In the future Windows will enable Control-flow Enforcement Technology
(CET aka Shadow Stacks). To protect the path where the context is
updated during exception handling, the binary is required to enumerate
valid unwind entrypoints in a dedicated section which is validated when
the context is being set during exception handling.
The required support for EHCONT has already been merged into LLVM,
long ago. This change adds the Rust codegen option to enable it.
Reference:
* https://reviews.llvm.org/D40223
This also adds a new `ehcont-guard` option to the bootstrap config which
enables EHCont Guard when building std.
The AST and HIR versions of `State::print_ident` are textually
identical, but the types differ slightly. This commit factors out the
common code they both have by replacing `print_ident` with `ann_post`,
which is a smaller function that still captures the type difference.
`PrintState` is a trait containing code that can be used by both AST and
HIR pretty-printing. But several of its methods are only used by AST
printing.
This commit moves those methods out of the trait and into the AST
`State` impl, so they are not exposed unnecessarily. This commit also
removes four unused methods: `param_to_string`,
`foreign_item_to_string`, `assoc_item_to_string`, and
`print_inner_attributes_inline`.
The search sorting code already sorts by item type discriminant,
putting things with smaller discriminants first. There was
also a special case for sorting keywords and primitives earlier,
and this commit removes it by giving them lower discriminants.
The sorting code has another criteria where items with descriptions
appear earlier than items without, and that criteria has higher
priority than the item type. This shouldn't matter, though,
because primitives and keywords normally only appear in the
standard library, and it always gives them descriptions.
This computes the same result with less code by computing many of
the old checks at once:
* It won't enter the loop if clength > length, because then the
result of length - clength will be negative and the
loop conditional will fail.
* i + clength will never be greater than length, because it
starts out as i = length - clength, implying that i + clength
equals length, and it only goes down from there.
* The aborted variable is replaced with control flow.
When building the standard library with Miri, it appears that cargo-miri
does not respect the RUSTFLAGS and even if they did it would be wrong
since they are created for ToolRustc not Std. To avoid errors ignore
RUSTC_HOST_FLAGS for Miri.
Update backtrace submodule
This PR updates the backtrace submodule, avoiding panics when resolving backtraces on Windows 7:
- https://github.com/rust-lang/backtrace-rs/pull/578Fixes#117941
I ran the std unit tests locally on a Windows7 machine, and can confirm that this indeed fixes#117941.
Remove i686-apple-darwin cross-testing
The Xcode SDK no longer ships with 32-bit Intel (i686-apple-darwin) support as of [Xcode 14](https://developer.apple.com/news/upcoming-requirements/?id=06062022a) (related, #112753). On an up-to-date Intel Mac, `x.py test --bless` fails.
r? ``@rust-lang/bootstrap``
Fix early param lifetimes in generic_const_exprs
In cases like below, we never actually be able to capture region name for two reasons, first `'static` becomes anonymous lifetime and second we never capture region if it doesn't have a name so this results in ICE.
```
struct DataWrapper<'static> {
data: &'a [u8; Self::SIZE],
}
impl DataWrapper<'a> {
```
Fixes https://github.com/rust-lang/rust/issues/118021
Expand Miri's BorTag GC to a Provenance GC
As suggested in https://github.com/rust-lang/miri/issues/3080#issuecomment-1732505573
We previously solved memory growth issues associated with the Stacked Borrows and Tree Borrows runtimes with a GC. But of course we also have state accumulation associated with whole allocations elsewhere in the interpreter, and this PR starts tackling those.
To do this, we expand the visitor for the GC so that it can visit a BorTag or an AllocId. Instead of collecting all live AllocIds into a single HashSet, we just collect from the Machine itself then go through an accessor `InterpCx::is_alloc_live` which checks a number of allocation data structures in the core interpreter. This avoids the overhead of all the inserts that collecting their keys would require.
r? ``@RalfJung``
Remove `--check-cfg` checking of command line `--cfg` args
Back in https://github.com/rust-lang/rust/pull/100574 we added to the `unexpected_cfgs` lint the checking of `--cfg` CLI arguments and emitted unexpected names and values for them.
The implementation works as expected, but it's usability in particular when using it in combination with Cargo+`RUSTFLAGS` as people who set `RUSTFLAGS=--cfg=tokio_unstable` (or whatever) have `unexpected_cfgs` warnings on all of their crates is debatable. ~~To fix this issue this PR proposes that we split the CLI argument checking into it's own separate allow-by-default lint: `unexpected_cli_cfgs`.~~
~~This has the advantage of letting people who want CLI warnings have them (although not by default anymore), while still linting on every unexpected cfg name and values in the code.~~
After some discussion with the Cargo team ([Zulip thread](https://rust-lang.zulipchat.com/#narrow/stream/246057-t-cargo/topic/check-cfg.20and.20RUSTFLAGS.20interaction)) and member of the compiler team (see below), I propose that we follow the suggestion from `@epage:` never check `--cfg` arguments, but still reserve us the possibility to do it later.
We would still lint on unexpected cfgs found in the source code no matter the `--cfg` args passed. This mean reverting https://github.com/rust-lang/rust/pull/100574 but NOT https://github.com/rust-lang/rust/pull/99519.
r? `@petrochenkov`
rustdoc-search: add support for traits and associated types
# Summary
Trait associated type queries work in rustdoc's type driven search. The data is included in the search-index.js file, and the queries are designed to "do what I mean" when users type them in, so, for example, `Iterator<Item=T> -> Option<T>` includes `Iterator::next` in the SERP[^SERP], and `Iterator<T> -> Option<T>` also includes `Iterator::next` in the SERP.
[^SERP]: search engine results page
## Sample searches
* [`iterator<Item=T>, fnmut -> T`][iterreduce]
* [`iterator<T>, fnmut -> T`][iterreduceterse]
[iterreduce]: http://notriddle.com/rustdoc-html-demo-5/associated-types/std/index.html?search=iterator%3CItem%3DT%3E%2C%20fnmut%20-%3E%20T&filter-crate=std
[iterreduceterse]: http://notriddle.com/rustdoc-html-demo-5/associated-types/std/index.html?search=iterator%3CT%3E%2C%20fnmut%20-%3E%20T&filter-crate=std
# Motivation
My primary motivation for working on search.js at all is to make it easier to use highly generic APIs, like the Iterator API. The type signature describes these functions pretty well, while the names are almost arbitrary.
Before this PR, type bindings were not consistently included in search-index.js at all (you couldn't find Iterator::next by typing in its function signature) and you couldn't explicitly search for them. This PR fixes both of these problems.
# Guide-level explanation
*Excerpt from [the Rustdoc book](http://notriddle.com/rustdoc-html-demo-5/associated-types/rustdoc/read-documentation/search.html), included in this PR.*
> Function signature searches can query generics, wrapped in angle brackets, and traits will be normalized like types in the search engine if no type parameters match them. For example, a function with the signature `fn my_function<I: Iterator<Item=u32>>(input: I) -> usize` can be matched with the following queries:
>
> * `Iterator<Item=u32> -> usize`
> * `Iterator<u32> -> usize` (you can leave out the `Item=` part)
> * `Iterator -> usize` (you can leave out iterator's generic entirely)
> * `T -> usize` (you can match with a generic parameter)
>
> Each of the above queries is progressively looser, except the last one would not match `dyn Iterator`, since that's not a type parameter.
# Reference-level explanation
Inside the angle brackets, you can choose whether to write a name before the parameter and the equal sign. This syntax is called [`GenericArgsBinding`](https://doc.rust-lang.org/reference/paths.html#paths-in-expressions) in the Rust Reference, and it allows you to constrain a trait's associated type.
As a convenience, you don't actually have to put the name in (Rust requires it, but Rustdoc Search doesn't). This works about the same way unboxing already works in Search: the terse `Iterator<u32>` is a match for `Iterator<Item=u32>`, but the opposite is not true, just like `u32` is a match for `Iterator<u32>`.
When converting a trait method for the search index, the trait is substituted for `Self`, and all associated types are bound to generics. This way, if you have the following trait definition:
```rust
pub trait MyTrait {
type Output;
fn method(self) -> Self::Output;
}
```
The following queries will match its method:
* `MyTrait<Output=T> -> T`
* `MyTrait<T> -> T`
* `MyTrait -> T`
But these queries will not match it:
* <i>`MyTrait<Output=u32> -> u32`</i>
* <i>`MyTrait<Output> -> Output`</i>
* <i>`MyTrait -> MyTrait::Output`</i>
# Drawbacks
It's a little bit bigger:
```console
$ du before/search-index1.74.0.js after/search-index1.74.0.js
4020 before/search-index1.74.0.js
4068 after/search-index1.74.0.js
```
# Rationale and alternatives
I don't want to just not do this. On it's own, it's not terribly useful, but in addition to searching by normal traits, this is also intended as a desugaring target for closures. That's why it needs to actually distinguish the two: it allows the future desugaring to distinguish function output and input.
The other alternative would be to not allow users to leave out the name, so `iterator<u32>` doesn't work. That would be unfortunate, because mixing up which ones have out params and which ones are plain generics is an easy enough mistake that the Rust compiler itself helps people out with it.
# Prior art
* <http://neilmitchell.blogspot.com/2020/06/hoogle-searching-overview.html>
The current Rustdoc algorithm, both before this PR and after it, has a fairly expensive matching algorithm over a fairly simple file format. Luckily, we aren't trying to scale to all of crates.io, so it's usable, but it's not great when I throw it at docs.servo.org
# Unresolved questions
Okay, but *how do we want to handle closures?* I know the system will desugar `FnOnce(T) -> U` into `trait:FnOnce<Output=U, primitive:tuple<T>>`, but what if I don't know what trait I'm looking for? This PR can merge with nothing, but it'd be nice to have a plan.
Specifically, how should the special form used to handle all varieties of basic callable: primitive:fn (function pointers), and trait:Fn, trait:FnOnce, and trait:FnMut should all be searchable using a single syntax, because I'm always forgetting which one is used in the function I'm looking for.
The essential question is how closely we want to copy Rust's own syntax. The tersest way to expression Option::map might be:
Option<T>, (T -> U) -> Option<U>
That's the approach I would prefer, but nobody's going to attempt it without being told, so maybe this would be better?
Option<T>, (fn(T) -> U) -> Option<U>
It does require double parens, but at least it's mostly unambiguous. Unfortunately, it looks like the syntax you'd use for function pointers, implying that if you specifically wanted to limit your search to function pointers, you'd need to use `primitive:fn(T) -> U`. Then again, searching is normally case-insensitive, so you'd want that anyway to disambiguate from `trait:Fn(T) -> U`.
# Future possibilities
## This thing really needs a ranking algorithm
That is, this PR increases the number of matches for some type-based queries. They're usually pretty good matches, but there's still more of them, and it's evident that if you have two functions, `foo(MyTrait<u8>)` and `bar(MyTrait<Item=u8>)`, if the user typed `MyTrait<u8>` then `foo` should show up first.
A design choice that these PRs have followed is that adding more stuff to the search query always reduces the number of functions that get matched. The advantage of doing it that way is that you can rank them by just counting how many atoms are in the function's signature (lowest score goes on top). Since it's impossible for a matching function to have fewer atoms than the search query, if there's a function with exactly the same set of atoms in it, then that'll be on top.
More complicated ranking algos tend to penalize long documents anyway, if the [distance metrics](https://www.benfrederickson.com/distance-metrics/?utm_source=flipboard&utm_content=other) I found through [Flipboard](https://flipboard.com/`@arnie0426/building-recommender-systems-nvue3iqtgrn10t45)` (and postgresql's `ts_rank_cd`) are anything to go by. Real-world data sets tend to have weird outliers, like they have God Functions with zillions of arguments of all sorts of types, and Rustdoc ought to put such a function at the bottom.
The other natural choice would be interleaving with `unifyFunctionTypes` to count the number of unboxings and reorderings. This would compute a distance function, and would do a fine job of ranking the results, as [described here](https://ndmitchell.com/downloads/slides-hoogle_finding_functions_from_types-16_may_2011.pdf) by the Hoogle dev, but is more complicated than it sounds. The current algorithm returns when it finds a result that *exists at all*, but a distance function should find an *optimal solution* to find the smallest sequence of edits.
## This could also use a benchmark suite and some optimization
This approach also lends itself to layering a bloom filter in front of the backtracking unification engine.
* At load time, hash the typeNameIdMap ID for each atom and set the matching entry in a fixed-size byte array for each function to 1. Call it `fnType.bloomFilter`
* At search time, do the same for the atoms in the query (excluding special forms like `[]` that can match more than one thing). Call it `parsedQuery.bloomFilter`
* For each function, `if (fnType.bloomFilter | (~parsedQuery.bloomFilter) !== ~0) { return false; }`
There's also room to optimize the unification engine itself, by using stacks and persistent data structures instead of copying arrays around, or by using hashing instead of linear scans (the current algorithm was rewritten from one that tried to do that, but was too much to fit in my head and had a bunch of bugs). The advantage of Just Backtracking Better over the bloom filter is that it doesn't require the engine to retain any special algebraic properties.
But, first, we need a set of benchmarks to be able to judge if such a thing will actually help.
## Referring to associated types by path
*I don't want to implement this one, but if I did, this is how I'd do it.*
In Rust, this is represented by a structure called a qualified path, or QPath. They look like this:
<Self as Iterator>::Item
<F as FnOnce>::Output
They can also, if it's unambiguous, use a plain path and just let the system figure it out:
Self::Item
F::Output
In Rustdoc Type-Driven Search, we don't want to force people to be unambiguous. Instead, we should try *all reasonable interpretations*, return results whenever any of them match, and let users make their query more specific if too many results are matches.
To enable associated type path searches in Rustdoc, we need to:
1. When lowering a trait method to a search-index.js function signature, Self should be explicitly represented as a generic argument. It should always be assigned `-1`, so that if the user uses `Self` in their search query, we can ensure it always matches the real Self and not something else. Any functions that don't *have* a Self should drop a `0` into the first position of the where clause, to express that there isn't one and reserve the `-1` position.
* Reminder: generics are negative, concrete types are positive, and zero is a reserved sentinel.
* Right now, `Iterator::next` is lowered as if it were `fn next<T>(self: Iterator<Item=T>) -> Option<T>`.
It should become `fn next<Self, T>(self: Self) -> Option<T> where Self: Iterator<Item=T>` instead.
3. Add another backtracking edge to the unification engine, so that when the user writes something like `some::thing`, the interpretation where `some` is a module and `thing` is a standalone item becomes one possible match candidate, while the interpretation where `some` is a trait and `thing` is an associated type is a separate match candidate. The backtracking engine is basically powerful enough to do this already, since unboxing generic type parameters into their traits already requires the ability to do this kind of thing.
* When interpreting `some::thing` where `some` is a trait and `thing` is an associated type, it should be treated equivalently to `<self as some>::thing`. If you want to bind it to some generic parameter other than `Self`, you need to explicitly say so.
* If no trait called `some` actually exists, treat it as a generic type parameter instead. Track every trait mentioned in the current working function signature, and add a match candidate for each one.
* A user that explicitly wants the trait-associated-type interpretation could write a qpath (like `<self as trait>::type`), and a user that explicitly wants the module-item interpretation should use an item type filter (like `struct:module::type`).
4. To actually do the matching, maintain a `Map<(QueryGenericParamId, TraitId), FnGenericParamId>` alongside the existing `Map<QueryGenericParamId, FnGenericParamId>` that is already used to handle plain generic parameters. This works, because, when a trait function signature is lowered to search-index.js, the `rustdoc` backend always generates an FnGenericParamId for every trait associated type it sees mentioned in the function's signature.
5. Parse QPaths. Specifically,
* QueryElem adds three new fields. `isQPath` is a boolean flag, and `traitNameId` contains an entry for `typeNameIdMap` corresponding to the trait part of a qpath, and `parentId` may contain either a concrete type ID or a negative number referring to a generic type parameter. The actual `id` of the query elem will always be a negative number, because this is essentially a funny way to add a generic type constraint.
* If it's a QPath, then both of those IDs get filled in with the respective parts of the map. The unification engine will check the where clause to ensure the trait actually applies to the generic parameter in question, will check the type parameter constraint, and will add a mapping to `mgens` recording this as a solution.
* If it's just a regular path, then `isQPath` is false, and the parser will fill in both `traitNameId` and `parentId` based on the same path. The unification engine, seeing isQPath is false and that these IDs were filled in, will try all three solutions: the path might be part of a concrete type name, or it might be referring to a trait, or it might be referring to a generic type parameter.
### Why not implement QPath searches?
I'm not sure if anybody really wants to write such complicated queries. You can do a pretty good job of describing the generic functions in the standard library without resorting to FQPs.
These two queries, for example, would both match the Iterator::map function if we added support for higher order function queries and a rule that allows a type to match its *notable traits*.
// I like this version, because it's identical to how `Option::map` would be written.
// There's a reason why Iterator::map and Option::map have the same name.
Iterator<T>, (T -> U) -> Iterator<U>
// This version explicitly uses the type parameter constraints.
Iterator<Item=T>, (T -> U) -> Iterator<Item=U>
If I try to write this one using FQP, however, the results seem worse:
// This one is less expressive than the versions that don't use associated type paths.
// It matches `Iterator::filter`, while the above two example queries don't.
Iterator, (Iterator::Item -> Iterator::Item) -> Iterator
// This doesn't work, because the return type of `Iterator::map` is not a generic
// parameter with an `Iterator` trait bound. It's a concrete type that
// implements `Iterator`. Return-Position-Impl-Trait is the same way.
//
// There's a difference between something like `map`, whose return value
// implements Iterator, and something like `collect`, where the caller
// gets to decide what the concrete type is going to be.
//Self, (Self::Item -> I::Item) -> I where Self: Iterator, I: Iterator
// This works, but it seems subjectively ugly, complex, and counterintuitive to me.
Self, (<Self as Iterator>::Item -> T) -> Iterator<Item=T>
Add `HashStable_NoContext` to simplify `HashStable` implementations in `rustc_type_ir`
adds `derive(HashStable_NoContext)` which is a derived `HashStable` implementation that has no `HashStableContext` bound, and which adds `where` bounds for `HashStable` based off of *fields* and not generics.
This means we can `derive(HashStable_NoContext)` in more places in `rustc_type_ir` rather than having to hand-roll implementations.
Update books
## rust-lang/book
2 commits in 5b6c1ceaa62ecbd6caef08df39b33b3938e99deb..71352deb20727b4dda9ebfe8182709d5bf17dfea
2023-11-09 14:49:45 UTC to 2023-11-09 14:49:16 UTC
- Fixed 'Devtools' link (rust-lang/book#3770)
- Fix mdBook links (rust-lang/book#3769)
## rust-lang/rust-by-example
7 commits in 311b84962016b28c75525c86e7b3f49fd9101a39..a6581246f96837113968c02187db24f742af3908
2023-11-18 21:45:20 UTC to 2023-11-07 22:32:53 UTC
- rename `y` to `_y` to get the correct compile error (rust-lang/rust-by-example#1769)
- fix test name in cargo/test.md (rust-lang/rust-by-example#1768)
- Various minor edits for typo fixes, formatting fixes, and clarifications (rust-lang/rust-by-example#1765)
- Update closures.md to correct a typo (rust-lang/rust-by-example#1763)
- Link to the Bulgarian translation (rust-lang/rust-by-example#1764)
- Fix asm example explanation for `inlateout` usage (22.1 Inline Assembly) (rust-lang/rust-by-example#1766)
- Update index.md: Added descriptions for the 'leftover' points (rust-lang/rust-by-example#1767)
## rust-lang/rustc-dev-guide
3 commits in 77dbe5782b2488af3bb489ad702eaff438f465bf..ddb8b1309f9e905804cea1e248a4572fed6b464b
2023-11-18 21:08:13 UTC to 2023-11-08 14:43:50 UTC
- Add link for unsize.md (rust-lang/rustc-dev-guide#1825)
- Fix typo in contribution walkthrough (rust-lang/rustc-dev-guide#1824)
- Update documentation for coverage tests (rust-lang/rustc-dev-guide#1823)
feat: specialize `SpecFromElem` for `()`
# Description
This PR adds a specialization `SpecFromElem for ()` which allows to significantly reduce `vec![(), N]` time in debug builds (specifically, tests) turning it from observable $O(n)$ to $O(1)$.
# Observing the change
The problem this PR aims to fix explicitly is slow `vec![(), N]` on big `N`s which may appear in tests (see [Background section](#Background) for more details).
The minimal example to see the problem:
```rust
#![feature(test)]
extern crate test;
#[cfg(test)]
mod tests {
const HUGE_SIZE: usize = i32::MAX as usize + 1;
#[bench]
fn bench_vec_literal(b: &mut test::Bencher) {
b.iter(|| vec![(); HUGE_SIZE]);
}
#[bench]
fn bench_vec_repeat(b: &mut test::Bencher) {
b.iter(|| [(); 1].repeat(HUGE_SIZE));
}
}
```
<details><summary>Output</summary>
<p>
```bash
cargo +nightly test -- --report-time -Zunstable-options
Compiling huge-zst-vec-literal-bench v0.1.0 (/home/progrm_jarvis/RustroverProjects/huge-zst-vec-literal-bench)
Finished test [unoptimized + debuginfo] target(s) in 0.31s
Running unittests src/lib.rs (target/debug/deps/huge_zst_vec_literal_bench-e43b1ef287ba8b36)
running 2 tests
test tests::bench_vec_repeat ... ok <0.000s>
test tests::bench_vec_literal ... ok <14.382s>
test result: ok. 2 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 14.38s
Doc-tests huge-zst-vec-literal-bench
running 0 tests
test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s
```
</p>
</details>
> [!IMPORTANT]
> This problem is only observable in Debug (unoptimized) builds, while Release (optimized) ones do not observe this problem. It still is worth fixing it, IMO, since the original scenario observes the problem in tests for which optimizations are disabled by default and it seems unreasonable to override this for the whole project while the problem is very local.
# Background
While working on a crate for a custom data format which has an `i32::MAX` limit on its list's sizes, I wrote the following test to ensure that this invariant is upheld:
```rust
#[test]
fn lists_should_have_i32_size() {
assert!(
RawNbtList::try_from(vec![(); i32::MAX as usize]).is_ok(),
"lists should permit the size up to {}",
i32::MAX
);
assert!(
RawNbtList::try_from(vec![(); i32::MAX as usize + 1]).is_err(),
"lists should have the size of at most {}",
i32::MAX
);
}
```
Soon I discovered that this takes $\approx 3--4s$ per assertion on my machine, almost all of which is spent on `vec![..]`.
While this would be logical for a non-ZST vector (which would require actual $O(n)$ allocation), here `()` was used intentionally considering that for ZSTs size-changing operations should anyway be $O(1)$ (at least from allocator perspective). Yet, this "overhead" is logical if we consider that in general case `clone()` (which is used by `Vec` literal) may have a non-trivial implementation and thus each element has to actually be visited (even if they occupy no space).
In my specific case, the following (contextual) equivalent solved the issue:
```rust
#[test]
fn lists_should_have_i32_size() {
assert!(
RawNbtList::try_from([(); 1].repeat(i32::MAX as usize)).is_ok(),
"lists should permit the size up to {}",
i32::MAX
);
assert!(
RawNbtList::try_from([(); 1].repeat(i32::MAX as usize + 1)).is_err(),
"lists should have the size of at most {}",
i32::MAX
);
}
```
which works since `repeat` explicitly uses `T: Copy` and so does not have to think about non-trivial `Clone`.
But it still may be counter-intuitive for users to observe such long time on the "canonical" vec literal thus the PR.
# Generic solution
This change is explicitly non-generic. Initially I considered it possible to implement in generically, but this would require the specialization to have the following type requirements:
- ✅ the type must be a ZST: easily done via
```rust
if core::mem::size_of::<T>() == 0 {
todo!("specialization")
}
```
or
```rust
use core::mem::SizedTypeProperties;
if T::IS_ZST {
todo!("specialization")
}
```
- :white_check_mark`: the type must implement `Copy`: implementable non-conflictable via a separate specialization:
```rust
trait IsCopyZst: Sized {
fn is_copy_zst() -> bool;
}
impl<T> IsCopyZst for T {
fn is_copy_zst() -> bool {
false
}
}
impl<T: Copy> IsCopyZst for T {
fn is_copy_zst() -> bool {
Self::IS_ZST
}
}
```
- ❌ the type should have a trivial `Clone` implementation: since `vec![t; n]` is specified to use `clone()`, we can only use this "performance optimization" when we are guaranteed that `clone()` does nothing except for copying.
The latter is the real blocker for a generic fix since I am unaware of any way to get this information in a compiler-guaranteed way.
While there may be a fix for this (my friend `@CertainLach` has suggested a potential solution by an perma-unstable fn in `Clone` like `is_trivially_cloneable()` defaulting to `false` and only overridable by `rustc` on derive), this is surely out of this PRs scope.
