rustdoc-search: add support for nested generics
This change allows `search.js` to parse nested generics (which look `Like<This<Example>>`) and match them. It maintains the existing "bag semantics", so that the order of type parameters is ignored but the number is required to be greater than or equal to what's in the query.
For example, a function with the signature `fn read_all(&mut self: impl Read) -> Result<Vec<u8>, Error>` will match these queries:
* `Read -> Result<Vec<u8>, Error>`
* `Read -> Result<Error, Vec>`
* `Read -> Result<Vec<u8>>`
But it *does not* match `Result<Vec, u8>` or `Result<u8<Vec>>`.
Do not attempt to commute comparison and cast to codegen discriminants
The general algorithm to compute a discriminant is:
```
relative_tag = tag - niche_start
is_niche = relative_tag <= (ule) relative_max
discr = if is_niche {
cast(relative_tag) + niche_variants.start()
} else {
untagged_variant
}
```
We have an optimization branch which attempts to merge the addition and the subtraction by commuting them with the cast. We currently get this optimization wrong.
This PR takes the easiest and safest way: remove the optimization, and let LLVM handle it. (Perf may not agree with that course of action 😅)
There may be a less invasive solution, but I don't have the necessary knowledge of LLVM semantics to find it. Cranelift has the same optimization, which should be handled similarly.
cc `@nikic` and `@bjorn3` if you have a better solution.
Fixes https://github.com/rust-lang/rust/issues/110128
Reformulate `point_at_expr_source_of_inferred_type` to be more accurate
Be more accurate when deducing where along the several usages of a binding it is constrained to be some type that is incompatible with an expectation.
This also renames the method to `note_source_of_type_mismatch_constraint` because I prefer that name, though I guess I can revert that. (Also drive-by rename `note_result_coercion` -> `suggest_coercing_result_via_try_operator`, because it's suggesting, not noting!)
This PR is (probably?) best reviewed per commit, but it does regress a bit only to fix it later on, so it could also be reviewed as a whole if that makes the final results more clear.
r? `@estebank`
Add a stable MIR way to get the main function
This is useful for analysis tools that only analyze the code paths that a specific program actually goes through. Or for code generators built on top of stable MIR.
rustdoc: Correctly handle built-in compiler proc-macros as proc-macro and not macro
Part of https://github.com/rust-lang/rust/issues/110111.
There were actually one issue split in two parts:
* Compiler built-in proc-macro were incorrectly considered as macros and not proc-macros.
* Re-exports of compiler built-in proc-macros were considering them as macros.
Both issues can be fixed by looking at the `MacroKind` variant instead of just relying on information extracted later on.
r? ``@fmease``
Remove all but one of the spans in `BoundRegionKind::BrAnon`
There are only three places where `BoundRegionKind::BrAnon` uses `Some(span)` instead of `None`. Two of them are easy to remove, which this PR does.
r? ```@jackh726```
don't uniquify regions when canonicalizing
uniquifying causes a bunch of issues, most notably it causes `AliasEq(<?x as Trait<'a>>::Assoc, <?x as Trait<'a>>::Assoc)` to result in ambiguity because both `normalizes-to` paths result in ambiguity and substs equate should trivially succeed but doesn't because we uniquified `'a` to two different regions.
I originally added uniquification to make it easier to deal with requirement 6 from the dev-guide: https://rustc-dev-guide.rust-lang.org/solve/trait-solving.html#requirements
> ### 6. Trait solving must be (free) lifetime agnostic
>
> Trait solving during codegen should have the same result as during typeck. As we erase
> all free regions during codegen we must not rely on them during typeck. A noteworthy example
> is special behavior for `'static`.
cc https://github.com/rust-lang/rustc-dev-guide/pull/1671
Relying on regions being identical may cause ICE during MIR typeck, but even without this PR we can end up relying on that as type inference vars can resolve to types which contain an identical region. Let's land this and deal with any ICE that crop up as we go. Will look at this issue again before stabilization.
r? ```@compiler-errors```
Improve safe transmute error reporting
This patch updates the error reporting when Safe Transmute is not possible between 2 types by including the reason.
Also, fix some small bugs that occur when computing the `Answer` for transmutability.
The `std` test straightforwardly can't work without file descriptors;
#99417 tracks moving it out of tests/ui.
`issue-13560.rs` requires the target to support dynamic linking.
`extern-mod-syntax` is interesting. The original test was added to check
if `extern mod` could be parsed correctly and used `extern mod std` and
an import:
138dc3048a (diff-73700e1e851b7a37bc92174635dab726124c82e5bfabbbc45b4a3c2e8e14fadd)
At some point `std::json::Object` was moved out of std to an unstable
rustc-only `extras` crate, and rather than just changing the import it
got changed to use the unstable crate. When `extras` was removed, people
assumed the test was meant to also test rustc_private and changed it to
another unstable crate rather than using something in std.
This changes the test to remove the `rustc_private` import, to allow it
to work properly when cross-compiling.
This patch updates the error reporting when Safe Transmute is not
possible between 2 types by including the reason.
Also, fix some small bugs that occur when computing the `Answer` for
transmutability.
Added diagnostic for pin! macro in addition to Box::pin if Unpin isn't implemented
I made a PR earlier, but accidentally renamed a branch and that deleted the PR... sorry for the duplicate
Currently, if an operation on `Pin<T>` is performed that requires `T` to implement `Unpin`, the diagnostic suggestion is to use `Box::pin` ("note: consider using `Box::pin`").
This PR suggests pin! as well, as that's another valid way of pinning a value, and avoids a heap allocation. Appropriate diagnostic suggestions were included to highlight the difference in semantics (local pinning for pin! vs non-local for Box::pin).
Fixes#109964
Check for body owner fallibly in error reporting
Sometimes the "body id" we use for an obligation cause is not actually a body owner, like when we're doing WF checking on items.
Fixes#110157
Fix diff option conflict in UI test
Trivial fix for test case `tests/run-make/rustdoc-verify-output-files`,
it's failing on MacOS, the `-u` option specifies the unified context format, while the `-q` option specifies only brief output. These two options are incompatible, since the unified context format produces a more detailed output than the brief output format.
Add inline assembly support for m68k
I believe this should be correct, to the extent I understand the logic around inline assembly. M68k is fairly straightforward here, other than having separate address registers.
Erase lifetimes above `ty::INNERMOST` when probing ambiguous types
Turns out that `TyCtxt::replace_escaping_bound_vars_uncached` only erases bound vars exactly at `ty::INNERMOST`, and not everything above. This regresses the suggestions for non-lifetime binders, but oh well, I don't really care about those.
Fixes#110052
Preserve argument indexes when inlining MIR
We store argument indexes on VarDebugInfo. Unlike the previous method of relying on the variable index to know whether a variable is an argument, this survives MIR inlining.
We also no longer check if var.source_info.scope is the outermost scope. When a function gets inlined, the arguments to the inner function will no longer be in the outermost scope. What we care about though is whether they were in the outermost scope prior to inlining, which we know by whether we assigned an argument index.
Fixes#83217
I considered using `Option<NonZeroU16>` instead of `Option<u16>` to store the index. I didn't because `TypeFoldable` isn't implemented for `NonZeroU16` and because it looks like due to padding, it currently wouldn't make any difference. But I indexed from 1 anyway because (a) it'll make it easier if later it becomes worthwhile to use a `NonZeroU16` and because the arguments were previously indexed from 1, so it made for a smaller change.
This is my first PR on rust-lang/rust, so apologies if I've gotten anything not quite right.
Fix `--extend-css` option
Fixes#110002.
The file was generated in the wrong folder so I moved it into `static.files` as it made more sense to have there instead of changing the path in the templates.
I also added a GUI test to ensure that this option won't break unexpectedly again.
Do not use ImplDerivedObligationCause for inherent impl method error reporting
We were constructing a `TraitRef` out of impl substs, for an *inherent* impl that has no corresponding trait. Instead of doing that, let's construct a meaningful obligation cause code, and instead adjust the error reporting machinery to handle that correctly.
Fixes#110131
cc #106702, which introduced this regression
fix: ensure bad `#[test]` invocs retain correct AST
Fixes#109816
Ensures that a `StmtKind::Item` doesn't get converted into a plain `Item` (causing the ICE from the linked issue) Also unifies the error path a bit.
Replace rustdoc-ui/{c,z}-help tests with a stable run-make test
This make rustdoc resilient to changes in the debugging options while still testing that it matches rustc.
Fixes https://github.com/rust-lang/rust/issues/109391.
Rollup of 8 pull requests
Successful merges:
- #110153 (Fix typos in compiler)
- #110165 (rustdoc: use CSS `overscroll-behavior` instead of JavaScript)
- #110175 (Symbol cleanups)
- #110203 (Remove `..` from return type notation)
- #110205 (rustdoc: make settings radio and checks thicker, less contrast)
- #110222 (Improve the error message when forwarding a matched fragment to another macro)
- #110237 (Split out a separate feature gate for impl trait in associated types)
- #110241 (tidy: Issue an error when UI test limits are too high)
Failed merges:
- #110218 (Remove `ToRegionVid`)
r? `@ghost`
`@rustbot` modify labels: rollup
Split out a separate feature gate for impl trait in associated types
in https://github.com/rust-lang/rust/issues/107645 it was decided that we'll take a new route for type alias impl trait. The exact route isn't clear yet, so while I'm working on implementing some of these proposed changes (e.g. in https://github.com/rust-lang/rust/pull/110010) to be able to experiment with them, I will also work on stabilizing another sugar version first: impl trait in associated types. Similarly I'll look into creating feature gates for impl trait in const/static types.
This PR does nothing but split the feature gate, so that you need to enable a different feature gate for
```rust
impl Trait for Type {
type Assoc = impl SomeTrait;
}
```
than what you need for `type Foo = impl SomeTrait;`
Improve the error message when forwarding a matched fragment to another macro
Adds a link to [Forwarding a matched fragment](https://doc.rust-lang.org/nightly/reference/macros-by-example.html#forwarding-a-matched-fragment) section of the Rust Reference, and suggests a possible fix (using `:tt` instead in the macro definition).
Also removes typos from the original message, it should be `:lifetime` instead of `$lifetime`.
## Motivation
When trying to write a macro which uses a literal in the matcher from the outer macro, like the following one, using a fragment specified that isn't one of `:ident`, `:lifetime`, or `:tt` currently results in a hard to understand message.
```rs
macro_rules! make_t_for_all_tokens {
($($name:literal as $variant:expr,)*) => {
macro_rules! t {
$(
($name) => {
$variant
};
)*
}
};
}
make_t_for_all_tokens! {
"fn" as Token::Fn,
"return" as Token::Return,
"let" as Token::Let,
}
// This creates
//
// macro_rules! t {
// ("fn") => {
// Token::Fn
// };
// ("return") => {
// Token::Return
// };
// ("let") => {
// Token::Let
// };
// }
t!["fn"];
```
### Before
```
error: no rules expected the token `"fn"`
--> src/main.rs:103:10
|
32 | macro_rules! t {
| -------------- when calling this macro
...
103 | t!["fn"];
| ^^^^ no rules expected this token in macro call
|
note: while trying to match `"fn"`
--> src/main.rs:34:6
|
34 | ($name) => {
| ^^^^^
...
58 | / make_t_for_all_tokens! {
59 | | "fn" as Token::Fn,
60 | | "return" as Token::Return,
61 | | "let" as Token::Let,
62 | | }
| |_- in this macro invocation
= note: captured metavariables except for `$tt`, `$ident` and `$lifetime` cannot be compared to other tokens
= note: this error originates in the macro `make_t_for_all_tokens` (in Nightly builds, run with -Z macro-backtrace for more info)
```
### After
```
error: no rules expected the token `"fn"`
--> src/main.rs:103:10
|
32 | macro_rules! t {
| -------------- when calling this macro
...
103 | t!["fn"];
| ^^^^ no rules expected this token in macro call
|
note: while trying to match `"fn"`
--> src/main.rs:34:6
|
34 | ($name) => {
| ^^^^^
...
58 | / make_t_for_all_tokens! {
59 | | "fn" as Token::Fn,
60 | | "return" as Token::Return,
61 | | "let" as Token::Let,
62 | | }
| |_- in this macro invocation
= note: captured metavariables except for `:tt`, `:ident` and `:lifetime` cannot be compared to other tokens
= note: see https://doc.rust-lang.org/nightly/reference/macros-by-example.html#forwarding-a-matched-fragment for more information
= help: try using `:tt` instead in the macro definition
= note: this error originates in the macro `make_t_for_all_tokens` (in Nightly builds, run with -Z macro-backtrace for more info)
```
## Unresolved questions
- Preferrably the suggestion should be attached to the `$name:literal` part of the outer macro, instead of being in the notes section at the end. But I'm not familiar with how the compiler works at all, and I have no idea how to approach this kind of solution.
- `@Nilstrieb` raised a question that the suggestion of adding `:tt` isn't accurate when there's more than `tt` being matched, for example when the input is an `item`.
Remove `..` from return type notation
`@nikomatsakis` and I decided that using `..` in the return-type notation syntax is probably overkill.
r? `@eholk` since you reviewed the last one
Since this is piggybacking now totally off of a pre-existing syntax (parenthesized generics), let me know if you need any explanation of the logic here, since it's a bit more complicated now.
Fixes the desktop scrolling weirdness mentioned in
https://github.com/rust-lang/rust/pull/98775#issuecomment-1182575603
As described in the MDN page for this property:
* The current Firefox ESR is 102, and the first Firefox version
to support this feature is 59.
* The current Chrome version 112, and the first version to support
this is 63.
* Edge is described as having a minor bug in `none` mode, but we
use `contain` mode anyway, so it doesn't matter.
* Safari 16, released September 2022, is the last browser to
add this feature, and is also the oldest version we officially
support.
This is very dependent on subjectivity and what screen you use,
but this change makes the radio buttons' outer circle less ugly.
This is because I could see the pixels very clearly, thanks to the
very thin line and high contrast. This change makes both less
severe, giving your browser's antialiasing algorithm more to
work with. Since it's thicker, lowering the contrast shouldn't
impact visibility.
Custom MIR: Support `BinOp::Offset`
Since offset doesn't have an infix operator, a new function `Offset` is added which is lowered to `Rvalue::BinaryOp(BinOp::Offset, ..)`
r? ```@oli-obk``` or ```@tmiasko``` or ```@JakobDegen```
Fix transmute intrinsic mir validation ICE
I stumbled across this at work, the minimal reproducer is included as a test which ICEs before this change.
I'm not 100% sure this is the right fix, but it matches what we do in `mir_assign_valid_types` so seems reasonable at least.
fixes#110151
r? `@lcnr` since they've been keeping the relevant logic correct, cc `@scottmcm`
incr.comp.: Make sure dependencies are recorded when feeding queries during eval-always queries.
This PR makes sure we don't drop dependency edges when feeding queries during an eval-always query.
Background: During eval-always queries, no dependencies are recorded because the system knows to unconditionally re-evaluate them regardless of any actual dependencies. This works fine for these queries themselves but leads to a problem when feeding other queries: When queries are fed, we set up their dependency edges by copying the current set of dependencies of the feeding query. But because this set is empty for eval-always queries, we record no edges at all -- which has the effect that the fed query instances always look "green" to the system, although they should always be "red".
The fix is to explicitly add a dependency on the artificial "always red" dep-node when feeding during eval-always queries.
Fixes https://github.com/rust-lang/rust/issues/108481
Maybe also fixes issue https://github.com/rust-lang/rust/issues/88488.
cc `@jyn514`
r? `@cjgillot` or `@oli-obk`
Adds a link to the relevant part of The Rust Reference in the eror
message, and suggests a possible fix (replacing the fragment specifier
with :tt in the macro definition).
Fixes typos in the original message.
Signed-off-by: Lena Milizé <me@lvmn.org>
Split implied and super predicate queries, then allow elaborator to filter only supertraits
Split the `super_predicates_of` query into a new `implied_predicates_of` query. The former now only returns the *real* supertraits of a trait alias, and the latter now returns the implied predicates (which include all of the `where` clauses of the trait alias). The behavior of these queries is identical for regular traits.
Now that the two queries are split, we can add a new filter method to the elaborator, `filter_only_self()`, which can be used in instances that we need only the *supertrait* predicates, such as during the elaboration used in closure signature deduction. This toggles the usage of `super_predicates_of` instead of `implied_predicates_of` during elaboration of a trait predicate.
This supersedes #104745, and fixes the four independent bugs identified in that PR.
Fixes#104719Fixes#106238Fixes#110023Fixes#109514
r? types
Support safe transmute in new solver
Basically copies the same implementation as the old solver, but instead of looking for param types, we look for type or const placeholders.
Rename tests/ui/unique to tests/ui/box/unit
fixes https://github.com/rust-lang/rust/issues/109878
Since tests/ui/box already exists, I have temporarily named it boxed-box, but if another name sounds better, please let me know.
Initial support for loongarch64-unknown-linux-gnu
Hi, We hope to add a new port in rust for LoongArch.
LoongArch intro
LoongArch is a RISC style ISA which is independently designed by Loongson
Technology in China. It is divided into two versions, the 32-bit version (LA32)
and the 64-bit version (LA64). LA64 applications have application-level
backward binary compatibility with LA32 applications. LoongArch is composed of
a basic part (Loongson Base) and an expanded part. The expansion part includes
Loongson Binary Translation (LBT), Loongson VirtualiZation (LVZ), Loongson SIMD
EXtension (LSX) and Loongson Advanced SIMD EXtension(LASX).
Currently the LA464 processor core supports LoongArch ISA and the Loongson
3A5000 processor integrates 4 64-bit LA464 cores. LA464 is a four-issue 64-bit
high-performance processor core. It can be used as a single core for high-end
embedded and desktop applications, or as a basic processor core to form an
on-chip multi-core system for server and high-performance machine applications.
Documentations:
ISA:
https://loongson.github.io/LoongArch-Documentation/LoongArch-Vol1-EN.html
ABI:
https://loongson.github.io/LoongArch-Documentation/LoongArch-ELF-ABI-EN.html
More docs can be found at:
https://loongson.github.io/LoongArch-Documentation/README-EN.html
Since last year, we have locally adapted two versions of rust, rust1.41 and rust1.57, and completed the test locally.
I'm not sure if I'm submitting all the patches at once, so I split up the patches and here's one of the commits
Update browser-ui-test version
This update add the support for expressions, so we can now do this:
```
assert: 1 > 2 && ["a"] != ["b", "c"]
```
It also improved commands naming and updated puppeteer version.
r? `@notriddle`
Migrate most of `rustc_builtin_macros` to diagnostic impls
cc #100717
This is a couple of days work, but I decided to stop for now before the PR becomes too big. There's around 50 unresolved failures when `rustc::untranslatable_diagnostic` is denied, which I'll finish addressing once this PR goes thtough
A couple of outputs have changed, but in all instances I think the changes are an improvement/are more consistent with other diagnostics (although I'm happy to revert any which seem worse)
rustc_metadata: Filter encoded data more aggressively using `DefKind`
I focused on data that contains spans, because spans are expensive to encode/decode/hash, but also touched `should_encode_visibility` too.
One incorrect use of impl visibility in diagnostics is also replaced with trait visibility.
These don't optimize with debug assertions. For one of them, this
is due to the new alignment checks, for the other I'm not sure
what specifically blocks it.
this test was added for rust 0.4 and doesn't test anything specific.
The repro originally relied on extern functions which are now just
ordinary methods. It's also a run pass test even though `main` has
been commented out.
Rollup of 8 pull requests
Successful merges:
- #109527 (Set up standard library path substitution in rust-gdb and gdbgui)
- #109752 (Stall auto trait assembly in new solver for int/float vars)
- #109860 (Add support for RISC-V relax target feature)
- #109923 (Update `error [E0449]: unnecessary visibility qualifier` to be more clear)
- #110070 (The `wrapping_neg` example for unsigned types shouldn't use `i8`)
- #110146 (fix(doc): do not parse inline when output is json for external crate)
- #110147 (Add regression test for #104916)
- #110149 (Update books)
Failed merges:
r? `@ghost`
`@rustbot` modify labels: rollup
Add regression test for #104916Closes#104916
I haven't tested if it still passes with debug assertions enabled so it'd be better to wait for CI to be green.
r? compiler-errors
Update `error [E0449]: unnecessary visibility qualifier` to be more clear
This updates the error message `error[E0449]: unnecessary visibility qualifier` by clearly indicating that visibility qualifiers already inherit their visibility from a parent item. The error message previously implied that the qualifiers were permitted, which is not the case anymore.
Resolves#109822.
We store argument indexes on VarDebugInfo. Unlike the previous method of
relying on the variable index to know whether a variable is an argument,
this survives MIR inlining.
We also no longer check if var.source_info.scope is the outermost scope.
When a function gets inlined, the arguments to the inner function will
no longer be in the outermost scope. What we care about though is
whether they were in the outermost scope prior to inlining, which we
know by whether we assigned an argument index.
Add suggestion to remove `derive()` if invoked macro is non-derive
Adds to the existing `expected derive macro, found {}` error message:
```
help: remove the surrounding "derive()":
--> $DIR/macro-path-prelude-fail-4.rs:1:3
|
LL | #[derive(inline)]
| ^^^^^^^ ^
```
This suggestion will either fix the issue, in the case that the macro was valid, or provide a better error message if not
Not ready for merge yet, as the highlighted span is only valid for trivial formatting. Is there a nice way to get the parent span of the macro path within `smart_resolve_macro_path`?
Closes#109589
Fix a couple ICEs in the new `CastKind::Transmute` code
Check the sizes of the immediates, rather than the overall types, when deciding whether we can convert types without going through memory.
Fixes#110005Fixes#109992Fixes#110032
cc `@matthiaskrgr`
Migrate remainder of rustc_ty_utils to `SessionDiagnostic`
This moves the remaining errors in `rust_ty_utils` to `SessionsDiagnostic`.
r? ``@davidtwco``
Instantiate instead of erasing binder when probing param methods
Fixes#108836
There is a really old comment saying that a `WhereClauseCandidate` probe candidate "should not contain any inference variables", but I'm not really confident that that comment applies anymore. In contrast, other candidates that we assemble during method probe contain inference variables in their substitutions (e.g. `InherentImplCandidate`)...
Since this change is made only to support a nightly feature, I'm happy to gate the new behavior behind this feature flag or discuss it further.
r? types
Better diagnostic when pattern matching tuple structs
Fixes#108284
When trying to pattern match a tuple struct we might get a flawed error message if there are missing fields. E.g.
```
let x = Foo(100, 200);
if let Foo { 0: bar } = x { ... }
```
Produces this error:
```
error[E0769]: tuple variant `Foo` written as struct variant
--> hello.rs:5:12
|
5 | if let Foo { 0: foo } = x {
| ^^^^^^^^^^^^^^
|
help: use the tuple variant pattern syntax instead
|
5 | if let Foo(_, _) = x {
| ~~~~~~
```
Which doesn't highlight that we can still use the struct syntax but we need to fill missing fields. This pr changes this error to:
```
error[E0027]: pattern does not mention field `1`
--> hello.rs:5:12
|
5 | if let Foo { 0: foo } = x {
| ^^^^^^^^^^^^^^ missing field `1`
|
help: include the missing field in the pattern
|
5 | if let Foo { 0: foo, 1: _ } = x {
| ~~~~~~~~
help: if you don't care about this missing field, you can explicitly ignore it
|
5 | if let Foo { 0: foo, .. } = x {
| ~~~~~~
```
Suggest defining const parameter when appropriate
Helps a bit with #91119.
Following #105523's lead, I use placeholder `/* Type */` instead of `_` in the suggestion.
It should be easier for newcomers to parse.
`@rustbot` label A-diagnostics
r? diagnostics
Enforce that `PointerLike` requires a pointer-like ABI
At least temporarily, let's ban coercing things that are pointer-sized and pointer-aligned but *not* `Abi::Scalar(..)` into `dyn*`. See: https://github.com/rust-lang/rust/pull/104694#discussion_r1142522073
This can be lifted in the future if we decie that we *want* to be able to coerce something `repr(C)` into a `dyn*`, but we'll have to figure out what to do with Miri and codegen...
r? compiler
Add ability to transmute (somewhat) with generic consts in arrays
Previously if the expression contained generic consts and did not have a directly equivalent type, transmuting the type in this way was forbidden, despite the two sizes being identical. Instead, we should be able to lazily tell if the two consts are identical, and if so allow them to be transmuted.
This is done by normalizing the forms of expressions into sorted order of multiplied terms, which is not generic over all expressions, but should handle most cases.
This allows for some _basic_ transmutations between types that are equivalent in size without requiring additional stack space at runtime.
I only see one other location at which `SizeSkeleton` is being used, and it checks for equality so this shouldn't affect anywhere else that I can tell.
See [this Stackoverflow post](https://stackoverflow.com/questions/73085012/transmute-nested-const-generic-array-rust) for what was previously necessary to convert between types. This PR makes converting nested `T -> [T; 1]` transmutes possible, and `[uB*2; N] -> [uB; N * 2]` possible as well.
I'm not sure whether this is something that would be wanted, and if it is it definitely should not be insta-stable, so I'd add a feature gate.
