foo
All references to the rust value of `foo` then reference `_some_internal_symbol`
instead of the symbol `_foo` itself. This allows us to guarantee that the
address of `foo` will never be null while the value may sometimes be null.
An example was implemented in `std::rt::thread` to determine if
`__pthread_get_minstack()` is available at runtime, and a test is checked in to
use it for a static value as well. Function pointers a little odd because you
still need to transmute the pointer value to a function pointer, but it's
thankfully better than not having this capability at all.
* `Ord` inherits from `Eq`
* `TotalOrd` inherits from `TotalEq`
* `TotalOrd` inherits from `Ord`
* `TotalEq` inherits from `Eq`
This is a partial implementation of #12517.
When using tasks in Rust, the expectation is that the runtime does not exit
before all tasks have exited. This is enforced in libgreen through the
`SchedPool` type, and it is enforced in libnative through a `bookkeeping` module
and a global count/mutex pair. Unfortunately, this means that a process which
originates with libgreen will not wait for spawned native tasks.
In order to fix this problem, the bookkeeping module was moved from libnative to
libstd so the runtime itself can wait for native tasks to exit. Green tasks do
not manage themselves through this bookkeeping module, but native tasks will
continue to manage themselves through this module.
Closes#12684
Closes#8506.
The `trans::adt` code for statics uses fields with `C_undef` values to
insert alignment padding (because LLVM's own alignment padding isn't
always sufficient for aggregate constants), and assumes that all fields
in the actual Rust value are represented by non-undef LLVM values, to
distinguish them from that padding.
But for nullable pointer enums, if non-null variant has fields other
than the pointer used as the discriminant, they would be set to undef in
the null case, to reflect that they're never accessed.
To avoid the obvious conflict between these two items, the latter undefs
were wrapped in unary LLVM structs to distinguish them from the former
undefs. Except this doesn't actually work -- LLVM, not unreasonably,
treats the "wrapped undef" as a regular undef.
So this commit just sets all fields to null in the null pointer case of
a nullable pointer enum static, because the other fields don't really
need to be undef in the first place.
When the timer_helper thread exited, it would attempt to re-acquire the global
task count mutex, but the mutex had previously been deallocated, leading to
undefined behavior of the mutex, and in some cases deadlock.
Another mutex is used to coordinate shutting down the timer helper thread.
Closes#12699
- Added `TraitObject` representation to `std::raw`.
- Added doc to `std::raw`.
- Removed `Any::as_void_ptr()` and `Any::as_mut_void_ptr()`
methods as they are uneccessary now after the removal of
headers on owned boxes. This reduces the number of virtual calls needed.
- Made the `..Ext` implementations work directly with the repr of
a trait object.
- Removed `Any`-related traits from the prelude.
- Added bench for `Any`
A couple of syntax extensions manually expanded expressions, but it
wasn't done universally, most noticably inside of asm!().
There's also a bit of random cleanup.
We weren't passing the node id for the enum and hence it couldn't retrieve the field types for the struct variant we were trying to destructure.
Fixes#11577.
Formatting via reflection has been a little questionable for some time now, and
it's a little unfortunate that one of the standard macros will silently use
reflection when you weren't expecting it. This adds small bits of code bloat to
libraries, as well as not always being necessary. In light of this information,
this commit switches assert_eq!() to using {} in the error message instead of
{:?}.
In updating existing code, there were a few error cases that I encountered:
* It's impossible to define Show for [T, ..N]. I think DST will alleviate this
because we can define Show for [T].
* A few types here and there just needed a #[deriving(Show)]
* Type parameters needed a Show bound, I often moved this to `assert!(a == b)`
* `Path` doesn't implement `Show`, so assert_eq!() cannot be used on two paths.
I don't think this is much of a regression though because {:?} on paths looks
awful (it's a byte array).
Concretely speaking, this shaved 10K off a 656K binary. Not a lot, but sometime
significant for smaller binaries.
Previously, format!("{a}{b}", a=foo(), b=bar()) has foo() and bar() run in a
nondeterminisc order. This is clearly a non-desirable property, so this commit
uses iteration over a list instead of iteration over a hash map to provide
deterministic code generation of these format arguments.
This PR includes:
- Create an iterator for ```List<T>``` called ```Items<T>```;
- Move all list operations inside ```List<T>``` impl;
- Removed functions that are already provided by ```Iterator``` trait;
- Refactor on ```len()``` and ```is_empty``` using ```Container``` trait;
- Bunch of minor fixes;
A replacement for using @ is intended, but still in discussion.
Closes#12344.
The printing of the error message on stack overflow had two sometimes false
assumptions previously. The first is that a local task was always available (it
called Local::take) and the second is that it used `println!` instead of
manually writing.
The first assumption isn't necessarily true because while stack overflow will
likely only be detected in situations that a local task is available, it's not
guaranteed to always be in TLS. For example, during a `println!` call a task
may be blocking, causing it to be unavailable. By using Local::try_take(), we
can be resilient against these occurrences.
The second assumption could lead to odd behavior because the stdout logger can
be overwritten to run arbitrary code. Currently this should be possible, but the
utility is much diminished because a stack overflow translates to an abort()
instead of a failure.
The printing of the error message on stack overflow had two sometimes false
assumptions previously. The first is that a local task was always available (it
called Local::take) and the second is that it used println! instead of
manually writing.
The first assumption isn't necessarily true because while stack overflow will
likely only be detected in situations that a local task is available, it's not
guaranteed to always be in TLS. For example, during a println! call a task
may be blocking, causing it to be unavailable. By using Local::try_take(), we
can be resilient against these occurrences.
The second assumption could lead to odd behavior because the stdout logger can
be overwritten to run arbitrary code. Currently this should be possible, but the
utility is much diminished because a stack overflow translates to an abort()
instead of a failure.
This updates a number of ignore-test tests, and removes a few completely
outdated tests due to the feature being tested no longer being supported.
This brings a number of bench/shootout tests up to date so they're compiling
again. I make no claims to the performance of these benchmarks, it's just nice
to not have bitrotted code.
Closes#2604Closes#9407
This patch series does a couple things:
* replaces manual `Hash` implementations with `#[deriving(Hash)]`
* adds `Hash` back to `std::prelude`
* minor cleanup of whitespace and variable names.
This commit removes deriving(ToStr) in favor of deriving(Show), migrating all impls of ToStr to fmt::Show.
Most of the details can be found in the first commit message.
Closes#12477
The std::run module is a relic from a standard library long since past, and
there's not much use to having two modules to execute processes with where one
is slightly more convenient. This commit merges the two modules, moving lots of
functionality from std::run into std::io::process and then deleting
std::run.
New things you can find in std::io::process are:
* Process::new() now only takes prog/args
* Process::configure() takes a ProcessConfig
* Process::status() is the same as run::process_status
* Process::output() is the same as run::process_output
* I/O for spawned tasks is now defaulted to captured in pipes instead of ignored
* Process::kill() was added (plus an associated green/native implementation)
* Process::wait_with_output() is the same as the old finish_with_output()
* destroy() is now signal_exit()
* force_destroy() is now signal_kill()
Closes#2625Closes#10016
The std::run module is a relic from a standard library long since past, and
there's not much use to having two modules to execute processes with where one
is slightly more convenient. This commit merges the two modules, moving lots of
functionality from std::run into std::io::process and then deleting
std::run.
New things you can find in std::io::process are:
* Process::new() now only takes prog/args
* Process::configure() takes a ProcessConfig
* Process::status() is the same as run::process_status
* Process::output() is the same as run::process_output
* I/O for spawned tasks is now defaulted to captured in pipes instead of ignored
* Process::kill() was added (plus an associated green/native implementation)
* Process::wait_with_output() is the same as the old finish_with_output()
* destroy() is now signal_exit()
* force_destroy() is now signal_kill()
Closes#2625Closes#10016
This commit changes the ToStr trait to:
impl<T: fmt::Show> ToStr for T {
fn to_str(&self) -> ~str { format!("{}", *self) }
}
The ToStr trait has been on the chopping block for quite awhile now, and this is
the final nail in its coffin. The trait and the corresponding method are not
being removed as part of this commit, but rather any implementations of the
`ToStr` trait are being forbidden because of the generic impl. The new way to
get the `to_str()` method to work is to implement `fmt::Show`.
Formatting into a `&mut Writer` (as `format!` does) is much more efficient than
`ToStr` when building up large strings. The `ToStr` trait forces many
intermediate allocations to be made while the `fmt::Show` trait allows
incremental buildup in the same heap allocated buffer. Additionally, the
`fmt::Show` trait is much more extensible in terms of interoperation with other
`Writer` instances and in more situations. By design the `ToStr` trait requires
at least one allocation whereas the `fmt::Show` trait does not require any
allocations.
Closes#8242Closes#9806
Makes labelled loops hygiene by performing renaming of the labels defined in e.g. `'x: loop { ... }` and then used in break and continue statements within loop body so that they act hygienically when used with macros.
Closes#12262.
Makes labelled loops hygiene by performing renaming of the labels
defined in e.g. `'x: loop { ... }` and then used in break and continue
statements within loop body so that they act hygienically when used with
macros.
Closes#12262.
With the stability attributes we can put public-but unstable modules next to others, so this moves `intrinsics` and `raw` out of the `unstable` module (and marks both as `#[experimental]`).
These two containers are indeed collections, so their place is in
libcollections, not in libstd. There will always be a hash map as part of the
standard distribution of Rust, but by moving it out of the standard library it
makes libstd that much more portable to more platforms and environments.
This conveniently also removes the stuttering of 'std::hashmap::HashMap',
although 'collections::HashMap' is only one character shorter.
This PR merges `IterBytes` and `Hash` into a trait that allows for generic non-stream-based hashing. It makes use of @eddyb's default type parameter support in order to have a similar usage to the old `Hash` framework.
Fixes#8038.
Todo:
- [x] Better documentation
- [ ] Benchmark
- [ ] Parameterize `HashMap` on a `Hasher`.
This patch merges IterBytes and Hash traits, which clears up the
confusion of using `#[deriving(IterBytes)]` to support hashing.
Instead, it now is much easier to use the new `#[deriving(Hash)]`
for making a type hashable with a stream hash.
Furthermore, it supports custom non-stream-based hashers, such as
if a value's hash was cached in a database.
This does not yet replace the old IterBytes-hash with this new
version.
This works towards a complete rewrite and ultimate removal of the `std::num::strconv` module (see #6220), and the removal of the `ToStrRadix` trait in favour of using the `std::fmt` functionality directly. This should make for a cleaner API, encourage less allocation, and make the implementation far more comprehensible.
The `Formatter::pad_integral` method has also been refactored make it easier to understand.
The formatting tests for integers have been moved out of `run-pass/ifmt.rs` in order to provide more immediate feedback when building using `make check-stage2-std NO_REBUILD=1`.
The benchmarks have been standardised between std::num::strconv and std::num::fmt to make it easier to compare the performance of the different implementations.
Arbitrary radixes are now easier to use in format strings. For example:
~~~
assert_eq!(format!("{:04}", radix(3, 2)), ~"0011");
~~~
Currently, the format_args! macro and its downstream macros in turn
expand to series of let statements, one for each of its arguments, and
then the invocation of the macro function. If one or more of the
arguments are RefCell's, the enclosing statement for the temporary of
the let is the let itself, which leads to scope problem. This patch
changes let's to a match expression.
Closes#12239.
Now that fold_item can return multiple items, this is pretty trivial. It
also recursively expands generated items so ItemDecorators can generate
items that are tagged with ItemDecorators!
Closes#4913
The first setp for #9880 is to add a new `crate` keyword. This PR does exactly that. I took a chance to refactor `parse_item_foreign_mod` and I broke it down into 2 separate methods to isolate each feature.
The next step will be to push a new stage0 snapshot and then get rid of all `extern mod` around the code.
If you were writing to something along the lines of `self.foo` then with the new
closure rules it meant that you were borrowing `self` for the entirety of the
closure, meaning that you couldn't format other fields of `self` at the same
time as writing to a buffer contained in `self`.
By lifting the borrow outside of the closure the borrow checker can better
understand that you're only borrowing one of the fields at a time. This had to
use type ascription as well in order to preserve trait object coercions.
This patch adds a new keyword `crate` which is intended to replace mod
in the context of `extern mod` as part of the issue #9880. The patch
doesn't replace all `extern mod` cases since it is necessary to first
push a new snapshot 0.
The implementation could've been less invasive than this. However I
preferred to take this chance to split the `parse_item_foreign_mod`
method and pull the `extern crate` part out of there, hence the new
method `parse_item_foreign_crate`.
While working on #11363 I stumbled over a couple of ignored tests, that seem to be fixed or invalid.
* src/test/run-pass/issue-3559.rs was fixed in #4726
* src/test/compile-fail/borrowck-call-sendfn.rs was fixed in #2978
* update src/test/compile-fail/issue-5500-1.rs to work with current Rust (I'm not 100% sure if the original condition is tested as mentioned in #5500, but I think so)
* removed src/test/compile-fail/issue-5500.rs because it is tested in
src/test/run-fail/issue-5500.rs (they are the same test cases, I just renamed src/test/run-fail/addr-of-bot.rs to be consistent with the other issue name
* src/test/run-pass/issue-3559.rs was fixed in #4726
* src/test/compile-fail/borrowck-call-sendfn.rs was fixed in #2978
* update src/test/compile-fail/issue-5500-1.rs to work with current Rust
* removed src/test/compile-fail/issue-5500.rs because it is tested in
src/test/run-fail/issue-5500.rs
* src/test/compile-fail/view-items-at-top.rs fixed
* #897 fixed
* compile-fail/issue-6762.rs issue was closed as dup of #6801
* deleted compile-fail/issue-2074.rs because it became irelevant and is
irrelevant #2074, a test covering this was added in
4f92f452bd
This, the Nth rewrite of channels, is not a rewrite of the core logic behind
channels, but rather their API usage. In the past, we had the distinction
between oneshot, stream, and shared channels, but the most recent rewrite
dropped oneshots in favor of streams and shared channels.
This distinction of stream vs shared has shown that it's not quite what we'd
like either, and this moves the `std::comm` module in the direction of "one
channel to rule them all". There now remains only one Chan and one Port.
This new channel is actually a hybrid oneshot/stream/shared channel under the
hood in order to optimize for the use cases in question. Additionally, this also
reduces the cognitive burden of having to choose between a Chan or a SharedChan
in an API.
My simple benchmarks show no reduction in efficiency over the existing channels
today, and a 3x improvement in the oneshot case. I sadly don't have a
pre-last-rewrite compiler to test out the old old oneshots, but I would imagine
that the performance is comparable, but slightly slower (due to atomic reference
counting).
This commit also brings the bonus bugfix to channels that the pending queue of
messages are all dropped when a Port disappears rather then when both the Port
and the Chan disappear.
This resolves issue #12157. Does that do it already or is there something else that needs taking care of?
As a side note, there seems to be some documentation, in which the old existence of the do keyword is explained. The list of keywords is not up-to-date either. But these are certainly separate issues.
Resolves issue #12157. `do` is hereby reinstated as a keyword; no syntax is
associated with it though. Along the way, a unit test had to be adapted, since
it was using `do` as a method identifier.
Breaking changes:
- Any code using `do` as an identifier will no longer work.
Declare a `type SendStr = MaybeOwned<'static>` to ease readibility of
types that needed the old SendStr behavior.
Implement all the traits for MaybeOwned that SendStr used to implement.
This has been a long time coming. Conditions in rust were initially envisioned
as being a good alternative to error code return pattern. The idea is that all
errors are fatal-by-default, and you can opt-in to handling the error by
registering an error handler.
While sounding nice, conditions ended up having some unforseen shortcomings:
* Actually handling an error has some very awkward syntax:
let mut result = None;
let mut answer = None;
io::io_error::cond.trap(|e| { result = Some(e) }).inside(|| {
answer = Some(some_io_operation());
});
match result {
Some(err) => { /* hit an I/O error */ }
None => {
let answer = answer.unwrap();
/* deal with the result of I/O */
}
}
This pattern can certainly use functions like io::result, but at its core
actually handling conditions is fairly difficult
* The "zero value" of a function is often confusing. One of the main ideas
behind using conditions was to change the signature of I/O functions. Instead
of read_be_u32() returning a result, it returned a u32. Errors were notified
via a condition, and if you caught the condition you understood that the "zero
value" returned is actually a garbage value. These zero values are often
difficult to understand, however.
One case of this is the read_bytes() function. The function takes an integer
length of the amount of bytes to read, and returns an array of that size. The
array may actually be shorter, however, if an error occurred.
Another case is fs::stat(). The theoretical "zero value" is a blank stat
struct, but it's a little awkward to create and return a zero'd out stat
struct on a call to stat().
In general, the return value of functions that can raise error are much more
natural when using a Result as opposed to an always-usable zero-value.
* Conditions impose a necessary runtime requirement on *all* I/O. In theory I/O
is as simple as calling read() and write(), but using conditions imposed the
restriction that a rust local task was required if you wanted to catch errors
with I/O. While certainly an surmountable difficulty, this was always a bit of
a thorn in the side of conditions.
* Functions raising conditions are not always clear that they are raising
conditions. This suffers a similar problem to exceptions where you don't
actually know whether a function raises a condition or not. The documentation
likely explains, but if someone retroactively adds a condition to a function
there's nothing forcing upstream users to acknowledge a new point of task
failure.
* Libaries using I/O are not guaranteed to correctly raise on conditions when an
error occurs. In developing various I/O libraries, it's much easier to just
return `None` from a read rather than raising an error. The silent contract of
"don't raise on EOF" was a little difficult to understand and threw a wrench
into the answer of the question "when do I raise a condition?"
Many of these difficulties can be overcome through documentation, examples, and
general practice. In the end, all of these difficulties added together ended up
being too overwhelming and improving various aspects didn't end up helping that
much.
A result-based I/O error handling strategy also has shortcomings, but the
cognitive burden is much smaller. The tooling necessary to make this strategy as
usable as conditions were is much smaller than the tooling necessary for
conditions.
Perhaps conditions may manifest themselves as a future entity, but for now
we're going to remove them from the standard library.
Closes#9795Closes#8968
This has been a long time coming. Conditions in rust were initially envisioned
as being a good alternative to error code return pattern. The idea is that all
errors are fatal-by-default, and you can opt-in to handling the error by
registering an error handler.
While sounding nice, conditions ended up having some unforseen shortcomings:
* Actually handling an error has some very awkward syntax:
let mut result = None;
let mut answer = None;
io::io_error::cond.trap(|e| { result = Some(e) }).inside(|| {
answer = Some(some_io_operation());
});
match result {
Some(err) => { /* hit an I/O error */ }
None => {
let answer = answer.unwrap();
/* deal with the result of I/O */
}
}
This pattern can certainly use functions like io::result, but at its core
actually handling conditions is fairly difficult
* The "zero value" of a function is often confusing. One of the main ideas
behind using conditions was to change the signature of I/O functions. Instead
of read_be_u32() returning a result, it returned a u32. Errors were notified
via a condition, and if you caught the condition you understood that the "zero
value" returned is actually a garbage value. These zero values are often
difficult to understand, however.
One case of this is the read_bytes() function. The function takes an integer
length of the amount of bytes to read, and returns an array of that size. The
array may actually be shorter, however, if an error occurred.
Another case is fs::stat(). The theoretical "zero value" is a blank stat
struct, but it's a little awkward to create and return a zero'd out stat
struct on a call to stat().
In general, the return value of functions that can raise error are much more
natural when using a Result as opposed to an always-usable zero-value.
* Conditions impose a necessary runtime requirement on *all* I/O. In theory I/O
is as simple as calling read() and write(), but using conditions imposed the
restriction that a rust local task was required if you wanted to catch errors
with I/O. While certainly an surmountable difficulty, this was always a bit of
a thorn in the side of conditions.
* Functions raising conditions are not always clear that they are raising
conditions. This suffers a similar problem to exceptions where you don't
actually know whether a function raises a condition or not. The documentation
likely explains, but if someone retroactively adds a condition to a function
there's nothing forcing upstream users to acknowledge a new point of task
failure.
* Libaries using I/O are not guaranteed to correctly raise on conditions when an
error occurs. In developing various I/O libraries, it's much easier to just
return `None` from a read rather than raising an error. The silent contract of
"don't raise on EOF" was a little difficult to understand and threw a wrench
into the answer of the question "when do I raise a condition?"
Many of these difficulties can be overcome through documentation, examples, and
general practice. In the end, all of these difficulties added together ended up
being too overwhelming and improving various aspects didn't end up helping that
much.
A result-based I/O error handling strategy also has shortcomings, but the
cognitive burden is much smaller. The tooling necessary to make this strategy as
usable as conditions were is much smaller than the tooling necessary for
conditions.
Perhaps conditions may manifest themselves as a future entity, but for now
we're going to remove them from the standard library.
Closes#9795Closes#8968
