Feature: cargo build --frozen-registry

[kpcyrd]

Steps to reproduce:

• docker run -it --rm rust:latest
• `git clone https://github.com/some-repo-with-cargo-lock
• cargo build --release --frozen-registry

Expected behaviour:

The dependencies are downloaded right away, without updating the registry

This seems to be a common request (#4783), there's currently --locked, which is still cloning the registry, and --frozen, which refuses any http at all. What I'm looking for is a way in-between, that just downloads the dependencies as specified inside my Cargo.lock. Fetching the registry takes forever on slow networks. :)

[alexcrichton]

Thanks for the report! I think this is a dupe of #3479, however, so I'm going to close in favor of that.