Do not assemble candidates for default impls #121047
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@bors r+ rollup (specialization is unstable and default impls are unused) |
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Do not assemble candidates for default impls
There is no reason (as far as I can tell?) that we should assemble an impl candidate for a default impl. This candidate itself does not prove that the impl holds, and any time that it *does* hold, there will be a more specializing non-default impl that also is assembled.
This is because `default impl<T> Foo for T {}` actually expands to `impl<T> Foo for T where T: Foo {}`. The only way to satisfy that where clause (without coinduction) is via *another* implementation that does hold -- precisely an impl that specializes it.
This should fix the specialization related regressions for rust-lang#116494. That should lead to one root crate regression that doesn't have to do with specialization, which I think we can regress.
r? lcnr cc `@rust-lang/types`
cc rust-lang#31844
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…iaskrgr Rollup of 8 pull requests Successful merges: - rust-lang#118882 (Check normalized call signature for WF in mir typeck) - rust-lang#120999 (rustdoc: replace `clean::InstantiationParam` with `clean::GenericArg`) - rust-lang#121002 (remove unnecessary calls to `commit_if_ok`) - rust-lang#121005 (Remove jsha from the rustdoc review rotation) - rust-lang#121043 (add lcnr to the compiler-team assignment group) - rust-lang#121045 (Fix two UI tests with incorrect directive / invalid revision) - rust-lang#121046 (Fix incorrect use of `compile_fail`) - rust-lang#121047 (Do not assemble candidates for default impls) r? `@ghost` `@rustbot` modify labels: rollup
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…iaskrgr Rollup of 8 pull requests Successful merges: - rust-lang#118882 (Check normalized call signature for WF in mir typeck) - rust-lang#120999 (rustdoc: replace `clean::InstantiationParam` with `clean::GenericArg`) - rust-lang#121002 (remove unnecessary calls to `commit_if_ok`) - rust-lang#121005 (Remove jsha from the rustdoc review rotation) - rust-lang#121014 (Remove `force_print_diagnostic`) - rust-lang#121043 (add lcnr to the compiler-team assignment group) - rust-lang#121046 (Fix incorrect use of `compile_fail`) - rust-lang#121047 (Do not assemble candidates for default impls) r? `@ghost` `@rustbot` modify labels: rollup
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…iaskrgr Rollup of 8 pull requests Successful merges: - rust-lang#118882 (Check normalized call signature for WF in mir typeck) - rust-lang#120999 (rustdoc: replace `clean::InstantiationParam` with `clean::GenericArg`) - rust-lang#121002 (remove unnecessary calls to `commit_if_ok`) - rust-lang#121005 (Remove jsha from the rustdoc review rotation) - rust-lang#121014 (Remove `force_print_diagnostic`) - rust-lang#121043 (add lcnr to the compiler-team assignment group) - rust-lang#121046 (Fix incorrect use of `compile_fail`) - rust-lang#121047 (Do not assemble candidates for default impls) r? `@ghost` `@rustbot` modify labels: rollup
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…iaskrgr Rollup of 8 pull requests Successful merges: - rust-lang#118882 (Check normalized call signature for WF in mir typeck) - rust-lang#120999 (rustdoc: replace `clean::InstantiationParam` with `clean::GenericArg`) - rust-lang#121002 (remove unnecessary calls to `commit_if_ok`) - rust-lang#121005 (Remove jsha from the rustdoc review rotation) - rust-lang#121014 (Remove `force_print_diagnostic`) - rust-lang#121043 (add lcnr to the compiler-team assignment group) - rust-lang#121046 (Fix incorrect use of `compile_fail`) - rust-lang#121047 (Do not assemble candidates for default impls) r? `@ghost` `@rustbot` modify labels: rollup
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Rollup merge of rust-lang#121047 - compiler-errors:default-impls, r=lcnr Do not assemble candidates for default impls There is no reason (as far as I can tell?) that we should assemble an impl candidate for a default impl. This candidate itself does not prove that the impl holds, and any time that it *does* hold, there will be a more specializing non-default impl that also is assembled. This is because `default impl<T> Foo for T {}` actually expands to `impl<T> Foo for T where T: Foo {}`. The only way to satisfy that where clause (without coinduction) is via *another* implementation that does hold -- precisely an impl that specializes it. This should fix the specialization related regressions for rust-lang#116494. That should lead to one root crate regression that doesn't have to do with specialization, which I think we can regress. r? lcnr cc ``@rust-lang/types`` cc rust-lang#31844
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There is no reason (as far as I can tell?) that we should assemble an impl candidate for a default impl. This candidate itself does not prove that the impl holds, and any time that it does hold, there will be a more specializing non-default impl that also is assembled.
This is because
default impl<T> Foo for T {}actually expands toimpl<T> Foo for T where T: Foo {}. The only way to satisfy that where clause (without coinduction) is via another implementation that does hold -- precisely an impl that specializes it.This should fix the specialization related regressions for #116494. That should lead to one root crate regression that doesn't have to do with specialization, which I think we can regress.
r? lcnr cc @rust-lang/types
cc #31844