borrowck: Dev notes

gcc/rust/ChangeLog:

	* checks/errors/borrowck/dev-notes.md: New file.

Signed-off-by: Jakub Dupak <dev@jakubdupak.com>

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+# Borrow-checker Development Notes
+
+## Testing BIR building
+
+There is no way to test BIR building directly, since it is a dead branch of the compilation pipeline.
+The only way to verify its generations is through manual inspection.
+The best way to inspect the BIR is to compare it with rustc's MIR.
+
+The following command will compile a rust file into a library and dump its MIR:
+
+```shell
+rustc --crate-type=lib -A dead_code -A unused -Z dump-mir="" <file>
+```
+
+The MIR dump directory `mir_dump` contains a dump before and after each MIR pass.
+We are interested in the one used for borrow-checking, which is called `<crate>.<function>.002-000.analysis.after.mir`.
+
+BIR dump is emitted to a `bir_dump` directory. With the following naming scheme: `<crate>.<function>.bir.dump`.
+
+At this point, MIR dump contains helper constructions that BIR does not contain yet (like storage live/dead annotations). To remove them from the MIR dump, run the following command:
+
+```shell
+awk -i inplace '!/^\s*(\/\/|StorageLive|StorageDead|FakeRead)/' mir_dump/*
+```
+
+To get the BIR dump into a similar format, run the following command:
+
+```shell
+./crab1 <file> -frust-incomplete-and-experimental-compiler-do-not-use -frust-borrowcheck -frust-dump-bir -frust-compile-until=compilation
+```
+
+
+## TODO
+
+- scope handling, cleanup
+- switch coercions to adjustments from typechecking
+- operator overloading
+- match selection
+- let without an initializer
+- lifetime parameters
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