Auto merge of #2147 - RalfJung:readme, r=RalfJung

split flag section into common and advanced flags

As discussed with `@oli-obk` . However I was not always sure which flags to put where, so if you think some flags should be in the other category please let me know. :)

Author: bors

README.md

@@ -258,33 +258,12 @@ up the sysroot.  If you are using `miri` (the Miri driver) directly, see the
 [miri-flags]: #miri--z-flags-and-environment-variables
 
 Miri adds its own set of `-Z` flags, which are usually set via the `MIRIFLAGS`
-environment variable. Some of these are **unsound**, which means they can lead
-to Miri failing to detect cases of undefined behavior in a program.
+environment variable. We first document the most relevant and most commonly used flags:
 
-* `-Zmiri-check-number-validity` enables checking of integer and float validity
-  (e.g., they must be initialized and not carry pointer provenance) as part of
-  enforcing validity invariants. This has no effect when
-  `-Zmiri-disable-validation` is present.
 * `-Zmiri-compare-exchange-weak-failure-rate=<rate>` changes the failure rate of
   `compare_exchange_weak` operations. The default is `0.8` (so 4 out of 5 weak ops will fail).
   You can change it to any value between `0.0` and `1.0`, where `1.0` means it
   will always fail and `0.0` means it will never fail.
-* `-Zmiri-disable-abi-check` disables checking [function ABI]. Using this flag
-  is **unsound**.
-* `-Zmiri-disable-alignment-check` disables checking pointer alignment, so you
-  can focus on other failures, but it means Miri can miss bugs in your program.
-  Using this flag is **unsound**.
-* `-Zmiri-disable-data-race-detector` disables checking for data races.  Using
-  this flag is **unsound**.
-* `-Zmiri-disable-stacked-borrows` disables checking the experimental
-  [Stacked Borrows] aliasing rules.  This can make Miri run faster, but it also
-  means no aliasing violations will be detected.  Using this flag is **unsound**
-  (but the affected soundness rules are experimental).
-* `-Zmiri-disable-validation` disables enforcing validity invariants, which are
-  enforced by default.  This is mostly useful to focus on other failures (such
-  as out-of-bounds accesses) first.  Setting this flag means Miri can miss bugs
-  in your program.  However, this can also help to make Miri run faster.  Using
-  this flag is **unsound**.
 * `-Zmiri-disable-isolation` disables host isolation.  As a consequence,
   the program has access to host resources such as environment variables, file
   systems, and randomness.
@@ -299,12 +278,50 @@ to Miri failing to detect cases of undefined behavior in a program.
   cannot be accessed by the program. Can be used multiple times to exclude several variables. The
   `TERM` environment variable is excluded by default to [speed up the test
   harness](https://github.com/rust-lang/miri/issues/1702). This has no effect unless
-  `-Zmiri-disable-validation` is also set.
+  `-Zmiri-disable-isolation` is also set.
 * `-Zmiri-env-forward=<var>` forwards the `var` environment variable to the interpreted program. Can
   be used multiple times to forward several variables. This has no effect if
-  `-Zmiri-disable-validation` is set.
+  `-Zmiri-disable-isolation` is set.
 * `-Zmiri-ignore-leaks` disables the memory leak checker, and also allows some
   remaining threads to exist when the main thread exits.
+* `-Zmiri-seed=<hex>` configures the seed of the RNG that Miri uses to resolve
+  non-determinism. This RNG is used to pick base addresses for allocations. When
+  isolation is enabled (the default), this is also used to emulate system
+  entropy. The default seed is 0. You can increase test coverage by running Miri
+  multiple times with different seeds.
+  **NOTE**: This entropy is not good enough for cryptographic use! Do not
+  generate secret keys in Miri or perform other kinds of cryptographic
+  operations that rely on proper random numbers.
+* `-Zmiri-strict-provenance` enables [strict
+  provenance](https://github.com/rust-lang/rust/issues/95228) checking in Miri. This means that
+  casting an integer to a pointer yields a result with 'invalid' provenance, i.e., with provenance
+  that cannot be used for any memory access. Also implies `-Zmiri-tag-raw-pointers` and
+  `-Zmiri-check-number-validity`.
+
+The remaining flags are for advanced use only, and more likely to change or be removed.
+Some of these are **unsound**, which means they can lead
+to Miri failing to detect cases of undefined behavior in a program.
+
+* `-Zmiri-check-number-validity` enables checking of integer and float validity
+  (e.g., they must be initialized and not carry pointer provenance) as part of
+  enforcing validity invariants. This has no effect when
+  `-Zmiri-disable-validation` is present.
+* `-Zmiri-disable-abi-check` disables checking [function ABI]. Using this flag
+  is **unsound**.
+* `-Zmiri-disable-alignment-check` disables checking pointer alignment, so you
+  can focus on other failures, but it means Miri can miss bugs in your program.
+  Using this flag is **unsound**.
+* `-Zmiri-disable-data-race-detector` disables checking for data races.  Using
+  this flag is **unsound**.
+* `-Zmiri-disable-stacked-borrows` disables checking the experimental
+  [Stacked Borrows] aliasing rules.  This can make Miri run faster, but it also
+  means no aliasing violations will be detected.  Using this flag is **unsound**
+  (but the affected soundness rules are experimental).
+* `-Zmiri-disable-validation` disables enforcing validity invariants, which are
+  enforced by default.  This is mostly useful to focus on other failures (such
+  as out-of-bounds accesses) first.  Setting this flag means Miri can miss bugs
+  in your program.  However, this can also help to make Miri run faster.  Using
+  this flag is **unsound**.
 * `-Zmiri-measureme=<name>` enables `measureme` profiling for the interpreted program.
    This can be used to find which parts of your program are executing slowly under Miri.
    The profile is written out to a file with the prefix `<name>`, and can be processed
@@ -329,17 +346,6 @@ to Miri failing to detect cases of undefined behavior in a program.
   memory/pointers which is subject to these operations. Also note that this flag
   is currently incompatible with Stacked Borrows, so you will have to also pass
   `-Zmiri-disable-stacked-borrows` to use this.
-* `-Zmiri-seed=<hex>` configures the seed of the RNG that Miri uses to resolve
-  non-determinism.  This RNG is used to pick base addresses for allocations.
-  When isolation is enabled (the default), this is also used to emulate system
-  entropy.  The default seed is 0.  **NOTE**: This entropy is not good enough
-  for cryptographic use!  Do not generate secret keys in Miri or perform other
-  kinds of cryptographic operations that rely on proper random numbers.
-* `-Zmiri-strict-provenance` enables [strict
-  provenance](https://github.com/rust-lang/rust/issues/95228) checking in Miri. This means that
-  casting an integer to a pointer yields a result with 'invalid' provenance, i.e., with provenance
-  that cannot be used for any memory access. Also implies `-Zmiri-tag-raw-pointers` and
-  `-Zmiri-check-number-validity`.
 * `-Zmiri-symbolic-alignment-check` makes the alignment check more strict.  By
   default, alignment is checked by casting the pointer to an integer, and making
   sure that is a multiple of the alignment.  This can lead to cases where a