rustc_typeck: gate AnonConst's generics on feature(const_generics). #66883
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I think we've always implicitly understood this would be the case, so I have no issue making that explicit. |
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@bors r+ |
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📌 Commit 584ede5 has been approved by |
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🌲 The tree is currently closed for pull requests below priority 1000, this pull request will be tested once the tree is reopened |
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…li-obk rustc_typeck: gate AnonConst's generics on feature(const_generics). This PR employs the fix for rust-lang#43408 when `#![feature(const_generics)]` is enabled, making the feature-gate the opt-in for all the possible breakage this may incur. For example, if this PR lands, this will cause a cycle error (due to rust-lang#60471): ```rust #![feature(const_generics)] fn foo<T: Into<[u8; 4]>>() {} ``` And so will anything with type-level const expressions, in its bounds. Surprisingly, `impl`s don't seem to be affected (if they were, even libcore wouldn't compile). One thing I'm worried about is not knowing how much unstable code out there, using const-generics, will be broken. But types like `Foo<{N+1}>` never really worked, and do after this PR, just not in bounds - so ironically, it's type-level const expressions that don't depend on generics, which will break (in bounds). Also, if we do this, we'll have effectively blocked stabilization of const generics on rust-lang#60471. r? @oli-obk cc @varkor @yodaldevoid @nikomatsakis
| // HACK(eddyb) when substs contain e.g. inference variables, | ||
| // attempt using identity substs instead, that will succeed | ||
| // when the expression doesn't depend on any parameters. | ||
| // FIXME(eddyb) make `const_eval` a canonical query instead, |
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I also discovered that we need to make const_eval a canonical query as part of my work on lazy normalization(https://rust-lang.zulipchat.com/#narrow/stream/144729-wg-traits/topic/lazy-normalization.20and.20const.20generics/near/180906840). That's also partly my motivation behind #66877.
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Rollup of 9 pull requests Successful merges: - #66612 (Initial implementation of or-pattern usefulness checking) - #66705 (Atomic as_mut_ptr) - #66759 (impl TrustedLen for vec::Drain) - #66858 (Use LLVMAddAnalysisPasses instead of Rust's wrapper) - #66870 (SimplifyArmIdentity only for locals with the same type) - #66883 (rustc_typeck: gate AnonConst's generics on feature(const_generics).) - #66889 (Make python-generated source files compatible with rustfmt) - #66894 (Remove unneeded prelude imports in libcore tests) - #66895 (Feature gating *declarations* => new crate `rustc_feature`) Failed merges: - #66905 (rustc_plugin: Remove some remaining plugin features) r? @ghost
This was referenced Dec 31, 2019
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This PR employs the fix for #43408 when
#![feature(const_generics)]is enabled, making the feature-gate the opt-in for all the possible breakage this may incur.For example, if this PR lands, this will cause a cycle error (due to #60471):
And so will anything with type-level const expressions, in its bounds.
Surprisingly,
impls don't seem to be affected (if they were, even libcore wouldn't compile).One thing I'm worried about is not knowing how much unstable code out there, using const-generics, will be broken. But types like
Foo<{N+1}>never really worked, and do after this PR, just not in bounds - so ironically, it's type-level const expressions that don't depend on generics, which will break (in bounds).Also, if we do this, we'll have effectively blocked stabilization of const generics on #60471.
r? @oli-obk cc @varkor @yodaldevoid @nikomatsakis