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Force different metadata for sysroot crates #269

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Merged
merged 2 commits into from
Dec 3, 2019
Merged

Force different metadata for sysroot crates #269

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f23ae8d
Force sysroot to have different hashes
roblabla Mar 16, 2018
394d272
Add some documentation on why we use __CARGO_DEFAULT_LIB_METADATA
roblabla Nov 29, 2019
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24 changes: 24 additions & 0 deletions src/sysroot.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -126,6 +126,30 @@ version = "0.0.0"
cmd.env("RUSTFLAGS", flags);
cmd.env_remove("CARGO_TARGET_DIR");

// Workaround #261.
//
// If a crate is shared between the sysroot and a binary, we might
// end up with conflicting symbols. This is because both versions
// of the crate would get linked, and their metadata hash would be
// exactly the same.
//
// To avoid this, we need to inject some data that modifies the
// metadata hash. Fortunately, cargo already has a mechanism for
// this, the __CARGO_DEFAULT_LIB_METADATA environment variable.
// Unsurprisingly, rust's bootstrap (which has basically the same
// role as xargo of building the libstd) makes use of this
// environment variable to avoid exactly this problem. See here:
// https://github.com/rust-lang/rust/blob/73369f32621f6a844a80a8513ae3ded901e4a406/src/bootstrap/builder.rs#L876
//
// This relies on an **unstable cargo feature** that isn't meant to
// be used outside the bootstrap. This is explicitly stated in
// cargo's source:
// https://github.com/rust-lang/cargo/blob/14654f38d0819c47d7a605d6f1797ffbcdc65000/src/cargo/core/compiler/context/compilation_files.rs#L496
// Unfortunately, I don't see any other way out. We need to have a
// way to modify the crate's hash, and from the outside this is the
// only way to do so.
cmd.env("__CARGO_DEFAULT_LIB_METADATA", "xargo");

// As of rust-lang/cargo#4788 Cargo invokes rustc with a changed "current directory" so
// we can't assume that such directory will be the same as the directory from which
// Xargo was invoked. This is specially true when compiling the sysroot as the std
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