forbid impl Trait in generic param defaults
#83935
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r? @varkor (rust-highfive has picked a reviewer for you, use r? to override) |
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| error[E0720]: cannot resolve opaque type | ||
| --> $DIR/where-allowed.rs:56:49 | ||
| | | ||
| LL | fn in_impl_Fn_parameter_in_return() -> &'static impl Fn(impl Debug) { panic!() } | ||
| | ^^^^^^^^^^^^^^^^^^^ -------- this returned value is of `!` type | ||
| | | | ||
| | cannot resolve opaque type | ||
| | | ||
| = help: this error will resolve once the item's body returns a concrete type | ||
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| error[E0720]: cannot resolve opaque type | ||
| --> $DIR/where-allowed.rs:62:46 | ||
| error: defaults for type parameters are only allowed in `struct`, `enum`, `type`, or `trait` definitions | ||
| --> $DIR/where-allowed.rs:234:7 | ||
| | | ||
| LL | fn in_impl_Fn_return_in_return() -> &'static impl Fn() -> impl Debug { panic!() } | ||
| | ^^^^^^^^^^^^^^^^^^^^^^^ -------- this returned value is of `!` type | ||
| | | | ||
| | cannot resolve opaque type | ||
| LL | impl <T = impl Debug> T {} | ||
| | ^ | ||
| | | ||
| = help: this error will resolve once the item's body returns a concrete type | ||
| = note: `#[deny(invalid_type_param_default)]` on by default | ||
| = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release! | ||
| = note: for more information, see issue #36887 <https://github.com/rust-lang/rust/issues/36887> | ||
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| error: could not find defining uses | ||
| --> $DIR/where-allowed.rs:121:16 | ||
| error: defaults for type parameters are only allowed in `struct`, `enum`, `type`, or `trait` definitions | ||
| --> $DIR/where-allowed.rs:240:36 | ||
| | | ||
| LL | type Out = impl Debug; | ||
| | ^^^^^^^^^^ | ||
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| error: could not find defining uses | ||
| --> $DIR/where-allowed.rs:157:23 | ||
| LL | fn in_method_generic_param_default<T = impl Debug>(_: T) {} | ||
| | ^ | ||
| | | ||
| LL | type InTypeAlias<R> = impl Debug; | ||
| | ^^^^^^^^^^ |
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I think these errors disappeared because hitting the invalid_type_param_default future compat lint is making the compiler abort earlier.
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I agree that this ought always to have been an error; it must have been an oversight. Thanks for fixing this. @bors r+ |
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📌 Commit ee79f83 has been approved by |
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… r=varkor forbid `impl Trait` in generic param defaults Fixes rust-lang#83929 Forbid using `impl Trait` in the default types of generic parameters, e.g. `struct Foo<T = impl Trait>`. I assume this was never supposed to be allowed - it seems no UI test used it. Note that using `impl Trait` in this position did not hit a feature gate error; however, this *shouldn't* be a breaking change as any attempt to use it should have hit the ICE in rust-lang#83929 and/or failed to provide a defining use of the `impl Trait`.
This was referenced Apr 7, 2021
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Rollup of 8 pull requests Successful merges: - rust-lang#83476 (Add strong_count mutation methods to Rc) - rust-lang#83634 (Do not emit the advanced diagnostics on macros) - rust-lang#83816 (Trigger `unused_doc_comments` on macros at once) - rust-lang#83916 (Use AnonConst for asm! constants) - rust-lang#83935 (forbid `impl Trait` in generic param defaults) - rust-lang#83936 (Disable using non-ascii identifiers in extern blocks.) - rust-lang#83945 (Add suggestion to reborrow mutable references when they're moved in a for loop) - rust-lang#83954 (Do not ICE when closure is involved in Trait Alias Impl Trait) Failed merges: r? `@ghost` `@rustbot` modify labels: rollup
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Fixes #83929
Forbid using
impl Traitin the default types of generic parameters, e.g.struct Foo<T = impl Trait>. I assume this was never supposed to be allowed - it seems no UI test used it.Note that using
impl Traitin this position did not hit a feature gate error; however, this shouldn't be a breaking change as any attempt to use it should have hit the ICE in #83929 and/or failed to provide a defining use of theimpl Trait.