Drop tracking: track borrows of projections
Previous efforts to ignore partially consumed values meant we were also not considering borrows of a projection. This led to cases where we'd miss borrowed types which MIR expected to be there, leading to ICEs.
This PR also includes the `-Zdrop-tracking` flag from #93313. If that PR lands first, I'll rebase to drop the commit from this one.
Fixes#93648
rustc_query_impl: reduce visibility of some modules/fn's
Locally this reduces number of exported functions from 15221 -> 14952 and size a little.
Perf run please?
Remove defaultness from ImplItem.
This information is not really used anywhere, except HIR pretty-printing. This makes ImplItem and TraitItem more similar.
Rollup of 7 pull requests
Successful merges:
- #91950 (Point at type when a `static` `#[global_allocator]` doesn't `impl` `GlobalAlloc`)
- #92715 (Do not suggest char literal for zero-length strings)
- #92917 (Don't constrain projection predicates with inference vars in GAT substs)
- #93206 (Use `NtCreateFile` instead of `NtOpenFile` to open a file)
- #93732 (add fut/back compat tests for implied trait bounds)
- #93764 (⬆️ rust-analyzer)
- #93767 (deduplicate `lcnr` in mailmap)
Failed merges:
r? `@ghost`
`@rustbot` modify labels: rollup
This replaces the per-shard counters with a single global counter, simplifying
the JobId struct down to just a u64 and removing the need to pipe a DepKind
generic through a bunch of code. The performance implications on non-parallel
compilers are likely minimal (this switches to `Cell<u64>` as the backing
storage over a `u64`, but the latter was already inside a `RefCell` so it's not
really a significance divergence). On parallel compilers, the cost of a single
global u64 counter may be more significant: it adds a serialization point in
theory. On the other hand, we can imagine changing the counter to have a
thread-local component if it becomes worrisome or some similar structure.
The new design is sufficiently simpler that it warrants the potential for slight
changes down the line if/when we get parallel compilation to be more of a
default.
A u64 counter, instead of u32 (the old per-shard width), is chosen to avoid
possibly overflowing it and causing problems; it is effectively impossible that
we would overflow a u64 counter in this context.
Currently, any higher-ranked region errors involving opaque types
fall back to a generic "higher-ranked subtype error" message when
run under NLL. This PR adds better error message handling for this
case, giving us the same kinds of error messages that we currently
get without NLL:
```
error: implementation of `MyTrait` is not general enough
--> $DIR/opaque-hrtb.rs:12:13
|
LL | fn foo() -> impl for<'a> MyTrait<&'a str> {
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ implementation of `MyTrait` is not general enough
|
= note: `impl MyTrait<&'2 str>` must implement `MyTrait<&'1 str>`, for any lifetime `'1`...
= note: ...but it actually implements `MyTrait<&'2 str>`, for some specific lifetime `'2`
error: aborting due to previous error
```
To accomplish this, several different refactoring needed to be made:
* We now have a dedicated `InstantiateOpaqueType` struct which
implements `TypeOp`. This is used to invoke `instantiate_opaque_types`
during MIR type checking.
* `TypeOp` is refactored to pass around a `MirBorrowckCtxt`, which is
needed to report opaque type region errors.
* We no longer assume that all `TypeOp`s correspond to canonicalized
queries. This allows us to properly handle opaque type instantiation
(which does not occur in a query) as a `TypeOp`.
A new `ErrorInfo` associated type is used to determine what
additional information is used during higher-ranked region error
handling.
* The body of `try_extract_error_from_fulfill_cx`
has been moved out to a new function `try_extract_error_from_region_constraints`.
This allows us to re-use the same error reporting code between
canonicalized queries (which can extract region constraints directly
from a fresh `InferCtxt`) and opaque type handling (which needs to take
region constraints from the pre-existing `InferCtxt` that we use
throughout MIR borrow checking).
Don't constrain projection predicates with inference vars in GAT substs
cc #91762
Not a fix, but a mitigation to prevent a backwards-compatible hazard where we normalize using a predicate only because it's the only one available, but shouldn't. This would constrain an inference variable which didn't really want. We already do this when selecting a projection candidate, which isn't always correct. But changing that is a problem for a different day.
Also found out that a suggestion for `await`ing a future was using the wrong substs.
r? ``@nikomatsakis``
Do not suggest char literal for zero-length strings
PR #92507 adds a hint to switch to single quotes when a char is expected and a single-character string literal is provided.
The check to ensure the string literal is one character long missed the 0-char case, and would incorrectly offer the hint.
This PR adds the missing check, and a test case to confirm the new behavior.
Currently, we are thinking to use *-unknown-none targets instead
to define for every platform our own one (see hermitcore/rusty-hermit#197).
However, the current target aarch64-unknown-none-softfloat doesn't support
dynamic relocation. Our kernel uses this feature and consequently
we define a new target aarch64-unknown-hermitkernel to support it.
Add in ValuePair::Term
This adds in an enum when matching on positions which can either be types or consts.
It will default to emitting old special cased error messages for types.
r? `@oli-obk`
cc `@matthiaskrgr`
Fixes#93578
update comment wrt const param defaults
after #93669 i looked through all other uses of `GenericParamKind::Const` again to detect if we missed the `default` there as well, but afaict we really only missed lifetime resolution '^^ at least i found an outdated comment :3
Add more *-unwind ABI variants
The following *-unwind ABIs are now supported:
- "C-unwind"
- "cdecl-unwind"
- "stdcall-unwind"
- "fastcall-unwind"
- "vectorcall-unwind"
- "thiscall-unwind"
- "aapcs-unwind"
- "win64-unwind"
- "sysv64-unwind"
- "system-unwind"
cc `@rust-lang/wg-ffi-unwind`
Previous efforts to ignore partially consumed values meant we were also
not considering borrows of a projection. This led to cases where we'd
miss borrowed types which MIR expected to be there, leading to ICEs.
Lazy type-alias-impl-trait
Previously opaque types were processed by
1. replacing all mentions of them with inference variables
2. memorizing these inference variables in a side-table
3. at the end of typeck, resolve the inference variables in the side table and use the resolved type as the hidden type of the opaque type
This worked okayish for `impl Trait` in return position, but required lots of roundabout type inference hacks and processing.
This PR instead stops this process of replacing opaque types with inference variables, and just keeps the opaque types around.
Whenever an opaque type `O` is compared with another type `T`, we make the comparison succeed and record `T` as the hidden type. If `O` is compared to `U` while there is a recorded hidden type for it, we grab the recorded type (`T`) and compare that against `U`. This makes implementing
* https://github.com/rust-lang/rfcs/pull/2515
much simpler (previous attempts on the inference based scheme were very prone to ICEs and general misbehaviour that was not explainable except by random implementation defined oddities).
r? `@nikomatsakis`
fixes#93411fixes#88236
Update tracking issue for `const_fn_trait_bound`
It previously pointed to #57563, the conglomerate issue for `const fn` (presumably under the feature gate `const_fn`). This tracking issue doesn't mention anything about `const_fn_trait_bound`(the only occurrence of "trait bound" is for the now-removed `?const Trait` syntax), which can be confusing to people who want to find out more about trait bounds on `const fn`s. This pull request changes the tracking issue to one meant specifically for `const_fn_trait_bound`, #93706, which can help collect information on this feature's stabilization and point users towards `const_trait_impl` if they're looking for const-in-const-contexts trait bounds.
Fixes#93679.
`````@rustbot````` modify labels +A-const-fn +F-const_trait_impl
Suggest 1-tuple parentheses on exprs without existing parens
A follow-on from #86116, split out from #90677.
This alters the suggestion to add a trailing comma to create a 1-tuple - previously we would only apply this if the relevant expression was parenthesised. We now make the suggestion regardless of parentheses, which reduces the fragility of the check (w.r.t formatting).
e.g.
```rust
let a: Option<(i32,)> = Some(3);
```
gets the below suggestion:
```rust
let a: Option<(i32,)> = Some((3,));
// ^ ^^
```
This change also improves the suggestion in other ways, such as by only making the suggestion if the types would match after the suggestion is applied and making the suggestion a multipart suggestion.
This adds in an enum when matching on positions which can either be types or consts.
It will default to emitting old special cased error messages for types.
use `fold_list` in `try_super_fold_with` for `SubstsRef`
split out from #93505 as this by itself is responsible for most of the perf improvements there
r? `@michaelwoerister`
These statistics are computable from the self-profile data and/or ad-hoc
collectable as needed, and in the meantime contribute to rustc bootstrap times.
Fix invalid special casing of the unreachable! macro
This pull-request fix an invalid special casing of the `unreachable!` macro in the same way the `panic!` macro was solved, by adding two new internal only macros `unreachable_2015` and `unreachable_2021` edition dependent and turn `unreachable!` into a built-in macro that do dispatching. This logic is stolen from the `panic!` macro.
~~This pull-request also adds an internal feature `format_args_capture_non_literal` that allows capturing arguments from formatted string that expanded from macros. The original RFC #2795 mentioned this as a future possibility. This feature is [required](https://github.com/rust-lang/rust/issues/92137#issuecomment-1018630522) because of concatenation that needs to be done inside the macro:~~
```rust
$crate::concat!("internal error: entered unreachable code: ", $fmt)
```
**In summary** the new behavior for the `unreachable!` macro with this pr is:
Edition 2021:
```rust
let x = 5;
unreachable!("x is {x}");
```
```
internal error: entered unreachable code: x is 5
```
Edition <= 2018:
```rust
let x = 5;
unreachable!("x is {x}");
```
```
internal error: entered unreachable code: x is {x}
```
Also note that the change in this PR are **insta-stable** and **breaking changes** but this a considered as being a [bug](https://github.com/rust-lang/rust/issues/92137#issuecomment-998441613).
If someone could start a perf run and then a crater run this would be appreciated.
Fixes https://github.com/rust-lang/rust/issues/92137
It previously pointed to #57563, the conglomerate issue for
`const fn` (presumably under the feature gate `const_fn`).
`const_fn_trait_bounds` weren't mentioned here, so this commit changes
its tracking issue to a new one.
Mark the panic_no_unwind lang item as nounwind
This has 2 effects:
- It helps LLVM when inlining since it doesn't need to generate landing pads for `panic_no_unwind`.
- It makes it sound for a panic handler to unwind even if `PanicInfo::can_unwind` returns true. This will simply cause another panic once the unwind tries to go past the `panic_no_unwind` lang item. Eventually this will cause a stack overflow, which is safe.
Require const stability attribute on all stable functions that are `const`
This PR requires all stable functions (of all kinds) that are `const fn` to have a `#[rustc_const_stable]` or `#[rustc_const_unstable]` attribute. Stability was previously implied if omitted; a follow-up PR is planned to change the fallback to be unstable.
Resolve lifetimes for const generic defaults
We weren't visiting the const generic default argument in `rustc_resolve::late::lifetimes`. This seems to fix the issue, and we deny any non-`'static` lifetimes anyways.
Fixes#93647
Add new target armv7-unknown-linux-uclibceabi (softfloat)
This adds the new target `armv7-unknown-linux-uclibceabi (softfloat)`. It is of course similar to `armv7-unknown-linux-uclibceabihf (hardfloat)` which was just recently added to rust except that it is `softfloat`.
My interest lies in the Broadcom BCM4707/4708/BCM4709 family, notably found in some Netgear and Asus consumer routers. The armv7 Cortex-A9 cpus found in these devices do not have an fpu or NEON support.
With this patch I've been able to bootstrap rustc, std and host tools `(extended = true)` to run on the target device for native compilation, allowing the target to be used as a development platform.
With the recent addition of `armv7-unknown-linux-uclibceabihf (hardfloat)` it looks like many of the edge cases of using the uclibc c-library are getting worked out nicely. I've been able to compile some complex projects. Some patching still needed in some crates, but getting there for sure. I think `armv7-unknown-linux-uclibceabi` is ready to be a tier 3 target.
I use a cross-toolchain from my project to bootstrap rust.
https://github.com/lancethepants/tomatoware
The goal of this project is to create a native development environment with support for various languages.
mips64-openwrt-linux-musl: Add Tier 3 target
Tier 3 tuple for Mips64 OpenWrt toolchain.
This add first-time support for OpenWrt. Future Tier3 targets will be added as I test them.
Signed-off-by: Donald Hoskins <grommish@gmail.com>
This doesn't handle `char` because it's a bit awkward to distinguish it
from u32 at this point in codegen.
Note that for some types (like `&Struct` and `&mut Struct`),
we already apply `dereferenceable`, which implies `noundef`,
so the IR does not change.
Stabilize `-Z instrument-coverage` as `-C instrument-coverage`
(Tracking issue for `instrument-coverage`: https://github.com/rust-lang/rust/issues/79121)
This PR stabilizes support for instrumentation-based code coverage, previously provided via the `-Z instrument-coverage` option. (Continue supporting `-Z instrument-coverage` for compatibility for now, but show a deprecation warning for it.)
Many, many people have tested this support, and there are numerous reports of it working as expected.
Move the documentation from the unstable book to stable rustc documentation. Update uses and documentation to use the `-C` option.
Addressing questions raised in the tracking issue:
> If/when stabilized, will the compiler flag be updated to -C instrument-coverage? (If so, the -Z variant could also be supported for some time, to ease migrations for existing users and scripts.)
This stabilization PR updates the option to `-C` and keeps the `-Z` variant to ease migration.
> The Rust coverage implementation depends on (and automatically turns on) -Z symbol-mangling-version=v0. Will stabilizing this feature depend on stabilizing v0 symbol-mangling first? If so, what is the current status and timeline?
This stabilization PR depends on https://github.com/rust-lang/rust/pull/90128 , which stabilizes `-C symbol-mangling-version=v0` (but does not change the default symbol-mangling-version).
> The Rust coverage implementation implements the latest version of LLVM's Coverage Mapping Format (version 4), which forces a dependency on LLVM 11 or later. A compiler error is generated if attempting to compile with coverage, and using an older version of LLVM.
Given that LLVM 13 has now been released, requiring LLVM 11 for coverage support seems like a reasonable requirement. If people don't have at least LLVM 11, nothing else breaks; they just can't use coverage support. Given that coverage support currently requires a nightly compiler and LLVM 11 or newer, allowing it on a stable compiler built with LLVM 11 or newer seems like an improvement.
The [tracking issue](https://github.com/rust-lang/rust/issues/79121) and the [issue label A-code-coverage](https://github.com/rust-lang/rust/labels/A-code-coverage) link to a few open issues related to `instrument-coverage`, but none of them seem like showstoppers. All of them seem like improvements and refinements we can make after stabilization.
The original `-Z instrument-coverage` support went through a compiler-team MCP at https://github.com/rust-lang/compiler-team/issues/278 . Based on that, `@pnkfelix` suggested that this needed a stabilization PR and a compiler-team FCP.
Fix ret > 1 bound if shadowed by const
Prior to a change, it would only look at types in bounds. When it started looking for consts,
shadowing type variables with a const would cause an ICE, so now defer looking at consts only if
there are no types present.
cc ``````@compiler-errors``````
Should Fix#93553
Windows: Disable LLVM crash dialog boxes.
This disables the crash dialog box on Windows. When LLVM hits an assertion, it will open a dialog box with Abort/Retry/Ignore. This is annoying on CI because CI will just hang until it times out (which can take hours).
Instead of opening a dialog box, it will print a message like this:
```
Assertion failed: isa<X>(Val) && "cast<Ty>() argument of incompatible type!", file D:\Proj\rust\rust\src\llvm-project\llvm\include\llvm/Support/Casting.h, line 255
```
Closes#92829
Prior to a change, it would only look at types in bounds. When it started looking for consts,
shadowing type variables with a const would cause an ICE, so now defer looking at consts only if
there are no types present.
The "CI" environment var isn't universal (for example, I think Azure
uses TF_BUILD). However, we are mostly concerned with rust-lang/rust's
own CI which currently is GitHub Actions which does set "CI". And I
think most other providers use "CI" as well.
Correct incorrect description of preorder traversals
The internal documentation for the `Preorder` type gave an incorrect description (the description is not even correct for the example provided, since C is visited after one of its successors). This corrects the description, and adds in a sentence explaining more precisely how the traversals are performed.
Make rustc use `RUST_BACKTRACE=full` by default
Compiler panics should be rare - when they do occur, we want the report
filed by the user to contain as much information as possible. This is
especially important when the panic is due to an incremental compilation
bug, since we may not have enough information to reproduce it.
This PR sets `RUST_BACKTRACE=full` inside `rustc` if the user has not
explicitly set `RUST_BACKTRACE`. This is more verbose than
`RUST_BACKTRACE=1`, but this may make it easier to debug incremental
compilation issues. Users who find this too verbose can still manually
set `RUST_BACKTRACE` before invoking the compiler.
This only affects `rustc` (and any tool using `rustc_driver::install_ice_hook`).
It does *not* affect any user crates or the standard library -
backtraces will continue to be off by default in any application
*compiled* by rustc.
Factor convenience functions out of main printer implementation
The pretty printer in rustc_ast_pretty has a section of methods commented "Convenience functions to talk to the printer". This PR pulls those out to a separate module. This leaves pp.rs with only the minimal API that is core to the pretty printing algorithm.
I found this separation to be helpful in https://github.com/dtolnay/prettyplease because it makes clear when changes are adding some fundamental new capability to the pretty printer algorithm vs just making it more convenient to call some already existing functionality.
rustdoc: Fix ICE report
The ICE report in rustdoc was confusing because it was returning an argument parse error:
```
thread 'rustc' panicked at 'aborting due to `-Z treat-err-as-bug=1`', compiler/rustc_errors/src/lib.rs:1212:27
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
error: internal compiler error: unexpected panic
error: Unrecognized option: 'crate-version'
```
This is because the ICE reporter was trying to parse the arguments as rustc, not rustdoc. Since an argument error is a fatal error, it was early-exiting with the argument error due to unwinding.
This changes it to be a more primitive scan of the arguments. The arguments being checked are pretty simple, and only have a small handful of forms that are easy to check for.
It now looks like this:
```
thread 'rustc' panicked at 'aborting due to `-Z treat-err-as-bug=1`', compiler/rustc_errors/src/lib.rs:1212:27
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
error: internal compiler error: unexpected panic
note: the compiler unexpectedly panicked. this is a bug.
note: we would appreciate a bug report: https://github.com/rust-lang/rust/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md
note: rustc 1.59.0-dev running on x86_64-apple-darwin
note: compiler flags: --crate-type lib -Z treat-err-as-bug
note: some of the compiler flags provided by cargo are hidden
query stack during panic:
end of query stack
```
It still says `rustc`, but I can live with that.
Temporary fix for the layout of aligned enums
Fix for the issue #92464
~~I was after this issue for quite some time now, I have a temporary fix for it.
I think the current problem is [here](e75f96763f/compiler/rustc_middle/src/ty/layout.rs (L1305-L1310)) created `tag` value might be wrong, because when I checked `min` and `max` values it's always between 0..1, which results in wrong size comparison in a few lines down below.
I think `min` and `max` values don't take `#[repr(aligned(8))]` into consideration and just act from base values assigned inside the enum. If what I am saying is true, aligned enums were created with the wrong layout for some time.~~
~~As stated in the title this is only a temporary fix and I think this needs further investigation, if someone wants to mentor it I would like to work on that too.~~ 😸
**Edit: Weird some tests fail now going to close this for now...**
**Edit2: I made it work again.**
I think I figured out the main problem of the issue, layout types of aligned enums with custom discriminant types were not handled, which resulted in confusing(such as this issue) behavior down the line, this is a kinda hacky fix for the issue.
Compress amount of hashed bytes for `isize` values in StableHasher
This is another attempt to land https://github.com/rust-lang/rust/pull/92103, this time hopefully with a correct implementation w.r.t. stable hashing guarantees. The previous PR was [reverted](https://github.com/rust-lang/rust/pull/93014) because it could produce the [same hash](https://github.com/rust-lang/rust/pull/92103#issuecomment-1014625442) for different values even in quite simple situations. I have since added a basic [test](https://github.com/rust-lang/rust/pull/93193) that should guard against that situation, I also added a new test in this PR, specialised for this optimization.
## Why this optimization helps
Since the original PR, I have tried to analyze why this optimization even helps (and why it especially helps for `clap`). I found that the vast majority of stable-hashing `i64` actually comes from hashing `isize` (which is converted to `i64` in the stable hasher). I only found a single place where is this datatype used directly in the compiler, and this place has also been showing up in traces that I used to find out when is `isize` being hashed. This place is `rustc_span::FileName::DocTest`, however, I suppose that isizes also come from other places, but they might not be so easy to find (there were some other entries in the trace). `clap` hashes about 8.5 million `isize`s, and all of them fit into a single byte, which is why this optimization has helped it [quite a lot](https://github.com/rust-lang/rust/pull/92103#issuecomment-1005711861).
Now, I'm not sure if special casing `isize` is the correct solution here, maybe something could be done with that `isize` inside `DocTest` or in other places, but that's for another discussion I suppose. In this PR, instead of hardcoding a special case inside `SipHasher128`, I instead put it into `StableHasher`, and only used it for `isize` (I tested that for `i64` it doesn't help, or at least not for `clap` and other few benchmarks that I was testing).
## New approach
Since the most common case is a single byte, I added a fast path for hashing `isize` values which positive value fits within a single byte, and a cold path for the rest of the values.
To avoid the previous correctness problem, we need to make sure that each unique `isize` value will produce a unique hash stream to the hasher. By hash stream I mean a sequence of bytes that will be hashed (a different sequence should produce a different hash, but that is of course not guaranteed).
We have to distinguish different values that produce the same bit pattern when we combine them. For example, if we just simply skipped the leading zero bytes for values that fit within a single byte, `(0xFF, 0xFFFFFFFFFFFFFFFF)` and `(0xFFFFFFFFFFFFFFFF, 0xFF)` would send the same hash stream to the hasher, which must not happen.
To avoid this situation, values `[0, 0xFE]` are hashed as a single byte. When we hash a larger (treating `isize` as `u64`) value, we first hash an additional byte `0xFF`. Since `0xFF` cannot occur when we apply the single byte optimization, we guarantee that the hash streams will be unique when hashing two values `(a, b)` and `(b, a)` if `a != b`:
1) When both `a` and `b` are within `[0, 0xFE]`, their hash streams will be different.
2) When neither `a` and `b` are within `[0, 0xFE]`, their hash streams will be different.
3) When `a` is within `[0, 0xFE]` and `b` isn't, when we hash `(a, b)`, the hash stream will definitely not begin with `0xFF`. When we hash `(b, a)`, the hash stream will definitely begin with `0xFF`. Therefore the hash streams will be different.
r? `@the8472`
Rollup of 8 pull requests
Successful merges:
- #92528 (Make `Fingerprint::combine_commutative` associative)
- #93221 ([borrowck] Fix help on mutating &self in async fns)
- #93542 (Prevent lifetime elision in type alias)
- #93546 (Validate that values in switch int terminator are unique)
- #93571 (better suggestion for duplicated `where` clause)
- #93574 (don't suggest adding `let` due to bad assignment expressions inside of `while` loop)
- #93590 (More let_else adoptions)
- #93592 (Remove unused dep from rustc_arena)
Failed merges:
r? `@ghost`
`@rustbot` modify labels: rollup
don't suggest adding `let` due to bad assignment expressions inside of `while` loop
adds a check that our `lhs` expression is actually within the conditional part of the `while` loop, instead of anywhere in the `while` body.
fixes#93486
[borrowck] Fix help on mutating &self in async fns
Previously, when rustc was provided an async function that tried to
mutate through a shared reference to an implicit self (as shown in the
ui test), rustc would suggest modifying the parameter signature
to `&mut` + the fully qualified name of the ty (in the case of the repro
`S`). If a user modified their code to match the suggestion, the
compiler would not accept it.
This commit modifies the suggestion so that when rustc is provided the
ui test that is also attached in this commit, it suggests (correctly)
`&mut self`. We try to be careful about distinguishing between implicit
and explicit self annotations, since the latter seem to be handled
correctly already.
This is my first PR here so I'm pretty sure I probably missed something/could use better terminology. I also didn't try to make the match exhaustive since implicit self is the only real special case that I need to handle (that I'm aware of), and I'm pretty sure there's a cleaner way to do this so any advice would be greatly appreciated! (I'm also not terribly confident about how I wrote the ui tests)
here is your cc as requested `@compiler-errors`
This is an attempt to fix#93093
Make `Fingerprint::combine_commutative` associative
The previous implementation swapped lower and upper 64-bits of a result
of modular addition, so the function was non-associative.
r? `@Aaron1011`
This means we stop supporting the case where a locally defined trait has only a single impl so we can always use that impl (see nested-tait-inference.rs).
by using an opaque type obligation to bubble up comparisons between opaque types and other types
Also uses proper obligation causes so that the body id works, because out of some reason nll uses body ids for logic instead of just diagnostics.
