There's now one unified way to return things from a macro, instead of
being able to choose the `AnyMacro` trait or the `MRItem`/`MRExpr`
variants of the `MacResult` enum. This does simplify the logic handling
the expansions, but the biggest value of this is it makes macros in (for
example) type position easier to implement, as there's this single thing
to modify.
By my measurements (using `-Z time-passes` on libstd and librustc etc.),
this appears to have little-to-no impact on expansion speed. There are
presumably larger costs than the small number of extra allocations and
virtual calls this adds (notably, all `macro_rules!`-defined macros have
not changed in behaviour, since they had to use the `AnyMacro` trait
anyway).
There is a broader revision (that does this across the board) pending
in #12675, but that is awaiting the arrival of more data (to decide
whether to keep OptVec alive by using a non-Vec internally).
For this code, the representation of lifetime lists needs to be the
same in both ScopeChain and in the ast and ty structures. So it
seemed cleanest to just use `vec_ng::Vec`, now that it has a cheaper
empty representation than the current `vec` code.
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.
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.
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.
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.
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.
Closes#11692. Instead of returning the original expression, a dummy expression
(with identical span) is returned. This prevents infinite loops of failed
expansions as well as odd double error messages in certain situations.
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 means that compilation continues for longer, and so we can see more
errors per compile. This is mildly more user-friendly because it stops
users having to run rustc n times to see n macro errors: just run it
once to see all of them.
Major changes:
- Define temporary scopes in a syntax-based way that basically defaults
to the innermost statement or conditional block, except for in
a `let` initializer, where we default to the innermost block. Rules
are documented in the code, but not in the manual (yet).
See new test run-pass/cleanup-value-scopes.rs for examples.
- Refactors Datum to better define cleanup roles.
- Refactor cleanup scopes to not be tied to basic blocks, permitting
us to have a very large number of scopes (one per AST node).
- Introduce nascent documentation in trans/doc.rs covering datums and
cleanup in a more comprehensive way.
r? @pcwalton
This means that compilation continues for longer, and so we can see more
errors per compile. This is mildly more user-friendly because it stops
users having to run rustc n times to see n macro errors: just run it
once to see all of them.
Major changes:
- Define temporary scopes in a syntax-based way that basically defaults
to the innermost statement or conditional block, except for in
a `let` initializer, where we default to the innermost block. Rules
are documented in the code, but not in the manual (yet).
See new test run-pass/cleanup-value-scopes.rs for examples.
- Refactors Datum to better define cleanup roles.
- Refactor cleanup scopes to not be tied to basic blocks, permitting
us to have a very large number of scopes (one per AST node).
- Introduce nascent documentation in trans/doc.rs covering datums and
cleanup in a more comprehensive way.
This extension can be used to concatenate string literals at compile time. C has
this useful ability when placing string literals lexically next to one another,
but this needs to be handled at the syntax extension level to recursively expand
macros.
The major use case for this is something like:
macro_rules! mylog( ($fmt:expr $($arg:tt)*) => {
error2!(concat!(file!(), ":", line!(), " - ", $fmt) $($arg)*);
})
Where the mylog macro will automatically prepend the filename/line number to the
beginning of every log message.
* Allow named parameters to specify width/precision
* Intepret the format string '0$' as "width is the 0th argument" instead of
thinking the lone '0' was the sign-aware-zero-padding flag. To get both you'd
need to put '00$' which makes more sense if you want both to happen.
Closes#9669
