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[idea] Add an option put the metadata in a separate file #57076
Description
Detailed design
When -Zsplit-metadata is passed to rustc, it will put the metadata normally inside a rlib or dylib in a separate rmeta file. The rlib/dylib will contain the SVH to prevent accidentally mixing the rmeta and rlib/dylib of different compilations.
Advantages
This would make the increase in size of #56987 easily fixable by deleting the rlib files, while keeping the rmeta files. Those are not necessary anyway, because rustc_driver.so already contains the code.
This will decrease staticlib, dylib and cdylib compilation time, because there is no need to copy all object files to a new archive to remove the rust.metadata.bin file from the rlib.
rust/src/librustc_codegen_llvm/back/link.rs
Lines 1340 to 1342 in 6d9640b
Disadvantages
Rustc and cargo get a bit more complex.
Activity
[-]Add an option put the metadata in a separate file[/-][+][idea] Add an option put the metadata in a separate file[/+]Xanewok commentedon Dec 28, 2018
If I understand this correctly, an another advantage is that we'd get a
cargo checkdone for free when we docargo {build, run, test}, which might speed up some workflows (no need to generate separate.rmetafor deps).bjorn3 commentedon Dec 28, 2018
@Xanewok If I read rust-lang/cargo#3501 correctly, it is technically possible to do so already, but cargo doesn't support it.
bjorn3 commentedon Dec 28, 2018
If this is accepted rust-lang/cargo#4831 could be done without invoking rustc twice. Just let rustc print a message when the metadata file is emitted and allow it to read
.rmetafiles before the.rlibfile is ready. When a crate is built which needs to run the linker (dyliborbin) either cargo or rustc could wait till all necessary.rlibfiles are generated.steveklabnik commentedon Nov 13, 2019
Triage: no real changes; sort of a duplicate of the cargo ticket, kinda.
Auto merge of rust-lang#120855 - bjorn3:split_metadata4, r=<try>
Auto merge of rust-lang#137535 - Kobzol:split-metadata, r=petrochenkov
Auto merge of #137535 - Kobzol:split-metadata, r=petrochenkov