Make Cargo aware of standard library dependencies

text/0000-cargo-libstd-awareness.md

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+- Feature Name: cargo_libstd_awareness
+- Start Date: 2015-05-26
+- RFC PR: (leave this empty)
+- Rust Issue: (leave this empty)
+
+# Summary
+
+Currently, all packages implicitly depend on libstd. This makes Cargo unsuitable for packages that
+need a custom-built libstd, or otherwise depend on crates with the same names as libstd and the
+crates behind the facade. The proposed fixes also open the door to a future where libstd can be
+Cargoized.
+
+# Motivation
+
+Bare-metal work cannot use a standard build of libstd. But since any crate built with Cargo can link
+with a system-installed libstd if the target matches, using Cargo for such projects can be irksome
+or impossible.
+
+Cargoizing libstd also generally simplifies the infrastructure, and makes cross compiling much
+slicker, but that is a separate discussion.
+
+Finally, I first raised this issue here: https://github.com/rust-lang/Cargo/issues/1096 Also, there
+are some (heavily bit-rotted) projects at https://github.com/RustOS-Fork-Holding-Ground that depend
+on each other in the way this RFC would make much more feasible.
+
+# Detailed design
+
+The current situation seems to be more of an accident of `rustc`'s pre-Cargo history than an
+explicit design decision. Cargo passes the location and name of all depended on crates to `rustc`.
+This is good because it means that that no undeclared dependencies on other Cargo packages can leak
+through. However, it also passes in `--sysroot /path/to/some/libdir`, the directory being were
+libstd is. This means packages are free to use libstd, the crates behind the facade, or none of the
+above, with Cargo being none the wiser.
+
+The only new interface proposed is a boolean field to the package meta telling Cargo that the
+package does not depend on libstd by default. This need not imply Rust's `no_std`, as one might want
+to `use` their own build of libstd by default. To disambiguate, this field is called
+q`no-implicit-deps`; please, go ahead and bikeshead the name. `no-implicit-deps` is false by
+default to maintain compatibility with existing packages.
+
+The meaning of this flag is defined in 3 phases, where each phase extends the last. The idea being
+is that while earlier phases are easier to implement, later phases yield a more elegant system.
+
+## Phase 1
+
+Add a `--no-sysroot` flag to `rustc`, and pass that to `rustc` is the case that `no-implicit-deps`
+is true.
+
+This hotfix is enough to allow us bare-metal devs to use Cargo for our own projects, but doesn't
+suffice for creating an ecosystem of packages that depend on crates behind the facade but not libstd
+itself. This is because the choices are all or nothing: Either one implicitly depends on libstd or
+the crates behind the facade, or they don't depend on them at all.
+
+## Phase 2
+
+Since, passing in a directory of crates is inherently more fragile than passing in a crate
+itself, make Cargo use `--no-sysroot` in all cases.
+
+Cargo would special case package names corresponding to the crates behind the facade, such that if
+the package don't exist, it would simply pass the corresponding system crate to `rustc`. I assume
+the names are blacklisted on crates.io already, so by default the packages won't exist. But users
+can use config files to extend the namespace so their own modded libstds can be used instead. Even
+if they don't want to change libstd but just cross-compile it, this is frankly the easiest way as
+Cargo will seemliest cross compile both their project and it's transitive dependencies.
+
+In this way we can put packages on crates.io that depend on the crates behind the facade. Some
+packages that already exist, like liblog and libbitflags, should be given features that optionally
+allow them to avoid libstd and just depend directly on the crates behind the facade they really
+need.
+
+## Phase 3
+
+If/when the standard library is built with Cargo and put on crates.io, all the specially-cased
+package names can be treated normally,
+
+The standard library is be downloaded and built from crates.io. Or equivalently, Cargo comes with a
+cache of that build, as Cargo should be able cache builds between projects at this point. Just as in
+phase 2, `no-implicit-deps` just prevents libstd from implicitly being appended to the list of
+dependencies.
+
+Again, to make this as least controversial as possible, this RFC does not propose outright that the
+standard library should be Cargoized. This 3rd phases just describes how this feature would work
+were that to happen.
+
+# Drawbacks
+
+I really don't know of any. Development for hosted environments would hardly be very affected.
+
+# Alternatives
+
+Make it so all dependencies, even libstd, must be explicit. C.f. Cabal and base.
+
+# Unresolved questions
+
+There are multiple lists of dependencies for different things (e.g. tests), Should libstd be append
+to all of them in phases 2 and 3?