Add information about package caching

[sbeckeriv]

Per #1609 "clarify how the cache works"

Added in a single line that states that the cached packages are only
removed by running the clean command.

Please let me know if this is the wrong spot, incorrect information or just silly.
Thanks
becker

[rust-highfive]

r? @huonw

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[alexcrichton]

Oh in the case of ~/.cargo, the clean command doesn't actually remove anything (only in the target directory), so perhaps it could clarify that Cargo never cleans out this cache?

[sbeckeriv]

Updated the text.

Is there a way to build via cargo with just the local cache crates or does the cache just prevent a duplicate download per project?

If I have no internet connect and remove a crate from my dependency list I can run cargo build fine.
If I add the same removed dependency <cached, same version and all> back to the list I can no longer build because it tries to update the registry.

[alexcrichton]

@bors: r+ 7695869

Thanks!

Yeah right now there's no way to say "use local cache, and only if that fails update the registry". That's covered by #1882

[bors]

⌛ Testing commit 7695869 with merge a2ca0ab...

[bors]

☀️ Test successful - cargo-linux-32, cargo-linux-64, cargo-mac-32, cargo-mac-64, cargo-win-gnu-32, cargo-win-gnu-64, cargo-win-msvc-32, cargo-win-msvc-64

[sbeckeriv]

Sorry it looks like I wrote the commit message incorrectly for bors to closed the related issue. Please close #1609