Improve opaque type higher-ranked region error message under NLL #92306
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| --> $DIR/opaque-hrtb.rs:12:13 | ||
| | | ||
| LL | fn foo() -> impl for<'a> MyTrait<&'a str> { | ||
| | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ implementation of `MyTrait` is not general enough |
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This span isn't great, but it's the same one that we get without #![feature(nll)]. I'll probably work on improving it in a follow-up PR.
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Most such spans will get better with my lazy TAIT PR
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☔ The latest upstream changes (presumably #92007) made this pull request unmergeable. Please resolve the merge conflicts. |
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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).
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Revert lazy TAIT PR Revert rust-lang#92306 (sorry `@Aaron1011,` will include your changes in the fix PR) Revert rust-lang#93783 Revert rust-lang#92007 fixes rust-lang#93788 fixes rust-lang#93794 fixes rust-lang#93821 fixes rust-lang#93831 fixes rust-lang#93841
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Revert lazy TAIT PR Revert rust-lang/rust#92306 (sorry `@Aaron1011,` will include your changes in the fix PR) Revert rust-lang/rust#93783 Revert rust-lang/rust#92007 fixes rust-lang/rust#93788 fixes rust-lang/rust#93794 fixes rust-lang/rust#93821 fixes rust-lang/rust#93831 fixes rust-lang/rust#93841
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…sakis
Lazy type-alias-impl-trait take two
### user visible change 1: RPIT inference from recursive call sites
Lazy TAIT has an insta-stable change. The following snippet now compiles, because opaque types can now have their hidden type set from wherever the opaque type is mentioned.
```rust
fn bar(b: bool) -> impl std::fmt::Debug {
if b {
return 42
}
let x: u32 = bar(false); // this errors on stable
99
}
```
The return type of `bar` stays opaque, you can't do `bar(false) + 42`, you need to actually mention the hidden type.
### user visible change 2: divergence between RPIT and TAIT in return statements
Note that `return` statements and the trailing return expression are special with RPIT (but not TAIT). So
```rust
#![feature(type_alias_impl_trait)]
type Foo = impl std::fmt::Debug;
fn foo(b: bool) -> Foo {
if b {
return vec![42];
}
std::iter::empty().collect() //~ ERROR `Foo` cannot be built from an iterator
}
fn bar(b: bool) -> impl std::fmt::Debug {
if b {
return vec![42]
}
std::iter::empty().collect() // Works, magic (accidentally stabilized, not intended)
}
```
But when we are working with the return value of a recursive call, the behavior of RPIT and TAIT is the same:
```rust
type Foo = impl std::fmt::Debug;
fn foo(b: bool) -> Foo {
if b {
return vec![];
}
let mut x = foo(false);
x = std::iter::empty().collect(); //~ ERROR `Foo` cannot be built from an iterator
vec![]
}
fn bar(b: bool) -> impl std::fmt::Debug {
if b {
return vec![];
}
let mut x = bar(false);
x = std::iter::empty().collect(); //~ ERROR `impl Debug` cannot be built from an iterator
vec![]
}
```
### user visible change 3: TAIT does not merge types across branches
In contrast to RPIT, TAIT does not merge types across branches, so the following does not compile.
```rust
type Foo = impl std::fmt::Debug;
fn foo(b: bool) -> Foo {
if b {
vec![42_i32]
} else {
std::iter::empty().collect()
//~^ ERROR `Foo` cannot be built from an iterator over elements of type `_`
}
}
```
It is easy to support, but we should make an explicit decision to include the additional complexity in the implementation (it's not much, see a721052457cf513487fb4266e3ade65c29b272d2 which needs to be reverted to enable this).
### PR formalities
previous attempt: rust-lang#92007
This PR also includes rust-lang#92306 and rust-lang#93783, as they were reverted along with rust-lang#92007 in rust-lang#93893
fixes rust-lang#93411
fixes rust-lang#88236
fixes rust-lang#89312
fixes rust-lang#87340
fixes rust-lang#86800
fixes rust-lang#86719
fixes rust-lang#84073
fixes rust-lang#83919
fixes rust-lang#82139
fixes rust-lang#77987
fixes rust-lang#74282
fixes rust-lang#67830
fixes rust-lang#62742
fixes rust-lang#54895
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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:
To accomplish this, several different refactoring needed to be made:
InstantiateOpaqueTypestruct whichimplements
TypeOp. This is used to invokeinstantiate_opaque_typesduring MIR type checking.
TypeOpis refactored to pass around aMirBorrowckCtxt, which isneeded to report opaque type region errors.
TypeOps correspond to canonicalizedqueries. This allows us to properly handle opaque type instantiation
(which does not occur in a query) as a
TypeOp.A new
ErrorInfoassociated type is used to determine whatadditional information is used during higher-ranked region error
handling.
try_extract_error_from_fulfill_cxhas 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 takeregion constraints from the pre-existing
InferCtxtthat we usethroughout MIR borrow checking).