Commit graph

38153 commits

Author SHA1 Message Date
Jakob Degen
d58d52a397 Fix handling of substitutions and binders when deciding whether to suggest references
When suggesting references, substitutions were being forgotten and some types were misused. This led to at
least one ICE and other incorrectly emitted diagnostics. This has been fixed; in some cases this leads to
diagnostics changing, and tests have been adjusted.
2021-11-13 16:28:41 -05:00
bors
62efba8a05 Auto merge of #90755 - scottmcm:spec-array-clone, r=jackh726
Specialize array cloning for Copy types

Because after PR 86041, the optimizer no longer load-merges at the LLVM IR level, which might be part of the perf loss.  (I'll run perf and see if this makes a difference.)

Also I added a codegen test so this hopefully won't regress in future -- it passes on stable and with my change here, but not on the 2021-11-09 nightly.

Example on current nightly: <https://play.rust-lang.org/?version=nightly&mode=release&edition=2021&gist=1f52d46fb8fc3ca3ac9f097390085ffa>
```rust
type T = u8;
const N: usize = 3;

pub fn demo_clone(x: &[T; N]) -> [T; N] {
    x.clone()
}

pub fn demo_copy(x: &[T; N]) -> [T; N] {
    *x
}
```
```llvm-ir
; playground::demo_clone
; Function Attrs: mustprogress nofree nosync nounwind nonlazybind uwtable willreturn
define i24 `@_ZN10playground10demo_clone17h98a4f11453d1a753E([3` x i8]* noalias nocapture readonly align 1 dereferenceable(3) %x) unnamed_addr #0 personality i32 (i32, i32, i64, %"unwind::libunwind::_Unwind_Exception"*, %"unwind::libunwind::_Unwind_Context"*)* `@rust_eh_personality` {
start:
  %0 = getelementptr [3 x i8], [3 x i8]* %x, i64 0, i64 0
  %1 = getelementptr inbounds [3 x i8], [3 x i8]* %x, i64 0, i64 1
  %.val.i.i.i.i.i.i.i.i.i = load i8, i8* %0, align 1, !alias.scope !2, !noalias !9
  %2 = getelementptr inbounds [3 x i8], [3 x i8]* %x, i64 0, i64 2
  %.val.i.i.i.i.i.1.i.i.i.i = load i8, i8* %1, align 1, !alias.scope !2, !noalias !20
  %.val.i.i.i.i.i.2.i.i.i.i = load i8, i8* %2, align 1, !alias.scope !2, !noalias !23
  %array.sroa.6.0.insert.ext.i.i.i.i = zext i8 %.val.i.i.i.i.i.2.i.i.i.i to i32
  %array.sroa.6.0.insert.shift.i.i.i.i = shl nuw nsw i32 %array.sroa.6.0.insert.ext.i.i.i.i, 16
  %array.sroa.5.0.insert.ext.i.i.i.i = zext i8 %.val.i.i.i.i.i.1.i.i.i.i to i32
  %array.sroa.5.0.insert.shift.i.i.i.i = shl nuw nsw i32 %array.sroa.5.0.insert.ext.i.i.i.i, 8
  %array.sroa.0.0.insert.ext.i.i.i.i = zext i8 %.val.i.i.i.i.i.i.i.i.i to i32
  %array.sroa.5.0.insert.insert.i.i.i.i = or i32 %array.sroa.5.0.insert.shift.i.i.i.i, %array.sroa.0.0.insert.ext.i.i.i.i
  %array.sroa.0.0.insert.insert.i.i.i.i = or i32 %array.sroa.5.0.insert.insert.i.i.i.i, %array.sroa.6.0.insert.shift.i.i.i.i
  %.sroa.4.0.extract.trunc.i.i.i.i = trunc i32 %array.sroa.0.0.insert.insert.i.i.i.i to i24
  ret i24 %.sroa.4.0.extract.trunc.i.i.i.i
}

; playground::demo_copy
; Function Attrs: mustprogress nofree norecurse nosync nounwind nonlazybind readonly uwtable willreturn
define i24 `@_ZN10playground9demo_copy17h7817453f9291d746E([3` x i8]* noalias nocapture readonly align 1 dereferenceable(3) %x) unnamed_addr #1 {
start:
  %.sroa.0.0..sroa_cast = bitcast [3 x i8]* %x to i24*
  %.sroa.0.0.copyload = load i24, i24* %.sroa.0.0..sroa_cast, align 1
  ret i24 %.sroa.0.0.copyload
}
```
2021-11-11 09:13:22 +00:00
bors
1d34cb4efc Auto merge of #89550 - lcnr:coherence-specialization, r=nikomatsakis
do not emit overlap errors for impls failing the orphan check

this should finally allow us to merge #86986, see https://github.com/rust-lang/rust/pull/86986#discussion_r716059345 for more details.

r? `@nikomatsakis` cc `@eddyb`
2021-11-11 05:47:37 +00:00
bors
9dbbbb12c0 Auto merge of #83846 - torhovland:issue-10971, r=davidtwco
Added the --temps-dir option

Fixes #10971.

The new `--temps-dir` option puts intermediate files in a user-specified directory. This provides a fix for the issue where parallel invocations of rustc would overwrite each other's intermediate files.

No files are kept in the intermediate directory unless `-C save-temps=yes`.

If additional files are specifically requested using `--emit asm,llvm-bc,llvm-ir,obj,metadata,link,dep-info,mir`, these will be put in the output directory rather than the intermediate directory.

This is a backward-compatible change, i.e. if `--temps-dir` is not specified, the behavior is the same as before.
2021-11-11 02:52:32 +00:00
bors
8e0293137f Auto merge of #90784 - matthiaskrgr:rollup-car8g12, r=matthiaskrgr
Rollup of 3 pull requests

Successful merges:

 - #89930 (Only use `clone3` when needed for pidfd)
 - #90736 (adjust documented inline-asm register constraints)
 - #90783 (Update Miri)

Failed merges:

r? `@ghost`
`@rustbot` modify labels: rollup
2021-11-10 23:13:06 +00:00
Matthias Krüger
a09115f3b4
Rollup merge of #89930 - cuviper:avoid-clone3, r=joshtriplett
Only use `clone3` when needed for pidfd

In #89522 we learned that `clone3` is interacting poorly with Gentoo's
`sandbox` tool. We only need that for the unstable pidfd extensions, so
otherwise avoid that and use a normal `fork`.

This is a re-application of beta #89924, now that we're aware that we need
more than just a temporary release fix. I also reverted 12fbabd27f, as
that was just fallout from using `clone3` instead of `fork`.

r? `@Mark-Simulacrum`
cc `@joshtriplett`
2021-11-10 23:04:25 +01:00
Matthias Krüger
858fea410d
Rollup merge of #88868 - calebzulawski:feature/simd_bitmask, r=workingjubilee
Allow simd_bitmask to return byte arrays

cc `@rust-lang/project-portable-simd` `@workingjubilee`
2021-11-10 18:52:27 +01:00
lcnr
1296719c06 no overlap errors after failing the orphan check 2021-11-10 15:10:19 +01:00
Tor Hovland
1793a7acc7 Changing cdylib to staticlib, as the former doesn't work with arm-none-eabi-gcc. 2021-11-10 08:25:35 +01:00
Scott McMurray
cc7d8014d7 Specialize array cloning for Copy types
Because after PR 86041, the optimizer no longer load-merges at the LLVM IR level, which might be part of the perf loss.  (I'll run perf and see if this makes a difference.)

Also I added a codegen test so this hopefully won't regress in future -- it passes on stable and with my change here, but not on the 2021-11-09 nightly.
2021-11-09 21:43:20 -08:00
bors
8b09ba6a5d Auto merge of #90734 - matthiaskrgr:rollup-e1euotp, r=matthiaskrgr
Rollup of 7 pull requests

Successful merges:

 - #89561 (Type inference for inline consts)
 - #90035 (implement rfc-2528 type_changing-struct-update)
 - #90613 (Allow to run a specific rustdoc-js* test)
 - #90683 (Make `compiler-docs` only control the default instead of being a hard off-switch)
 - #90685 (x.py: remove fixme by deleting code)
 - #90701 (Record more artifact sizes during self-profiling.)
 - #90723 (Better document `Box` and `alloc::alloc::box_free` connection)

Failed merges:

r? `@ghost`
`@rustbot` modify labels: rollup
2021-11-09 20:09:53 +00:00
Matthias Krüger
610b4e503c
Rollup merge of #90035 - SparrowLii:rfc2528, r=jackh726
implement rfc-2528 type_changing-struct-update

This PR implement rfc2528-type_changing-struct-update.
The main change process is as follows:
1. Move the processing part of `base_expr` into `check_expr_struct_fields` to avoid returning `remaining_fields` (a relatively complex hash table)
2. Before performing the type consistency check(`check_expr_has_type_or_error`), if the `type_changing_struct_update` feature is set, enter a different processing flow, otherwise keep the original flow
3. In the case of the same structure definition, check each field in `remaining_fields`. If the field in `base_expr` is not the suptype of the field in `adt_ty`, an error(`FeildMisMatch`) will be reported.

The MIR part does not need to be changed, because only the items contained in `remaining_fields` will be extracted from `base_expr` when MIR is generated. This means that fields with different types in `base_expr` will not be used
Updates #86618
cc `@nikomatsakis`
2021-11-09 19:00:41 +01:00
Matthias Krüger
fd74c93403
Rollup merge of #89561 - nbdd0121:const_typeck, r=nikomatsakis
Type inference for inline consts

Fixes #78132
Fixes #78174
Fixes #81857
Fixes #89964

Perform type checking/inference of inline consts in the same context as the outer def, similar to what is currently done to closure.

Doing so would require `closure_base_def_id` of the inline const to return the outer def, and since `closure_base_def_id` can be called on non-local crate (and thus have no HIR available), a new `DefKind` is created for inline consts.

The type of the generated anon const can capture lifetime of outer def, so we couldn't just use the typeck result as the type of the inline const's def. Closure has a similar issue, and it uses extra type params `CK, CS, U` to capture closure kind, input/output signature and upvars. I use a similar approach for inline consts, letting it have an extra type param `R`, and then `typeof(InlineConst<[paremt generics], R>)` would just be `R`. In borrowck region requirements are also propagated to the outer MIR body just like it's currently done for closure.

With this PR, inline consts in expression position are quitely usable now; however the usage in pattern position is still incomplete -- since those does not remain in the MIR borrowck couldn't verify the lifetime there. I have left an ignored test as a FIXME.

Some disucssions can be found on [this Zulip thread](https://rust-lang.zulipchat.com/#narrow/stream/260443-project-const-generics/topic/inline.20consts.20typeck).
cc `````@spastorino````` `````@lcnr`````
r? `````@nikomatsakis`````

`````@rustbot````` label A-inference F-inline_const T-compiler
2021-11-09 19:00:40 +01:00
bors
d6082292a6 Auto merge of #86041 - bstrie:unmagic-array-copy, r=jackh726
Replace Copy/Clone compiler magic on arrays with library impls

With const generics the compiler no longer needs to fake these impls.
2021-11-09 17:13:44 +00:00
Yuki Okushi
f1fef6be7f
Rollup merge of #90708 - NieDzejkob:feature-note, r=jackh726
Add a note about feature(explicit_generic_args_with_impl_trait) to the relevant error message

Fixes #90615
2021-11-09 22:02:25 +09:00
Yuki Okushi
c2f095a37b
Rollup merge of #90678 - JohnTitor:add-some-gats-tests, r=jackh726
Add some GATs-related regression tests

Closes #88595, closes #90014
r? `@jackh726`
2021-11-09 22:02:23 +09:00
Yuki Okushi
6e39d27a82
Rollup merge of #90591 - richlowe:illumos-ui-target, r=Mark-Simulacrum
treat illumos like solaris in failing ui tests which need it

Just adding the right cfg target for tests which fail because they don't know illumos is a thing.

(cc `````@jclulow)`````
2021-11-09 22:02:22 +09:00
bors
214cd1f228 Auto merge of #87337 - jyn514:lint-error, r=oli-obk,flip1995
Don't abort compilation after giving a lint error

The only reason to use `abort_if_errors` is when the program is so broken that either:
1. later passes get confused and ICE
2. any diagnostics from later passes would be noise

This is never the case for lints, because the compiler has to be able to deal with `allow`-ed lints.
So it can continue to lint and compile even if there are lint errors.

Closes https://github.com/rust-lang/rust/issues/82761. This is a WIP because I have a feeling it will exit with 0 even if there were lint errors; I don't have a computer that can build rustc locally at the moment.
2021-11-09 08:21:10 +00:00
bors
60952bc3da Auto merge of #90485 - camsteffen:fmt-args-less-bind, r=m-ou-se
Don't destructure args tuple in format_args!

This allows Clippy to parse the HIR more simply since `arg0` is changed to `_args.0`. (cc rust-lang/rust-clippy#7843). From rustc's perspective, I think this is something between a lateral move and a tiny improvement since there are fewer bindings.

r? `@m-ou-se`
2021-11-09 05:33:16 +00:00
Yuki Okushi
ebd401bc36
Add some GATs-related regression tests 2021-11-09 08:04:39 +09:00
Jakub Kądziołka
048e1c942d
Add a note about feature(explicit_generic_args_with_impl_trait) to the relevant error message 2021-11-08 19:49:41 +01:00
bstrie
ce1143e94d impl Copy/Clone for arrays in std, not in compiler 2021-11-08 13:11:58 -05:00
Guillaume Gomez
d9fc7d1041
Rollup merge of #90657 - GuillaumeGomez:one-char-last-line-removed, r=jyn514
Fix bug with `#[doc]` string single-character last lines

Fixes #90618.

This is because `.iter().all(|c| c == '*')` returns `true` if there is no character checked. And in case the last line has only one character, it simply returns `true`, making the last line behind removed.
2021-11-08 15:15:24 +01:00
bors
43192ca3d7 Auto merge of #89488 - c410-f3r:testsssssss, r=petrochenkov
Move some tests to more reasonable directories - 8

cc #73494
r? `@petrochenkov`
2021-11-08 02:04:31 +00:00
Joshua Nelson
0ac13bd430 Don't abort compilation after giving a lint error
The only reason to use `abort_if_errors` is when the program is so broken that either:
1. later passes get confused and ICE
2. any diagnostics from later passes would be noise

This is never the case for lints, because the compiler has to be able to deal with `allow`-ed lints.
So it can continue to lint and compile even if there are lint errors.
2021-11-08 01:22:28 +00:00
bors
5fa94f3c57 Auto merge of #88368 - jyn514:metadata-error, r=petrochenkov
Improve error when an .rlib can't be parsed

This usually describes either an error in the compiler itself or some
sort of IO error. Either way, we should report it to the user rather
than just saying "crate not found".

This only gives an error if the crate couldn't be loaded at all - if the
compiler finds another .rlib or .rmeta file which was valid, it will
continue to compile the crate.

Example output:
```
error[E0785]: found invalid metadata files for crate `foo`
 --> bar.rs:3:24
  |
3 |         println!("{}", foo::FOO_11_49[0]);
  |                        ^^^
  |
  = warning: failed to parse rlib '/home/joshua/test-rustdoc/libfoo.rlib': Invalid archive extended name offset
```

cc `@ehuss`
2021-11-07 23:03:55 +00:00
Tor Hovland
13dbdc6ad3 There is a -Clinker='arm-none-eabi-gcc' that seems to be causing trouble. 2021-11-07 20:08:42 +01:00
Tor Hovland
27464fa501 Almost all the other tests use '-include', so we'll do too. 2021-11-07 20:06:22 +01:00
Joshua Nelson
257ac1b498 Improve error when an .rlib can't be parsed
This usually describes either an error in the compiler itself or some
sort of IO error. Either way, we should report it to the user rather
than just saying "crate not found".

This only gives an error if the crate couldn't be loaded at all - if the
compiler finds another .rlib or .rmeta file which was valid, it will
continue to compile the crate.

Example output:
```
error[E0785]: found invalid metadata files for crate `foo`
 --> bar.rs:3:24
  |
3 |         println!("{}", foo::FOO_11_49[0]);
  |                        ^^^
  |
  = warning: failed to parse rlib '/home/joshua/test-rustdoc/libfoo.rlib': Invalid archive extended name offset
```
2021-11-07 15:03:40 +00:00
Vadim Petrochenkov
2834f57c45 ast: Fix naming conventions in AST structures
TraitKind -> Trait
TyAliasKind -> TyAlias
ImplKind -> Impl
FnKind -> Fn

All `*Kind`s in AST are supposed to be enums.

Tuple structs are converted to braced structs for the types above, and fields are reordered in syntactic order.

Also, mutable AST visitor now correctly visit spans in defaultness, unsafety, impl polarity and constness.
2021-11-07 21:38:17 +08:00
Tor Hovland
ede76c40d1 Made temps-dir an unstable option. 2021-11-07 09:32:05 +01:00
bors
90a273b785 Auto merge of #90348 - Amanieu:asm_feature_gates, r=joshtriplett
Add features gates for experimental asm features

This PR splits off parts of `asm!` into separate features because they are not ready for stabilization.

Specifically this adds:
- `asm_const` for `const` operands.
- `asm_sym` for `sym` operands.
- `asm_experimental_arch` for architectures other than x86, x86_64, arm, aarch64 and riscv.

r? `@nagisa`
2021-11-07 04:59:42 +00:00
Gary Guo
4060ed7eff Add a FIXME note about what's missing 2021-11-07 04:00:34 +00:00
Gary Guo
ff055e2135 Ensure closure requirements are proven for inline const 2021-11-07 04:00:34 +00:00
Gary Guo
1d32b20170 Add unit test for issue 78174 2021-11-07 04:00:34 +00:00
Gary Guo
4acef9ac19 Add unit test for issue 78132 2021-11-07 04:00:34 +00:00
Amanieu d'Antras
87d0d64b78 Restrict tests that use needs-asm-support to non-experimental
architectures
2021-11-07 03:44:42 +00:00
Amanieu d'Antras
eb32c00216 Add features gates for experimental asm features 2021-11-07 01:23:53 +00:00
Matthias Krüger
ec471de865
Rollup merge of #90649 - cjgillot:reveal-all-2, r=lcnr
Run reveal_all on MIR when inlining is activated.

Fix logic error in https://github.com/rust-lang/rust/pull/85254 which prevented the pass from running when needed.
Fixes https://github.com/rust-lang/rust/issues/78442
r? ``@lcnr``
2021-11-06 23:12:06 +01:00
Matthias Krüger
43fee0e0a9
Rollup merge of #90646 - BoxyUwU:funky_ice, r=estebank
type error go brrrrrrrr

Fixes #90444

when we relate something like:
`fn(fn((), (), u32))` with `fn(fn((), (), ()))`
we relate the inner fn ptrs:
`fn((), (), u32)` with `fn((), (), ())`
yielding a `TypeError::ArgumentSorts(_, 2)` which we then use as the `TypeError` for the `fn(fn(..))` which later causes the ICE as the `2` does not correspond to any input or output types in `fn(_)`

r? `@estebank`
2021-11-06 23:12:06 +01:00
Matthias Krüger
1d9fe9cd06
Rollup merge of #90627 - camelid:suggest-box-deref, r=davidtwco
Suggest dereference of `Box` when inner type is expected

For example:

    enum Ty {
        Unit,
        List(Box<Ty>),
    }

    fn foo(x: Ty) -> Ty {
        match x {
            Ty::Unit => Ty::Unit,
            Ty::List(elem) => foo(elem),
        }
    }

Before, the only suggestion was to rewrap `inner` with `Ty::Wrapper`,
which is unhelpful and confusing:

    error[E0308]: mismatched types
     --> src/test/ui/suggestions/boxed-variant-field.rs:9:31
      |
    9 |         Ty::List(elem) => foo(elem),
      |                               ^^^^
      |                               |
      |                               expected enum `Ty`, found struct `Box`
      |                               help: try using a variant of the expected enum: `Ty::List(elem)`
      |
      = note: expected enum `Ty`
               found struct `Box<Ty>`

Now, rustc will first suggest dereferencing the `Box`, which is most
likely what the user intended:

    error[E0308]: mismatched types
     --> src/test/ui/suggestions/boxed-variant-field.rs:9:31
      |
    9 |         Ty::List(elem) => foo(elem),
      |                               ^^^^ expected enum `Ty`, found struct `Box`
      |
      = note: expected enum `Ty`
               found struct `Box<Ty>`
    help: try dereferencing the `Box`
      |
    9 |         Ty::List(elem) => foo(*elem),
      |                               +
    help: try using a variant of the expected enum
      |
    9 |         Ty::List(elem) => foo(Ty::List(elem)),
      |                               ~~~~~~~~~~~~~~

r? ``@davidtwco``
2021-11-06 23:12:04 +01:00
Matthias Krüger
4c49db35fc
Rollup merge of #90508 - nbdd0121:issue-90483, r=davidtwco
Apply adjustments for field expression even if inaccessible

The adjustments are used later by ExprUseVisitor to build Place projections and without adjustments it can produce invalid result.

Fix #90483

``@rustbot`` label: T-compiler
2021-11-06 23:12:03 +01:00
Cameron Steffen
a9a24d5106 Don't destructure args tuple in format_args! 2021-11-06 14:28:08 -05:00
Guillaume Gomez
aa6f6f47cd Add test for removed one character last line in rustdoc 2021-11-06 20:21:29 +01:00
Caio
7fd15f0900 Move some tests to more reasonable directories 2021-11-06 15:35:20 -03:00
Noah Lev
d93f7f93c4 Suggest dereference of Box when inner type is expected
For example:

    enum Ty {
        Unit,
        List(Box<Ty>),
    }

    fn foo(x: Ty) -> Ty {
        match x {
            Ty::Unit => Ty::Unit,
            Ty::List(elem) => foo(elem),
        }
    }

Before, the only suggestion was to rewrap `elem` with `Ty::List`,
which is unhelpful and confusing:

    error[E0308]: mismatched types
     --> src/test/ui/suggestions/boxed-variant-field.rs:9:31
      |
    9 |         Ty::List(elem) => foo(elem),
      |                               ^^^^
      |                               |
      |                               expected enum `Ty`, found struct `Box`
      |                               help: try using a variant of the expected enum: `Ty::List(elem)`
      |
      = note: expected enum `Ty`
               found struct `Box<Ty>`

Now, rustc will first suggest dereferencing the `Box`, which is most
likely what the user intended:

    error[E0308]: mismatched types
     --> src/test/ui/suggestions/boxed-variant-field.rs:9:31
      |
    9 |         Ty::List(elem) => foo(elem),
      |                               ^^^^ expected enum `Ty`, found struct `Box`
      |
      = note: expected enum `Ty`
               found struct `Box<Ty>`
    help: try dereferencing the `Box`
      |
    9 |         Ty::List(elem) => foo(*elem),
      |                               +
    help: try using a variant of the expected enum
      |
    9 |         Ty::List(elem) => foo(Ty::List(elem)),
      |                               ~~~~~~~~~~~~~~
2021-11-06 11:06:17 -07:00
Camille GILLOT
a3776d99cc Run reveal_all on MIR more often. 2021-11-06 15:56:29 +01:00
Ellen
abb9a9853b type error go brrrrrrrr 2021-11-06 10:39:11 +00:00
bors
7276a6a117 Auto merge of #90297 - dtolnay:dotzero, r=petrochenkov
Append .0 to unsuffixed float if it would otherwise become int token

Previously the unsuffixed f32/f64 constructors of `proc_macro::Literal` would create literal tokens that are definitely not a float:

```rust
Literal::f32_unsuffixed(10.0)  // 10
Literal::f32_suffixed(10.0)    // 10f32
Literal::f64_unsuffixed(10.0)  // 10
Literal::f64_suffixed(10.0)    // 10f64
```

Notice that the `10` are actually integer tokens if you were to reparse them, not float tokens.

This diff updates `Literal::f32_unsuffixed` and `Literal::f64_unsuffixed` to produce tokens that unambiguously parse as a float. This matches longstanding behavior of the proc-macro2 crate's implementation of these APIs dating back at least 3.5 years, so it's likely an unobjectionable behavior.

```rust
Literal::f32_unsuffixed(10.0)  // 10.0
Literal::f32_suffixed(10.0)    // 10f32
Literal::f64_unsuffixed(10.0)  // 10.0
Literal::f64_suffixed(10.0)    // 10f64
```

Fixes https://github.com/dtolnay/syn/issues/1085.
2021-11-06 07:15:05 +00:00
bors
9d39f6ab7d Auto merge of #89970 - jackh726:gats_diagnostics, r=nikomatsakis
Implementation of GATs outlives lint

See #87479 for background. Closes #87479

The basic premise of this lint/error is to require the user to write where clauses on a GAT when those bounds can be implied or proven from any function on the trait returning that GAT.

## Intuitive Explanation (Attempt) ##
Let's take this trait definition as an example:
```rust
trait Iterable {
    type Item<'x>;
    fn iter<'a>(&'a self) -> Self::Item<'a>;
}
```
Let's focus on the `iter` function. The first thing to realize is that we know that `Self: 'a` because of `&'a self`. If an impl wants `Self::Item` to contain any data with references, then those references must be derived from `&'a self`. Thus, they must live only as long as `'a`. Furthermore, because of the `Self: 'a` implied bound, they must live only as long as `Self`. Since it's `'a` is used in place of `'x`, it is reasonable to assume that any value of `Self::Item<'x>`, and thus `'x`, will only be able to live as long as `Self`. Therefore, we require this bound on `Item` in the trait.

As another example:
```rust
trait Deserializer<T> {
    type Out<'x>;
    fn deserialize<'a>(&self, input: &'a T) -> Self::Out<'a>;
}
```
The intuition is similar here, except rather than a `Self: 'a` implied bound, we have a `T: 'a` implied bound. Thus, the data on `Self::Out<'a>` is derived from `&'a T`, and thus it is reasonable to expect that the lifetime `'x` will always be less than `T`.

## Implementation Algorithm ##
* Given a GAT `<P0 as Trait<P1..Pi>>::G<Pi...Pn>` declared as `trait T<A1..Ai> for A0 { type G<Ai...An>; }` used in return type of one associated function `F`
* Given env `E` (including implied bounds) for `F`
* For each lifetime parameter `'a` in `P0...Pn`:
    * For each other type parameter `Pi != 'a` in `P0...Pn`: // FIXME: this include of lifetime parameters too
        * If `E => (P: 'a)`:
            * Require where clause `Ai: 'a`

## Follow-up questions ##
* What should we do when we don't pass params exactly?
For this example:
```rust
trait Des {
    type Out<'x, D>;
    fn des<'z, T>(&self, data: &'z Wrap<T>) -> Self::Out<'z, Wrap<T>>;
}
```
Should we be requiring a `D: 'x` clause? We pass `Wrap<T>` as `D` and `'z` as `'x`, and should be able to prove that `Wrap<T>: 'z`.

r? `@nikomatsakis`
2021-11-06 04:15:22 +00:00