Fix `vec![x; n]` with null raw fat pointer zeroing the pointer metadata
https://github.com/rust-lang/rust/pull/49496 introduced specialization based on:
```rust
unsafe impl<T: ?Sized> IsZero for *mut T {
fn is_zero(&self) -> bool {
(*self).is_null()
}
}
```
… to call `RawVec::with_capacity_zeroed` for creating `Vec<*mut T>`, which is incorrect for fat pointers since `<*mut T>::is_null` only looks at the data component. That is, a fat pointer can be “null” without being made entirely of zero bits.
This commit fixes it by removing the `?Sized` bound on this impl (and the corresponding `*const T` one). This regresses `vec![x; n]` with `x` a null raw slice of length zero, but that seems exceptionally uncommon. (Vtable pointers are never null, so raw trait objects would not take the fast path anyway.)
An alternative to keep the `?Sized` bound (or even generalize to `impl<U: Copy> IsZero for U`) would be to cast to `&[u8]` of length `size_of::<U>()`, but the optimizer seems not to be able to propagate alignment information and sticks with comparing one byte at a time:
https://rust.godbolt.org/z/xQFkwL
----
Without the library change, the new test fails as follows:
```rust
---- vec::vec_macro_repeating_null_raw_fat_pointer stdout ----
[src/liballoc/tests/vec.rs:1301] ptr_metadata(raw_dyn) = 0x00005596ef95f9a8
[src/liballoc/tests/vec.rs:1306] ptr_metadata(vec[0]) = 0x0000000000000000
thread 'vec::vec_macro_repeating_null_raw_fat_pointer' panicked at 'assertion failed: vec[0] == null_raw_dyn', src/liballoc/tests/vec.rs:1307:5
```
Graphviz debug output for generic dataflow analysis
A follow up to #64566.
This outputs graphviz diagrams in the generic dataflow engine when `#[rustc_mir(borrowck_graphviz_postflow="suffix.dot")]` is set on an item. This code is based on [`dataflow/graphviz.rs`](https://github.com/rust-lang/rust/blob/master/src/librustc_mir/dataflow/graphviz.rs), but displays different information since there are no "gen"/"kill" sets and transfer functions cannot be coalesced in the generic analysis. As a result, we show the dataflow state at the start and end of each block, as well as the changes resulting from each statement. I also render the state bitset in full (`{_1,_2}`) instead of hex-encoded as the current renderer does (`06`).
Use https for curl when building for linux
I noticed that the dist-x86_64-linux builder uses http to fetch curl and doesn't do any signature verification. It should probably use https.
r? @alexcrichton @Mark-Simulacrum
This changes the default parallelism for parallel compilers to one,
instead of the previous default, which was "num cpus". This is likely
not an optimal default long-term, but it is a good default for testing
whether parallel compilers are not a significant regression over a
sequential compiler.
Notably, this in theory makes a parallel-enabled compiler behave
exactly like a sequential compiler with respect to the jobserver.
Remove manual unrolling from slice::Iter(Mut)::try_fold
While this definitely helps sometimes (particularly for trivial closures), it's also a pessimization sometimes, so it's better to leave this to (hypothetical) future LLVM improvements instead of forcing this on everyone.
I think it's better for the advice to be that sometimes you need to unroll manually than you sometimes need to not-unroll manually (like #64545).
---
For context see https://github.com/rust-lang/rust/pull/64572#issuecomment-532961046
