mirror of
https://github.com/rust-lang/rust.git
synced 2024-11-22 06:44:35 +00:00
4e510daed7
Allow instantiating object trait binder when upcasting
This PR fixes two bugs (that probably need an FCP).
### We use equality rather than subtyping for upcasting dyn conversions
This code should be valid:
```rust
#![feature(trait_upcasting)]
trait Foo: for<'h> Bar<'h> {}
trait Bar<'a> {}
fn foo(x: &dyn Foo) {
let y: &dyn Bar<'static> = x;
}
```
But instead:
```
error[E0308]: mismatched types
--> src/lib.rs:7:32
|
7 | let y: &dyn Bar<'static> = x;
| ^ one type is more general than the other
|
= note: expected existential trait ref `for<'h> Bar<'h>`
found existential trait ref `Bar<'_>`
```
And so should this:
```rust
#![feature(trait_upcasting)]
fn foo(x: &dyn for<'h> Fn(&'h ())) {
let y: &dyn FnOnce(&'static ()) = x;
}
```
But instead:
```
error[E0308]: mismatched types
--> src/lib.rs:4:39
|
4 | let y: &dyn FnOnce(&'static ()) = x;
| ^ one type is more general than the other
|
= note: expected existential trait ref `for<'h> FnOnce<(&'h (),)>`
found existential trait ref `FnOnce<(&(),)>`
```
Specifically, both of these fail because we use *equality* when comparing the supertrait to the *target* of the unsize goal. For the first example, since our supertrait is `for<'h> Bar<'h>` but our target is `Bar<'static>`, there's a higher-ranked type mismatch even though we *should* be able to instantiate that supertrait binder when upcasting. Similarly for the second example.
### New solver uses equality rather than subtyping for no-op (i.e. non-upcasting) dyn conversions
This code should be valid in the new solver, like it is with the old solver:
```rust
// -Znext-solver
fn foo<'a>(x: &mut for<'h> dyn Fn(&'h ())) {
let _: &mut dyn Fn(&'a ()) = x;
}
```
But instead:
```
error: lifetime may not live long enough
--> <source>:2:11
|
1 | fn foo<'a>(x: &mut dyn for<'h> Fn(&'h ())) {
| -- lifetime `'a` defined here
2 | let _: &mut dyn Fn(&'a ()) = x;
| ^^^^^^^^^^^^^^^^^^^ type annotation requires that `'a` must outlive `'static`
|
= note: requirement occurs because of a mutable reference to `dyn Fn(&())`
```
Specifically, this fails because we try to coerce `&mut dyn for<'h> Fn(&'h ())` to `&mut dyn Fn(&'a ())`, which registers an `dyn for<'h> Fn(&'h ()): dyn Fn(&'a ())` goal. This fails because the new solver uses *equating* rather than *subtyping* in `Unsize` goals.
This is *mostly* not a problem... You may wonder why the same code passes on the new solver for immutable references:
```
// -Znext-solver
fn foo<'a>(x: &dyn Fn(&())) {
let _: &dyn Fn(&'a ()) = x; // works
}
```
That's because in this case, we first try to coerce via `Unsize`, but due to the leak check the goal fails. Then, later in coercion, we fall back to a simple subtyping operation, which *does* work.
Since `&T` is covariant over `T`, but `&mut T` is invariant, that's where the discrepancy between these two examples crops up.
---
r? lcnr or reassign :D
|
||
|---|---|---|
| .. | ||
| rustc | ||
| rustc_abi | ||
| rustc_arena | ||
| rustc_ast | ||
| rustc_ast_ir | ||
| rustc_ast_lowering | ||
| rustc_ast_passes | ||
| rustc_ast_pretty | ||
| rustc_attr | ||
| rustc_baked_icu_data | ||
| rustc_borrowck | ||
| rustc_builtin_macros | ||
| rustc_codegen_cranelift | ||
| rustc_codegen_gcc | ||
| rustc_codegen_llvm | ||
| rustc_codegen_ssa | ||
| rustc_const_eval | ||
| rustc_data_structures | ||
| rustc_driver | ||
| rustc_driver_impl | ||
| rustc_error_codes | ||
| rustc_error_messages | ||
| rustc_errors | ||
| rustc_expand | ||
| rustc_feature | ||
| rustc_fluent_macro | ||
| rustc_fs_util | ||
| rustc_graphviz | ||
| rustc_hir | ||
| rustc_hir_analysis | ||
| rustc_hir_pretty | ||
| rustc_hir_typeck | ||
| rustc_incremental | ||
| rustc_index | ||
| rustc_index_macros | ||
| rustc_infer | ||
| rustc_interface | ||
| rustc_lexer | ||
| rustc_lint | ||
| rustc_lint_defs | ||
| rustc_llvm | ||
| rustc_log | ||
| rustc_macros | ||
| rustc_metadata | ||
| rustc_middle | ||
| rustc_mir_build | ||
| rustc_mir_dataflow | ||
| rustc_mir_transform | ||
| rustc_monomorphize | ||
| rustc_next_trait_solver | ||
| rustc_parse | ||
| rustc_parse_format | ||
| rustc_passes | ||
| rustc_pattern_analysis | ||
| rustc_privacy | ||
| rustc_query_impl | ||
| rustc_query_system | ||
| rustc_resolve | ||
| rustc_sanitizers | ||
| rustc_serialize | ||
| rustc_session | ||
| rustc_smir | ||
| rustc_span | ||
| rustc_symbol_mangling | ||
| rustc_target | ||
| rustc_trait_selection | ||
| rustc_traits | ||
| rustc_transmute | ||
| rustc_ty_utils | ||
| rustc_type_ir | ||
| rustc_type_ir_macros | ||
| stable_mir | ||