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+This source package is unfortunately quite tricky and with
+several cutting edges, due to the complexity of rust-lang
+bootstrapping system and the high rate of language changes
+still ongoing.
+
+We try to describe here inner packaging details and the
+reasons behind them.
+
+
+Bootstrapping
+=============
+
+This package is composed of two upstream tarballs:
+ * the rust-lang source
+ * a minimal stage0 to start bootstrapping the system
+
+This is an interim solution, in the long term we plan to have
+rust bootstrap itself from an installed copy. However, we are 
+currently forced to ship a stage0.
+
+We have to that way, because:
+ * upstream periodically snapshots stage0. This happens multiple times 
+   between releases, so it is not currently possible to bootstrap using
+   (only) released versions
+ * requirements are pretty tight, and when a language feature change
+   rustc cannot self-bootstrap (you need a stage0 before the change,
+   which support the old set of features)
+ * stage0 are in strictly-chained linear series. This means we cannot 
+   miss even a single snapshot, otherwise next bootstrap could fail.
+   Moreover, also due to first point above, bootstrapping across 
+   non-adjacent snapshots/packages won't work
+
+On the other hand:
+ * stage0 is directly generated by the same infrastructure that
+   packages the source tarball
+ * Mozilla upstream directly provides it, and at least has some point 
+   you have to trust it in order to break the boostrap cycle
+ * builds are automated, and public build logs can be inspected at
+   http://buildbot.rust-lang.org/builders/snap3-linux
+ * snapshot artifacts have to be manually approved by upstream, and
+   are identified by the SHA1 of the resulting content
+
+Things should improve as the language is being stabilized. 
+We will re-evaluate this choice after 1.0.
+
+
+Embedded libraries
+==================
+
+This source package embeds several external libraries (foeked and managed 
+by rust upstream as git submodules).
+In early stages, many more libraries were forked/emebedded but we are steadily
+progressing in splitting them out.
+
+Here below the remaining ones, with the technical reasons.
+
+ * jemalloc from https://github.com/rust-lang/jemalloc
+   -> system-wide one can't be used due to rust using a "je_" prefix.
+
+   This is intentional upstream design and won't change soon, see:
+    - https://github.com/rust-lang/rust/pull/18678
+    - http://smallcultfollowing.com/babysteps/blog/2014/11/14/allocators-in-rust/
+
+ * compiler-rt from https://github.com/rust-lang/compiler-rt
+   -> system-wide compiler-rt fails during linkage
+
+   Bug reported upstream, still to be fixed, see:
+    - https://github.com/rust-lang/rust/issues/15054
+    - https://github.com/rust-lang/rust/issues/15708
+
+ * LLVM from https://github.com/rust-lang/llvm
+   -> minimum required LLVM is now 3.6, not yet released
+
+   Rust fork delta is minimal and mostly performance/optimizations only.
+   Declared goal is to get in synch with vanilla LLVM.
+   However, rust depends on some features not available in LLVM-3.5,
+   while LLVM-3.6 is still pending release. See:
+    - https://github.com/rust-lang/rust/issues/20010
+    - http://discuss.rust-lang.org/t/targeted-llvm-for-1-0/1371
+
+As a summary, we plan to:
+ * keep embedding jemalloc (probably forever)
+ * wait for a stable LLVM-3.6 to try the switch
+ * work with upstream to fix compiler-rt linkage soon.
+
+ -- Luca Bruno <lucab@debian.org>  Wed, 21 Jan 2015 14:24:35 +0100
+