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Clarify when dependencies are linked #64731

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dhardy opened this issue Sep 24, 2019 · 1 comment · Fixed by #73567
Closed

Clarify when dependencies are linked #64731

dhardy opened this issue Sep 24, 2019 · 1 comment · Fixed by #73567
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A-docs Area: documentation for any part of the project, including the compiler, standard library, and tools A-linkage Area: linking into static, shared libraries and binaries C-enhancement Category: An issue proposing an enhancement or a PR with one. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue.

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@dhardy
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dhardy commented Sep 24, 2019 •
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The reference documents that exern crate has some documentation on linkage of dependencies, but appears to relate to Edition 2015 (or at least not mention linking without extern crate).

We are considering use of extern "Rust" and #[no_mangle] for custom backends in getrandom (standalone demo), however before proceeding would like to know:

  1. if we can rely on this poorly documented aspect of the language
  2. why "multiple definition" errors are not always present when declaring multiple backend libs with extern crate
@jonas-schievink jonas-schievink added A-linkage Area: linking into static, shared libraries and binaries C-enhancement Category: An issue proposing an enhancement or a PR with one. A-docs Area: documentation for any part of the project, including the compiler, standard library, and tools labels Sep 24, 2019
@josephlr
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I think this is mainly an issue of documenting @alexcrichton's comment here: #64402 (comment)

To summarize: Cargo dependencies and the --extern rustc flag just make crates candidates for linking, you have to actually use the crate to link it.

@jonas-schievink jonas-schievink added the T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. label Mar 6, 2020
@tmiasko tmiasko mentioned this issue Jun 20, 2020
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@adetaylor adetaylor mentioned this issue Jun 21, 2020
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Manishearth added a commit to Manishearth/rust that referenced this issue Jun 23, 2020
@Manishearth
Rollup merge of rust-lang#73567 - adetaylor:extern-doc-fix, r=dtolnay
0560151 
Clarify --extern documentation.

Fixes rust-lang#64731, rust-lang#73531.

See also rust-lang#64402 (comment)
@bors bors closed this as completed in a63eb3c Jun 23, 2020
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A-docs Area: documentation for any part of the project, including the compiler, standard library, and tools A-linkage Area: linking into static, shared libraries and binaries C-enhancement Category: An issue proposing an enhancement or a PR with one. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue.
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Clarify --extern documentation. adetaylor/rust
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@dhardy @jonas-schievink @josephlr