[breaking-change]
What does this break? Basically, code that implements `Drop` and is
using `T:Copy` for one of its type parameters and is relying on the
Drop Check rule not applying to it.
Here is an example:
```rust
#![allow(dead_code,unused_variables,unused_assignments)]
struct D<T:Copy>(T);
impl<T:Copy> Drop for D<T> { fn drop(&mut self) { } }
trait UserT { fn c(&self) { } }
impl<T:Copy> UserT for T { }
struct E<T:UserT>(T);
impl<T:UserT> Drop for E<T> { fn drop(&mut self) { } }
// This one will start breaking.
fn foo() { let (d2, d1); d1 = D(34); d2 = D(&d1); }
#[cfg(this_one_does_and_should_always_break)]
fn bar() { let (e2, e1); e1 = E(34); e2 = E(&e1); }
fn main() {
foo();
}
```
Closes#17841.
The majority of the work should be done, e.g. trait and inherent impls, different forms of UFCS syntax, defaults, and cross-crate usage. It's probably enough to replace the constants in `f32`, `i8`, and so on, or close to good enough.
There is still some significant functionality missing from this commit:
- ~~Associated consts can't be used in match patterns at all. This is simply because I haven't updated the relevant bits in the parser or `resolve`, but it's *probably* not hard to get working.~~
- Since you can't select an impl for trait-associated consts until partway through type-checking, there are some problems with code that assumes that you can check constants earlier. Associated consts that are not in inherent impls cause ICEs if you try to use them in array sizes or match ranges. For similar reasons, `check_static_recursion` doesn't check them properly, so the stack goes ka-blooey if you use an associated constant that's recursively defined. That's a bit trickier to solve; I'm not entirely sure what the best approach is yet.
- Dealing with consts associated with type parameters will raise some new issues (e.g. if you have a `T: Int` type parameter and want to use `<T>::ZERO`). See rust-lang/rfcs#865.
- ~~Unused associated consts don't seem to trigger the `dead_code` lint when they should. Probably easy to fix.~~
Also, this is the first time I've been spelunking in rustc to such a large extent, so I've probably done some silly things in a couple of places.
An actual typeck error is the cause of many failed compilations but an
unrelated bug is being reported instead. It is triggered because a typeck
error is presumably not yet identified during compiler execution, which
would normally bypass an invariant in the presence of other errors. In
this particular situation, we delay the reporting of the bug until
abort_if_errors().
Closes#23827, closes#24356, closes#23041, closes#22897, closes#23966,
closes#24013, and closes#23729
**There is at least one situation where this bug may still be genuinely
triggered (#23437).**
Rather than storing the relations between free-regions in a global
table, introduce a `FreeRegionMap` data structure. regionck computes the
`FreeRegionMap` for each fn and stores the result into the tcx so that
borrowck can use it (this could perhaps be refactored to have borrowck
recompute the map, but it's a bid tedious to recompute due to the
interaction of closures and free fns). The main reason to do this is
because of #22779 -- using a global table was incorrect because when
validating impl method signatures, we want to use the free region
relationships from the *trait*, not the impl.
Fixes#22779.
PR #24242 added the ability to the compiler to directly give suggestions about
how to modify code to fix an error. The new errors look like this:
foobar.rs:5:12: 5:25 error: expected a path on the left-hand side of `+`,
not `&'static Copy` [E0178]
foobar.rs:5 let x: &'static Copy + 'static;
^~~~~~~~~~~~~
foobar.rs:5:12: 5:35 help: try adding parentheses (per RFC 438):
foobar.rs: let x: &'static (Copy + 'static);
foobar.rs:2:13: 2:23 error: cast to unsized type: `&_` as `core::marker::Copy`
foobar.rs:2 let x = &1 as Copy;
^~~~~~~~~~
foobar.rs:2:19: 2:23 help: try casting to a reference instead:
foobar.rs: let x = &1 as &Copy;
foobar.rs:7:24: 7:25 error: expected expression, found `;`
foobar.rs:7 let x = box (1 + 1);
^
foobar.rs:7:13: 7:16 help: try using `box()` instead:
foobar.rs: let x = box() (1 + 1);
This also modifies compiletest to give the ability to directly test suggestions
given by error messages.
table, introduce a `FreeRegionMap` data structure. regionck computes the
`FreeRegionMap` for each fn and stores the result into the tcx so that
borrowck can use it (this could perhaps be refactored to have borrowck
recompute the map, but it's a bid tedious to recompute due to the
interaction of closures and free fns). The main reason to do this is
because of #22779 -- using a global table was incorrect because when
validating impl method signatures, we want to use the free region
relationships from the *trait*, not the impl.
Fixes#22779.
This makes it illegal to have unconstrained lifetimes that appear in an associated type definition. Arguably, we should prohibit all unconstrained lifetimes -- but it would break various macros. It'd be good to evaluate how large a break change it would be. But this seems like the minimal change we need to do to establish soundness, so we should land it regardless. Another variant would be to prohibit all lifetimes that appear in any impl item, not just associated types. I don't think that's necessary for soundness -- associated types are different because they can be projected -- but it would feel a bit more consistent and "obviously" safe. I'll experiment with that in the meantime.
r? @aturon
Fixes#22077.
which get mentioned in an associated type are constrained. Arguably we
should just require that all regions are constrained, but that is more
of a breaking change.
typeck: Do high-level structural/signature checks before function body checks.
This avoids various ICEs, e.g. premature calls to cat_expr that yield the dreaded "cat_expr Errd" ICE.
However, it also means that some early error feedback is now not provided. This may be for the best, because the error feedback were were providing in some of those cases were false positives -- it was spurious feedback and a distraction from the real problem.
So it is not 100% clear whether we actually want to put this change in or not. I think its a net win, but others might disagree.
(Kudos to @arielb1 for suggesting this modification.)
An actual typeck error is the cause of many failed compilations but an
unrelated bug is being reported instead. It is triggered because a typeck
error is presumably not yet identified during compiler execution, which
would normally bypass an invariant in the presence of other errors. In
this particular situation, we delay the reporting of the bug until
abort_if_errors().
Closes#23827, closes#24356, closes#23041, closes#22897, closes#23966,
closes#24013, and closes#23729
Implements an intrinsic for extracting the value of the discriminant
enum variant values. For non-enum types, this returns zero, otherwise it
returns the value we use for discriminant comparisons. This means that
enum types that do not have a discriminant will also work in this
arrangement.
This is (at least part of) the work on Issue #24263
Modify the ExprUseVisitor to walk each part of an AutoRef, and in
particular to treat an AutoUnsize as as kind of \"instantaneous\" borrow
of the value being unsized. This prevents us from feeding uninitialized
data.
This caused a problem for the eager reborrow of comparison traits,
because that wound up introducing a \"double AutoRef\", which was not
being thoroughly checked before but turned out not to type check.
Fortunately, we can just remove that \"eager reborrow\" as it is no longer
needed now that `PartialEq` doesn't force both LHS and RHS to have the
same type (and even if we did have this problem, the better way would be
to lean on introducing a common supertype).
Fixes#20791.
r? @nrc
particular to treat an AutoUnsize as as kind of "instantaneous" borrow
of the value being unsized. This prevents us from feeding uninitialized
data.
This caused a problem for the eager reborrow of comparison traits,
because that wound up introducing a "double AutoRef", which was not
being thoroughly checked before but turned out not to type check.
Fortunately, we can just remove that "eager reborrow" as it is no longer
needed now that `PartialEq` doesn't force both LHS and RHS to have the
same type (and even if we did have this problem, the better way would be
to lean on introducing a common supertype).
In addition to being nicer, this also allows you to use `sum` and `product` for
iterators yielding custom types aside from the standard integers.
Due to removing the `AdditiveIterator` and `MultiplicativeIterator` trait, this
is a breaking change.
[breaking-change]
`Trait`, prefer the object. Also give a nice error for attempts to
manually `impl Trait for Trait`, since they will be ineffectual.
Fixes#24015.
Fixes#24051.
Fixes#24037.
Fixes#23853.
Fixes#21942.
cc #21756.
