Document sanitizers in unstable-book

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+# `sanitizer`
+
+The tracking issue for this feature is: [#39699](https://github.com/rust-lang/rust/issues/39699).
+
+------------------------
+
+This feature allows for use of one of following sanitizers:
+
+* [AddressSanitizer][clang-asan] a faster memory error detector. Can
+  detect out-of-bounds access to heap, stack, and globals, use after free, use
+  after return, double free, invalid free, memory leaks.
+* [LeakSanitizer][clang-lsan] a run-time memory leak detector.
+* [MemorySanitizer][clang-msan] a detector of uninitialized reads.
+* [ThreadSanitizer][clang-tsan] a fast data race detector.
+
+To enable a sanitizer compile with `-Zsanitizer=...` option, where value is one
+of `address`, `leak`, `memory` or `thread`.
+
+# Examples
+
+This sections show various issues that can be detected with sanitizers.  For
+simplicity, the examples are prepared under assumption that optimization level
+used is zero.
+
+## AddressSanitizer
+
+Stack buffer overflow:
+
+```shell
+$ cat a.rs
+fn main() {
+    let xs = [0, 1, 2, 3];
+    let _y = unsafe { *xs.as_ptr().offset(4) };
+}
+$ rustc -Zsanitizer=address a.rs
+$ ./a
+=================================================================
+==10029==ERROR: AddressSanitizer: stack-buffer-overflow on address 0x7ffcc15f43d0 at pc 0x55f77dc015c5 bp 0x7ffcc15f4390 sp 0x7ffcc15f4388
+READ of size 4 at 0x7ffcc15f43d0 thread T0
+    #0 0x55f77dc015c4 in a::main::hab3bd2a745c2d0ac (/tmp/a+0xa5c4)
+    #1 0x55f77dc01cdb in std::rt::lang_start::_$u7b$$u7b$closure$u7d$$u7d$::haa8c76d1faa7b7ca (/tmp/a+0xacdb)
+    #2 0x55f77dc90f02 in std::rt::lang_start_internal::_$u7b$$u7b$closure$u7d$$u7d$::hfeb9a1aef9ac820d /rustc/c27f7568bc74c418996892028a629eed5a7f5f00/src/libstd/rt.rs:48:12
+    #3 0x55f77dc90f02 in std::panicking::try::do_call::h12f0919717b8e0a6 /rustc/c27f7568bc74c418996892028a629eed5a7f5f00/src/libstd/panicking.rs:288:39
+    #4 0x55f77dc926c9 in __rust_maybe_catch_panic /rustc/c27f7568bc74c418996892028a629eed5a7f5f00/src/libpanic_unwind/lib.rs:80:7
+    #5 0x55f77dc9197c in std::panicking::try::h413b21cdcd6cfd86 /rustc/c27f7568bc74c418996892028a629eed5a7f5f00/src/libstd/panicking.rs:267:12
+    #6 0x55f77dc9197c in std::panic::catch_unwind::hc5cc8ef2fd73424d /rustc/c27f7568bc74c418996892028a629eed5a7f5f00/src/libstd/panic.rs:396:8
+    #7 0x55f77dc9197c in std::rt::lang_start_internal::h2039f418ab92218f /rustc/c27f7568bc74c418996892028a629eed5a7f5f00/src/libstd/rt.rs:47:24
+    #8 0x55f77dc01c61 in std::rt::lang_start::ha905d28f6b61d691 (/tmp/a+0xac61)
+    #9 0x55f77dc0163a in main (/tmp/a+0xa63a)
+    #10 0x7f9b3cf5bbba in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x26bba)
+    #11 0x55f77dc01289 in _start (/tmp/a+0xa289)
+
+Address 0x7ffcc15f43d0 is located in stack of thread T0 at offset 48 in frame
+    #0 0x55f77dc0135f in a::main::hab3bd2a745c2d0ac (/tmp/a+0xa35f)
+
+  This frame has 1 object(s):
+    [32, 48) 'xs' <== Memory access at offset 48 overflows this variable
+HINT: this may be a false positive if your program uses some custom stack unwind mechanism, swapcontext or vfork
+      (longjmp and C++ exceptions *are* supported)
+SUMMARY: AddressSanitizer: stack-buffer-overflow (/tmp/a+0xa5c4) in a::main::hab3bd2a745c2d0ac
+Shadow bytes around the buggy address:
+  0x1000182b6820: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
+  0x1000182b6830: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
+  0x1000182b6840: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
+  0x1000182b6850: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
+  0x1000182b6860: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
+=>0x1000182b6870: 00 00 00 00 f1 f1 f1 f1 00 00[f3]f3 00 00 00 00
+  0x1000182b6880: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
+  0x1000182b6890: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
+  0x1000182b68a0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
+  0x1000182b68b0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
+  0x1000182b68c0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
+Shadow byte legend (one shadow byte represents 8 application bytes):
+  Addressable:           00
+  Partially addressable: 01 02 03 04 05 06 07 
+  Heap left redzone:       fa
+  Freed heap region:       fd
+  Stack left redzone:      f1
+  Stack mid redzone:       f2
+  Stack right redzone:     f3
+  Stack after return:      f5
+  Stack use after scope:   f8
+  Global redzone:          f9
+  Global init order:       f6
+  Poisoned by user:        f7
+  Container overflow:      fc
+  Array cookie:            ac
+  Intra object redzone:    bb
+  ASan internal:           fe
+  Left alloca redzone:     ca
+  Right alloca redzone:    cb
+  Shadow gap:              cc
+==10029==ABORTING
+```
+
+## MemorySanitizer
+
+Use of uninitialized memory. Note that we are using `-Zbuild-std` to instrument
+standard library, and passing `-msan-track-origins=2` to the LLVM to track
+origins of uninitialized memory:
+
+```shell
+$ cat src/main.rs
+use std::mem::MaybeUninit;
+
+fn main() {
+    unsafe {
+        let a = MaybeUninit::<[usize; 4]>::uninit();
+        let a = a.assume_init();
+        println!("{}", a[2]);
+    }
+}
+
+$ env RUSTFLAGS="-Zsanitizer=memory -Cllvm-args=-msan-track-origins=2" cargo -Zbuild-std run --target x86_64-unknown-linux-gnu
+==9416==WARNING: MemorySanitizer: use-of-uninitialized-value
+    #0 0x560c04f7488a in core::fmt::num::imp::fmt_u64::haa293b0b098501ca $RUST/build/x86_64-unknown-linux-gnu/stage1/lib/rustlib/src/rust/src/libcore/fmt/num.rs:202:16
+...
+  Uninitialized value was stored to memory at
+    #0 0x560c04ae898a in __msan_memcpy.part.0 $RUST/src/llvm-project/compiler-rt/lib/msan/msan_interceptors.cc:1558:3
+    #1 0x560c04b2bf88 in memory::main::hd2333c1899d997f5 $CWD/src/main.rs:6:16
+
+  Uninitialized value was created by an allocation of 'a' in the stack frame of function '_ZN6memory4main17hd2333c1899d997f5E'
+    #0 0x560c04b2bc50 in memory::main::hd2333c1899d997f5 $CWD/src/main.rs:3
+```
+
+
+# Instrumentation of external dependencies and std
+
+The sanitizers to varying degrees work correctly with partially instrumented
+code. On the one extreme is LeakSanitizer that doesn't use any compile time
+instrumentation, on the other is MemorySanitizer that requires that all program
+code to be instrumented (failing to achieve that will inevitably result in
+false positives).
+
+It is strongly recommended to combine sanitizers with recompiled and
+instrumented standard library, for example using [cargo `-Zbuild-std`
+functionality][build-std].
+
+[build-std]: https://doc.rust-lang.org/nightly/cargo/reference/unstable.html#build-std
+
+# Build scripts and procedural macros
+
+Use of sanitizers together with build scripts and procedural macros is
+technically possible, but in almost all cases it would be best avoided.  This
+is especially true for procedural macros which would require an instrumented
+version of rustc.
+
+In more practical terms when using cargo always remember to pass `--target`
+flag, so that rustflags will not be applied to build scripts and procedural
+macros.
+
+# Additional Information
+
+* [Sanitizers project page](https://github.com/google/sanitizers/wiki/)
+* [AddressSanitizer in Clang][clang-asan]
+* [LeakSanitizer in Clang][clang-lsan]
+* [MemorySanitizer in Clang][clang-msan]
+* [ThreadSanitizer in Clang][clang-tsan]
+
+[clang-asan]: https://clang.llvm.org/docs/AddressSanitizer.html
+[clang-lsan]: https://clang.llvm.org/docs/LeakSanitizer.html
+[clang-msan]: https://clang.llvm.org/docs/MemorySanitizer.html
+[clang-tsan]: https://clang.llvm.org/docs/ThreadSanitizer.html