const_evaluatable_checked: extend predicate collection
We now walk the hir instead of using `ty` so that we get better spans here, While I am still not completely sure if that's
what we want in the end, it does seem a lot closer to the final goal than the previous version.
We also look into type aliases (and use a `TypeVisitor` here), about which I am not completely sure, but we will see how well this works.
We also look into fn decls, so the following should work now.
```rust
fn test<T>() -> [u8; std::mem::size_of::<T>()] {
[0; std::mem::size_of::<T>()]
}
```
Additionally, we visit the optional trait and self type of impls.
r? `@oli-obk`
Fix underflow when calculating the number of no-op jumps folded
When removing unwinds to no-op blocks and folding jumps to no-op blocks,
remove the unwind target first. Otherwise we cannot determine if target
has been already folded or not.
Previous implementation incorrectly assumed that all resume targets had
been folded already, occasionally resulting in an underflow:
```
remove_noop_landing_pads: removed 18446744073709551613 jumps and 3 landing pads
```
- Add `PrimTy::name` and `PrimTy::name_str`
- Use those new functions to distinguish between the name in scope and
the canonical name
- Fix diagnostics for primitive types
- Add tests for primitives
Rollup of 9 pull requests
Successful merges:
- #76898 (Record `tcx.def_span` instead of `item.span` in crate metadata)
- #76939 (emit errors during AbstractConst building)
- #76965 (Add cfg(target_has_atomic_equal_alignment) and use it for Atomic::from_mut.)
- #76993 (Changing the alloc() to accept &self instead of &mut self)
- #76994 (fix small typo in docs and comments)
- #77017 (Add missing examples on Vec iter types)
- #77042 (Improve documentation for ToSocketAddrs)
- #77047 (Miri: more informative deallocation error messages)
- #77055 (Add #[track_caller] to more panicking Cell functions)
Failed merges:
r? `@ghost`
MIR pass to remove unneeded drops on types not needing drop
This is heavily dependent on MIR inlining running to actually see the drop statement.
Do we want to special case replacing a call to std::mem::drop with a goto aswell?
Add cfg(target_has_atomic_equal_alignment) and use it for Atomic::from_mut.
Fixes some platform-specific problems with #74532 by using the actual alignment of the types instead of hardcoding a few `target_arch`s.
r? @RalfJung
emit errors during AbstractConst building
There changes are currently still untested, so I don't expect this to pass CI 😆
It seems to me like this is the direction we want to go in, though we didn't have too much of a discussion about this.
r? @oli-obk
Record `tcx.def_span` instead of `item.span` in crate metadata
This was missed in PR #75465. As a result, a few places have been using
the full body span of functions, instead of just the header span.
SimplifyComparisonIntegral: fix miscompilation
Fixes#76432
Only insert StorageDeads if we actually removed one.
Fixes an issue where we added StorageDead to a place with no StorageLive
r? `@oli-obk`
Remove `qualify_min_const_fn`
~~Blocked on #76807 (the first six commits).~~
With this PR, all checks in `qualify_min_const_fn` are replicated in `check_consts`, and the former is no longer invoked. My goal was to have as few changes to test output as possible, since making sweeping changes to the code *while* doing big batches of diagnostics updates turned out to be a headache. To this end, there's a few `HACK`s in `check_consts` to achieve parity with `qualify_min_const_fn`.
The new system that replaces `is_min_const_fn` is referred to as "const-stability" My end goal for the const-stability rules is this:
* Const-stability is only applicable to functions defined in `staged_api` crates.
* All functions not marked `rustc_const_unstable` are considered "const-stable".
- NB. This is currently not implemented. `#[unstable]` functions are also const-unstable. This causes problems when searching for feature gates.
- All "const-unstable" functions have an associated feature gate
* const-stable functions can only call other const-stable functions
- `allow_internal_unstable` can be used to circumvent this.
* All const-stable functions are subject to some additional checks (the ones that were unique to `qualify_min_const_fn`)
The plan is to remove each `HACK` individually in subsequent PRs. That way, changes to error message output can be reviewed in isolation.
use if let instead of single match arm expressions
use if let instead of single match arm expressions to compact code and reduce nesting (clippy::single_match)
Use const-checking to forbid use of unstable features in const-stable functions
First step towards #76618.
Currently this code isn't ever hit because `qualify_min_const_fn` runs first and catches pretty much everything. One exception is `const_precise_live_drops`, which does not use the newly added code since it runs as part of a separate pass.
Also contains some unrelated refactoring, which is split into separate commits.
r? @oli-obk
Don't use `zip` to compare iterators during pretty-print hack
If the right-hand iterator has exactly one more element than the
left-hand iterator, then both iterators will be fully consumed, but
the extra element will never be compared.
Split out from https://github.com/rust-lang/rust/pull/76130
If the right-hand iterator has exactly one more element than the
left-hand iterator, then both iterators will be fully consumed, but
the extra element will never be compared.
Make `ensure_sufficient_stack()` non-generic, using cargo-llvm-lines
Inspired by [this blog post](https://blog.mozilla.org/nnethercote/2020/08/05/how-to-speed-up-the-rust-compiler-some-more-in-2020/) from `@nnethercote,` I used [cargo-llvm-lines](https://github.com/dtolnay/cargo-llvm-lines/) on the rust compiler itself, to improve it's compile time. This PR contains only one low-hanging fruit, but I also want to share some measurements.
The function `ensure_sufficient_stack()` was monomorphized 1500 times, and with it the `stacker` and `psm` crates, for a total of 1.5% of all llvm IR lines. With some trickery I convert the generic closure into a dynamic one, and thus all that code is only monomorphized once.
# Measurements
Getting these numbers took some fiddling with CLI flags and I [modified](https://github.com/Julian-Wollersberger/cargo-llvm-lines/blob/master/src/main.rs#L115) cargo-llvm-lines to read from a folder instead of invoking cargo. Commands I used:
```
./x.py clean
RUSTFLAGS="--emit=llvm-ir -C link-args=-fuse-ld=lld -Z self-profile=profile" CARGOFLAGS_BOOTSTRAP="-Ztimings" RUSTC_BOOTSTRAP=1 ./x.py build -i --stage 1 library/std
# Then manually copy all .ll files into a folder I hardcoded in cargo-llvm-lines in main.rs#L115
cd ../cargo-llvm-lines
cargo run llvm-lines
```
The result is this list (see [first 500 lines](https://github.com/Julian-Wollersberger/cargo-llvm-lines/blob/master/llvm-lines-rustc-before.txt) ), before the change:
```
Lines Copies Function name
----- ------ -------------
16894211 (100%) 58417 (100%) (TOTAL)
2223855 (13.2%) 502 (0.9%) rustc_query_system::query::plumbing::get_query_impl::{{closure}}
1331918 (7.9%) 1287 (2.2%) hashbrown::raw::RawTable<T>::reserve_rehash
774434 (4.6%) 12043 (20.6%) core::ptr::drop_in_place
294170 (1.7%) 499 (0.9%) rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
245410 (1.5%) 1552 (2.7%) psm::on_stack::with_on_stack
210311 (1.2%) 1 (0.0%) rustc_target::spec::load_specific
200962 (1.2%) 513 (0.9%) rustc_query_system::query::plumbing::get_query_impl
190704 (1.1%) 1 (0.0%) rustc_middle::ty::query::<impl rustc_middle::ty::context::TyCtxt>::alloc_self_profile_query_strings
180272 (1.1%) 468 (0.8%) rustc_query_system::query::plumbing::load_from_disk_and_cache_in_memory
177396 (1.1%) 114 (0.2%) rustc_query_system::query::plumbing::force_query_impl
161134 (1.0%) 445 (0.8%) rustc_query_system::dep_graph::graph::DepGraph<K>::with_anon_task
141551 (0.8%) 186 (0.3%) rustc_query_system::query::plumbing::incremental_verify_ich
110191 (0.7%) 7 (0.0%) rustc_middle::ty::context::_DERIVE_rustc_serialize_Decodable_D_FOR_TypeckResults::<impl rustc_serialize::serialize::Decodable<__D> for rustc_middle::ty::context::TypeckResults>::decode::{{closure}}
108590 (0.6%) 420 (0.7%) core::ops::function::FnOnce::call_once
88488 (0.5%) 21 (0.0%) rustc_query_system::dep_graph::graph::DepGraph<K>::try_mark_previous_green
86368 (0.5%) 1 (0.0%) rustc_middle::ty::query::stats::query_stats
85654 (0.5%) 3973 (6.8%) <&T as core::fmt::Debug>::fmt
84475 (0.5%) 1 (0.0%) rustc_middle::ty::query::Queries::try_collect_active_jobs
81220 (0.5%) 862 (1.5%) <hashbrown::raw::RawIterHash<T> as core::iter::traits::iterator::Iterator>::next
77636 (0.5%) 54 (0.1%) core::slice::sort::recurse
66484 (0.4%) 461 (0.8%) <hashbrown::raw::RawIter<T> as core::iter::traits::iterator::Iterator>::next
```
All `.ll` files together had 4.4GB. After my change they had 4.2GB. So a few percent less code LLVM has to process. Hurray!
Sadly, I couldn't measure an actual wall-time improvement. Watching YouTube while compiling added to much noise...
Here is the top of the list after the change:
```
16460866 (100%) 58341 (100%) (TOTAL)
1903085 (11.6%) 504 (0.9%) rustc_query_system::query::plumbing::get_query_impl::{{closure}}
1331918 (8.1%) 1287 (2.2%) hashbrown::raw::RawTable<T>::reserve_rehash
777796 (4.7%) 12031 (20.6%) core::ptr::drop_in_place
551462 (3.4%) 1519 (2.6%) rustc_data_structures::stack::ensure_sufficient_stack::{{closure}}
```
Note that the total was reduced by 430 000 lines and `psm::on_stack::with_on_stack` has disappeared. Instead `rustc_data_structures::stack::ensure_sufficient_stack::{{closure}}` appeared. I'm confused about that one, but it seems to consist of inlined calls to `rustc_query_system::*` stuff.
Further note the other two big culprits in this list: `rustc_query_system` and `hashbrown`. These two are monomorphized many times, the query system summing to more than 20% of all lines, not even counting code that's probably inlined elsewhere.
Assuming compile times scale linearly with llvm-lines, that means a possible 20% compile time reduction.
Reducing eg. `get_query_impl` would probably need a major refactoring of the qery system though. _Everything_ in there is generic over multiple types, has associated types and passes generic Self arguments by value. Which means you can't simply make things `dyn`.
---------------------------------------
This PR is a small step to make rustc compile faster and thus make contributing to rustc less painful. Nonetheless I love Rust and I find the work around rustc fascinating :)
Remove DeclareMethods
Most of the `DeclareMethods` API was only used internally by rustc_codegen_llvm. As such, it makes no sense to require other backends to implement them.
(`get_declared_value` and `declare_cfn` were used, in one place, specific to the `main` symbol, which I've replaced with a more specialized function to allow more flexibility in implementation - the intent is that `declare_c_main` can go away once we do something more clever, e.g. @eddyb has ideas around having a MIR shim or somesuch we can explore in a follow-up PR)
Let user see the full type of type-length limit error
Seeing the full type of the error is sometimes essential to diagnosing the problem, but the type itself is too long to be displayed in the terminal in a useful fashion. This change solves this dilemma by writing the full offending type name to a file, and displays this filename as a note.
> note: the full type name been written to '$TEST_BUILD_DIR/issues/issue-22638/issue-22638.long-type.txt'
Closes#76777
Remove MMX from Rust
Follow-up to https://github.com/rust-lang/stdarch/pull/890
This removes most of MMX from Rust (tests pass with small changes), keeping stable `is_x86_feature_detected!("mmx")` working.
New MIR optimization pass to reduce branches on match of tuples of enums
Fixes#68867 by adding a new pass that turns something like
```rust
let x: Option<()>;
let y: Option<()>;
match (x,y) {
(Some(_), Some(_)) => {0},
_ => {1}
}
```
into something like
```rust
let x: Option<()>;
let y: Option<()>;
let discriminant_x = // get discriminant of x
let discriminant_y = // get discriminant of x
if discriminant_x != discriminant_y {1} else {0}
```
The opt-diffs still have the old basic blocks like
```
bb3: {
_8 = discriminant((*(_4.1: &ViewportPercentageLength))); // scope 0 at $DIR/early-otherwise-branch-68867.rs:21:21: 21:30
switchInt(move _8) -> [1_isize: bb7, otherwise: bb2]; // scope 0 at $DIR/early-otherwise-branch-68867.rs:21:21: 21:30
}
bb4: {
_9 = discriminant((*(_4.1: &ViewportPercentageLength))); // scope 0 at $DIR/early-otherwise-branch-68867.rs:22:23: 22:34
switchInt(move _9) -> [2_isize: bb8, otherwise: bb2]; // scope 0 at $DIR/early-otherwise-branch-68867.rs:22:23: 22:34
}
bb5: {
_10 = discriminant((*(_4.1: &ViewportPercentageLength))); // scope 0 at $DIR/early-otherwise-branch-68867.rs:23:23: 23:34
switchInt(move _10) -> [3_isize: bb9, otherwise: bb2]; // scope 0 at $DIR/early-otherwise-branch-68867.rs:23:23: 23:34
}
```
These do get removed on later passes. I'm not sure if I should include those passes in the test to make it clear?
Don't allow implementing trait directly on type-alias-impl-trait
This is specifically disallowed by the RFC, but we never added a check
for it.
Fixes#76202
transmute: use diagnostic item
closes#66075, we now have no remaining uses of `match_def_path` in the compiler while some uses still remain in `clippy`.
cc @RalfJung
Rollup of 15 pull requests
Successful merges:
- #76722 (Test and fix Send and Sync traits of BTreeMap artefacts)
- #76766 (Extract some intrinsics out of rustc_codegen_llvm)
- #76800 (Don't generate bootstrap usage unless it's needed)
- #76809 (simplfy condition in ItemLowerer::with_trait_impl_ref())
- #76815 (Fix wording in mir doc)
- #76818 (Don't compile regex at every function call.)
- #76821 (Remove redundant nightly features)
- #76823 (black_box: silence unused_mut warning when building with cfg(miri))
- #76825 (use `array_windows` instead of `windows` in the compiler)
- #76827 (fix array_windows docs)
- #76828 (use strip_prefix over starts_with and manual slicing based on pattern length (clippy::manual_strip))
- #76840 (Move to intra doc links in core/src/future)
- #76845 (Use intra docs links in core::{ascii, option, str, pattern, hash::map})
- #76853 (Use intra-doc links in library/core/src/task/wake.rs)
- #76871 (support panic=abort in Miri)
Failed merges:
r? `@ghost`
use `array_windows` instead of `windows` in the compiler
I do think these changes are beautiful, but do have to admit that using type inference for the window length
can easily be confusing. This seems like a general issue with const generics, where inferring constants adds an additional
complexity which users have to learn and keep in mind.
Remove redundant nightly features
Removes a bunch of redundant/outdated nightly features. The first commit removes a `core_intrinsics` use for which a stable wrapper has been provided since. The second commit replaces the `const_generics` feature with `min_const_generics` which might get stabilized this year. The third commit is the result of a trial/error run of removing every single feature and then adding it back if compile failed. A bunch of unused features are the result that the third commit removes.
Don't compile regex at every function call.
Use `SyncOnceCell` to only compile it once.
I believe this still adds some kind of locking mechanism?
Related issue: https://github.com/rust-lang/rust/issues/76817
Extract some intrinsics out of rustc_codegen_llvm
A significant amount of intrinsics do not actually need backend-specific behaviors to be implemented, instead relying on methods already in rustc_codegen_ssa. So, extract those methods out to rustc_codegen_ssa, so that each backend doesn't need to reimplement the same code.
Almost everything should be a pretty direct translation. A notable not-direct-translation is `add_with_overflow` and friends being changed to `bx.checked_binop`, but it's pretty simple.
I could have been a lot more aggressive here and pulled out way more methods, and add a few new methods in the rustc_codegen_ssa "API". However, because this is my second rustc PR, I thought that moving those to a follow-up PR and doing more incremental changes here would be better (and I guess ask if this work is even desired in the first place). I'm hoping to eventually remove the mess of intrinsic handling in the backend entirely, which would be hecking fantastic ✨
Validate constants during `const_eval_raw`
This PR implements the groundwork for https://github.com/rust-lang/rust/issues/72396
* constants are now validated during `const_eval_raw`
* to prevent cycle errors, we do not validate references to statics anymore beyond the fact that they are not dangling
* the `const_eval` query ICEs if used on `static` items
* as a side effect promoteds are now evaluated to `ConstValue::Scalar` again (since they are just a reference to the actual promoted allocation in most cases).
Some promotion cleanup
Based on top of both https://github.com/rust-lang/rust/pull/75502 and https://github.com/rust-lang/rust/pull/75585, this does some cleanup of the promotion code. The last 2 commits are new.
* Remove the remaining cases where `const fn` is treated different from `fn`. This means no longer promoting ptr-to-int casts, raw ptr operations, and union field accesses in `const fn` -- or anywhere, for that matter. These are all unstable in const-context so this should not break any stable code. Fixes https://github.com/rust-lang/rust/issues/75586.
* ~~Promote references to statics even outside statics (i.e., in functions) for consistency.~~
* Promote `&mut []` everywhere, not just in non-`const` functions, for consistency.
* Explain why we do not promote deref's of statics outside statics. ~~(This is the only remaining direct user of `const_kind`.)~~
This can only land once the other two PRs land; I am mostly putting this up already because I couldn't wait ;) and to get some feedback from `@rust-lang/wg-const-eval` .
shim: monomorphic `FnPtrShim`s during construction
Fixes#69925.
This PR adjusts MIR shim construction so that substitutions are applied to function pointer shims during construction, rather than during codegen (as determined by `substs_for_mir_body`).
r? `@eddyb`
Implement a generic Destination Propagation optimization on MIR
This takes the work that was originally started by `@eddyb` in https://github.com/rust-lang/rust/pull/47954, and then explored by me in https://github.com/rust-lang/rust/pull/71003, and implements it in a general (ie. not limited to acyclic CFGs) and dataflow-driven way (so that no additional infrastructure in rustc is needed).
The pass is configured to run at `mir-opt-level=2` and higher only. To enable it by default, some followup work on it is still needed:
* Performance needs to be evaluated. I did some light optimization work and tested against `tuple-stress`, which caused trouble in my last attempt, but didn't go much in depth here.
* We can also enable the pass only at `opt-level=2` and higher, if it is too slow to run in debug mode, but fine when optimizations run anyways.
* Debuginfo needs to be fixed after locals are merged. I did not look into what is required for this.
* Live ranges of locals (aka `StorageLive` and `StorageDead`) are currently deleted. We either need to decide that this is fine, or if not, merge the variable's live ranges (or remove these statements entirely – https://github.com/rust-lang/rust/issues/68622).
Some benchmarks of the pass were done in https://github.com/rust-lang/rust/pull/72635.
Rollup of 14 pull requests
Successful merges:
- #73963 (deny(unsafe_op_in_unsafe_fn) in libstd/path.rs)
- #75099 (lint/ty: move fns to avoid abstraction violation)
- #75502 (Use implicit (not explicit) rules for promotability by default in `const fn`)
- #75580 (Add test for checking duplicated branch or-patterns)
- #76310 (Add `[T; N]: TryFrom<Vec<T>>` (insta-stable))
- #76400 (Clean up vec benches bench_in_place style)
- #76434 (do not inline black_box when building for Miri)
- #76492 (Add associated constant `BITS` to all integer types)
- #76525 (Add as_str() to string::Drain.)
- #76636 (assert ScalarMaybeUninit size)
- #76749 (give *even better* suggestion when matching a const range)
- #76757 (don't convert types to the same type with try_into (clippy::useless_conversion))
- #76796 (Give a better error message when x.py uses the wrong stage for CI)
- #76798 (Build fixes for RISC-V 32-bit Linux support)
Failed merges:
r? `@ghost`
give *even better* suggestion when matching a const range
notice that the err already has "constant defined here"
so this is now *exceedingly clear*
extension to #76222
r? @estebank
assert ScalarMaybeUninit size
I noticed most low-level Miri types have such an assert but `ScalarMaybeUninit` does not, so let's add that. Good t see that the `Option`-like optimization kicks in and this is no bigger than `Scalar`. :)
r? @oli-obk
Add associated constant `BITS` to all integer types
Recently I've regularly come across this snippet (in a few different crates, including `core` and `std`):
```rust
std::mem::size_of<usize>() * 8
```
I think it's time for a `usize::BITS`.
lint/ty: move fns to avoid abstraction violation
This PR moves `transparent_newtype_field` and `is_zst` to `LateContext` where they are used, rather than being on the `VariantDef` and `TyS` types, hopefully addressing @eddyb's concern [from this comment](https://github.com/rust-lang/rust/pull/74340#discussion_r456534910).
Wrap recursive predicate evaluation with `ensure_sufficient_stack`
I haven't been able to come up with a minimized test case for #76770,
but this fixes a stack overflow in rustc as well.
As a side effect, we now represent most promoteds as `ConstValue::Scalar` again. This is useful because all implict promoteds are just references anyway and most explicit promoteds are numeric arguments to `asm!` or SIMD instructions.
Dogfood new_uninit and maybe_uninit_slice in rustc_arena
Dogfoods a few cool `MaybeUninit` related features in the compiler's rustc_arena crate.
Split off from #76821
r? `@oli-obk`
Some MIR statements and terminators have an (undocumented...) invariant
that some of their input and outputs must not overlap. This records
conflicts between locals used in these positions.
compare generic constants using `AbstractConst`s
This is a MVP of rust-lang/compiler-team#340. The changes in this PR should only be relevant if `feature(const_evaluatable_checked)` is enabled.
~~currently based on top of #76559, so blocked on that.~~
r? `@oli-obk` cc `@varkor` `@eddyb`
Issue 72408 nested closures exponential
This fixes#72408.
Nested closures were resulting in exponential compilation time.
This PR is enhancing asymptotic complexity, but also increasing the constant, so I would love to see perf run results.
[mir-opt] Disable the `ConsideredEqual` logic in SimplifyBranchSame opt
The logic is currently broken and we need to disable it to fix a beta
regression (see #76803)
r? `@oli-obk`
Mostly to fix ui/issues/issue-37311-type-length-limit/issue-37311.rs.
Most parts of the compiler can handle deeply nested types with a lot
of duplicates just fine, but some parts still attempt to naively
traverse type tree.
Before such problems were caught by type length limit check,
but now these places will have to be changed to handle
duplicated types gracefully.
This fixes#72408.
Nested closures were resulting in exponential compilation time.
As a performance optimization this change introduces MiniSet,
which is a simple small storage optimized set.
move guaranteed{ne,eq} implementation to compile-time machine
Currently, Miri needs a special hack to avoid using the core engine implementation of these intrinsics. That seems silly, so let's move them to the CTFE machine, which is the only machine that wants to use them.
I also added a reference to https://github.com/rust-lang/rust/issues/73722 as a warning to anyone who wants to adjust `guaranteed_eq`.
Improve E0118
- Changes the "base type" terminology to "nominal type" (according to the [reference](https://doc.rust-lang.org/stable/reference/items/implementations.html#inherent-implementations)).
- Suggests removing a reference, if one is present on the type.
- Clarify what is meant by a "nominal type".
closes#69392
This is my first not-entirely-trivial PR, so please let me know if I missed anything or if something could be improved. Though I probably won't be able to fix anything in the upcoming week.
Make graphviz font configurable
Alternative to PR #76776.
To change the graphviz output to use an alternative `fontname` value,
add a command line option like: `rustc --graphviz-font=monospace`.
r? @ecstatic-morse
Strip a single leading tab when rendering dataflow diffs
The `fmt_diff_with` formatter uses a tab to separate additions from subtractions. Strip it when rendering those diffs on separate lines.
r? @Mark-Simulacrum (since you're speedy)
Change error message for ty param in const
This PR introduces the following changes:
* Change error message for type param in a const expression when using
`min_const_generics`
* Change `ParamInNonTrivialAnonConst` to contain an extra `bool` used for
distinguishing whether the passed-in symbol is a type or a value.
Fixes#76701
Alternative to PR ##76776.
To change the graphviz output to use an alternative `fontname` value,
add a command line option like: `rustc --graphviz-font=monospace`.
improve const infer error
cc #72328
reduces it from
```
error[E0282]: type annotations needed
--> src/main.rs:17:5
|
17 | Foo.bar().bar().bar().bar().baz();
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
= note: unable to infer the value of a const parameter
```
to
```
error[E0282]: type annotations needed
--> $DIR/method-chain.rs:21:33
|
LL | Foo.bar().bar().bar().bar().baz();
| ^^^
|
= note: cannot infer the value of the const parameter `N`
```
r? @varkor
fix syntax error in suggesting generic constraint in trait parameter
suggest `where T: Foo` for the first bound on a trait, then suggest
`, T: Foo` when the suggested bound would add to an existing set of
`where` clauses. `where T: Foo` may be the first bound if `T` has a
default, because we'd rather suggest
```
trait A<T=()> where T: Copy
```
than
```
trait A<T: Copy=()>
```
for legibility reasons.
the test case i added here is derived from [this reproduction](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=0bf3ace9f2a183d0bdbd748c6b8e3971):
```
struct B<T: Copy> {
t: T
}
trait A<T = ()> {
fn returns_constrained_type(&self, t: T) -> B<T> {
B { t }
}
}
```
where the suggested fix,
```
trait A<T = ()>, T: Copy { ... }
```
is in fact invalid syntax!
i also found an error in the existing suggestion for `trait Base<T = String>: Super<T>` where rustc would suggest `trait Base<T = String>: Super<T>, T: Copy`, but `T: Copy` is the first of the trait's `where` clauses and should be `where T: Copy` as well. the test for that suggestion expects invalid syntax, and has been revised to a compiler-pleasing `trait Base<T = String>: Super<T> where T: Copy`.
judging by https://github.com/rust-lang/rust/pull/70009 i'll.. cc @estebank ?
