Delay gensym creation for "underscore items" (use foo as _/const _) until name resolution
#56392
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| let Export { ident, def, vis, span } = child; | ||
| // FIXME: We shouldn't create the gensym here, it should come from metadata, | ||
| // but metadata cannot encode gensyms currently, so we create it here. | ||
| // This is only a guess, two equivalent idents may incorrectly get different gensyms here. |
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Is this even a hypothetical problem for _?
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I'm... not entirely sure.
I haven't found an example where it would caused observable problems so far (assuming gensym creation is delayed until name resolution).
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#56303 (comment) has an example of gensyms being created incorrectly internally, but it's not observable from the outside.
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| // except according to those terms. | |||
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| const _: () = (); //~ ERROR is unstable | |||
| static _: () = (); //~ ERROR is unstable | |||
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I understand this wasn't in the RFC, but I see no reason not to support it.
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const _: T = expr; was permitted because it has a reasonable generalization to irrefutable patterns (e.g. const (A, B) = (1, 2);) but does static also have that with the same-address notion?
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At least the motivation in rust-lang/rfcs#2526 does not apply to statics.
Constants don't even exist in MIR (AFAIK), so they can be used for compile-time asserts, while statics end up in the resulting executable/library, or need to be optimized away.
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Updated with comments addressed. |
Prohibit `static _` Fis unused import warnings for `use foo as _` Add more tests for `use foo as _`
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📌 Commit eb1d2e6 has been approved by |
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Delay gensym creation for "underscore items" (`use foo as _`/`const _`) until name resolution So they cannot be cloned by macros. See #56303 for the discussion. Mostly fix cross-crate use of underscore items by inverting the "gensyms are lost in metadata" bug as described in #56303 (comment). Fix unused import warnings for single-segment imports (first commit) and `use crate_name as _` imports (as specified in #56303 (comment)). Prohibit accidentally implemented `static _: TYPE = EXPR;` (cc #55983). Add more tests for `use foo as _` imports.
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☀️ Test successful - status-appveyor, status-travis |
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cc @rust-lang/compiler, this is needed to backport #57160. |
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I'm removing the beta-nominated tag here because this is rolled into #57483 |
Do not encode gensymed imports in metadata (Unless they are underscore `_` imports which are re-gensymed on crate loading, see rust-lang#56392.) We cannot encode gensymed imports properly in metadata and if we encode them improperly, we can get erroneous name conflicts downstream. Gensymed imports are produced by the compiler, so we control their set, and can be sure that none of them needs being encoded for use from other crates. A workaround that fixes rust-lang#59243.
Do not encode gensymed imports in metadata (Unless they are underscore `_` imports which are re-gensymed on crate loading, see rust-lang#56392.) We cannot encode gensymed imports properly in metadata and if we encode them improperly, we can get erroneous name conflicts downstream. Gensymed imports are produced by the compiler, so we control their set, and can be sure that none of them needs being encoded for use from other crates. A workaround that fixes rust-lang#59243.
So they cannot be cloned by macros. See #56303 for the discussion.
Mostly fix cross-crate use of underscore items by inverting the "gensyms are lost in metadata" bug as described in #56303 (comment).
Fix unused import warnings for single-segment imports (first commit) and
use crate_name as _imports (as specified in #56303 (comment)).Prohibit accidentally implemented
static _: TYPE = EXPR;(cc #55983).Add more tests for
use foo as _imports.