Updating guide for Rand 0.8

src/update-0.8.md

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+# Updating to 0.8
+
+In the following, instructions are provided for porting your code from
+`rand 0.7` and `rand_distr 0.2` to `rand 0.8` and `rand_distr 0.3`.
+
+## Dependencies
+
+Rand crates now require `rustc` version 1.36.0 or later.
+This allowed us to remove some unsafe code and simplify the internal `cfg` logic.
+
+The dependency on `getrandom` was bumped to version 0.2. While this does not
+affect Rand's API, you may be affected by some of the breaking changes even if
+you use `getrandom` only as a dependency:
+
+-    You may have to update the `getrandom` features you are using. The
+     following features are now available:
+     -   `"rdrand"`: Use the RDRAND instruction on `no_std` `x86/x86_64`
+         targets.
+     -   `"js"`: Use JavaScript calls on `wasm32-unknown-unknown`. This
+         replaces the `stdweb` and `wasm-bindgen` features, which are
+         removed.
+     -   `"custom"`: Allows you to specify a custom implementation.
+-   Unsupported targets no longer compile. If you require the previous behavior
+    (panicking at runtime instead of failing to compile), you can use the
+    `custom` feature to provide a panicking implementation.
+-   Hermit, L4Re and UEFI are no longer officially supported. You can use the
+    `rdrand` feature on these platforms.
+-   The minimum supported Linux kernel version is now 2.6.32.
+
+If you are using `getrandom`'s API directly, there are further breaking changes
+that may affect you. See its
+[changelog](https://github.com/rust-random/getrandom/blob/master/CHANGELOG.md#020---2020-09-10).
+
+
+## Core features
+
+`ThreadRng` no longer implements `Copy`. This was necessary to fix a possible
+use-after-free in its thread-local destructor. Any code relying on `ThreadRng`
+being copied must be updated to use a mutable reference instead. For example,
+```
+let rng = thread_rng();
+let a: u32 = Standard.sample(rng);
+let b: u32 = Standard.sample(rng);
+```
+can be replaced with the following code:
+```
+let mut rng = thread_rng();
+let a: u32 = Standard.sample(&mut rng);
+let b: u32 = Standard.sample(&mut rng);
+```
+
+[`Rng::gen_range`] now takes a `Range` instead of two numbers. This requires
+replacing `gen_range(a, b)` with `gen_range(a..b)` in code written for `rand
+0.7`. We suggest to replace the regular expression
+`gen_range\(([^,]*),\s*([^)]*)\)` with `gen_range(\1..\2)` (or
+`gen_range($1..$2)` in some tools without support for backreferences).
+Additionally, if `a` or `b` were a reference, explicit dereferencing may now be
+required. Inclusive ranges are now supported: `gen_range(a, b + 1)` can be
+replaced with `gen_range(a..=b)`.
+
+The `AsByteSliceMut` trait was replaced with the [`Fill`] trait. This should
+only affect code implementing `AsByteSliceMut` on user-defined types, so they
+are supported by [`Rng::fill`] and [`Rng::try_fill`]. Now, the [`Fill`] trait
+has to be implemented instead: Rather than providing a mutable byte slice, the
+user-defined type must be filled with random data.
+
+The entire [`rand::rngs::adapter`] module is now restricted to the `std` feature.
+While this is technically a breaking change, it should only affect `no_std` code
+using [`ReseedingRng`], which is unlikely to exist in the wild.
+
+## PRNGs
+
+These have seen only small changes, but noteworthy is:
+
+-   [`StdRng`] and [`ThreadRng`] now use the ChaCha12 instead of the ChaCha20
+    algorithm. This improves performance and is a value-breaking change for
+    [`StdRng`].
+-   [`SmallRng`] now uses the Xoshiro128++ and Xoshiro256++ algorithm on 32-bit
+    and 64-bit platforms respectively. This reduces correlations of random data
+    generated from similar seeds, improves performance and is a value-breaking
+    change.
+-   [`StdRng`], [`SmallRng`], and [`StepRng`] now implement `PartialEq` and `Eq`.
+
+## Distributions
+
+The most widely used distributions ([`Standard`] and [`Uniform`]), were not
+significantly changed. Only the following distributions suffered breaking
+changes:
+
+-   The [`Alphanumeric`] distribution now samples bytes instead of chars. This
+    more closely reflects the internally used type, but old code likely has to
+    be adapted to perform the conversion from `u8` to `char`. For example, with
+    Rand 0.7 you could write:
+    ```
+    let chars: String = iter::repeat(())
+        .map(|()| rng.sample(Alphanumeric))
+        .take(7)
+        .collect();
+    ```
+    With Rand 0.8, this is equivalent to the following:
+    ```
+    let chars: String = iter::repeat(())
+        .map(|()| rng.sample(Alphanumeric))
+        .map(char::from)
+        .take(7)
+        .collect();
+    ```
+-   The alternative implementation of [`WeightedIndex`] employing the alias
+    method was moved from `rand` to [`rand_distr::WeightedAliasIndex`]. The
+    alias method is faster for large sizes, but it suffers from a slow
+    initialization, making it less generally useful.
+-   [`rand_distr::Dirchlet`] now uses boxed slices internally instead of `Vec`.
+    Therefore, the weights are taken as a slice instead of a `Vec` as input.
+    For example, the following `rand_distr 0.2` code
+    ```
+    Dirichlet::new(vec![1.0, 2.0, 3.0]).unwrap();
+    ```
+    can be replaced with the following `rand_distr 0.3` code:
+    ```
+    Dirichlet::new(&[1.0, 2.0, 3.0]).unwrap();
+    ```
+-   [`rand_distr::Poisson`] does no longer support sampling `u64` values directly.
+    Old code may have to be updated to perform the conversion from `f64`
+    explicitly.
+-   The custom `Float` trait in `rand_distr` was replaced with
+    `num_traits::Float`. Any implementations of `Float` for user-defined types
+    have to be migrated. Thanks to the math functions from `num_traits::Float`,
+    `rand_distr` now supports `no_std`.
+
+Additonally, there were some minor improvements:
+
+-   The treatment of rounding errors and NaN was improved for the
+    [`WeightedIndex`] distribution.
+-   The [`UniformInt`] and [`WeightedIndex`] distributions now support serialization
+    via the `serde1` feature.
+-   The [`rand_distr::Exp`] distribution now supports the `lambda = 0` parametrization.
+
+We also added several distributions:
+
+-   [`rand_distr::WeightedAliasIndex`] (moved from the `rand` crate)
+-   [`rand_distr::InverseGaussian`]
+-   [`rand_distr::NormalInverseGaussian`]
+
+The `nightly` feature no longer implies the `simd_support` feature. If you were
+relying on this for SIMD support, you will have to use `simd_support` feature
+directly.
+
+## Sequences
+
+Weighted sampling without replacement is now supported, see
+[`rand::seq::index::sample_weighted`] and
+[`SliceRandom::choose_multiple_weighted`].
+
+There have been value-breaking changes to [`IteratorRandom::choose`] improving
+accuracy and performance. Furthermore, [`IteratorRandom::choose_stable`] was
+added to provide an alternative that sacrifices performance for independence of
+iterator size hints.
+
+
+[`Fill`]: ../rand/rand/trait.Fill.html
+[`Rng::gen_range`]: ../rand/rand/trait.Rng.html#method.gen_range
+[`Rng::fill`]: ../rand/rand/trait.Rng.html#method.fill
+[`Rng::try_fill`]: ../rand/rand/trait.Rng.html#method.try_fill
+[`SmallRng`]: ../rand/rand/rngs/struct.SmallRng.html
+[`StdRng`]: ../rand/rand/rngs/struct.StdRng.html
+[`StepRng`]: ../rand/rand/rngs/struct.StepRng.html
+[`ThreadRng`]: ../rand/rand/rngs/struct.ThreadRng.html
+[`ReseedingRng`]: ../rand/rand/rngs/adapter/struct.ReseedingRng.html
+[`Standard`]: ../rand/rand/distributions/struct.Standard.html
+[`Uniform`]: ../rand/rand/distributions/struct.Uniform.html
+[`UniformInt`]: ../rand/rand/distributions/struct.UniformInt.html
+[`Alphanumeric`]: ../rand/rand/distributions/struct.Alphanumeric.html
+[`WeightedIndex`]: ../rand/rand/distributions/struct.WeightedIndex.html
+[`rand::rngs::adapter`]: ../rand/rand/rngs/adapter/index.html
+[`rand::seq::index::sample_weighted`]: ../rand/rand/seq/index/fn.sample_weighted.html
+[`SliceRandom::choose_multiple_weighted`]: ../rand/rand/seq/trait.SliceRandom.html#method.choose_multiple_weighted
+[`IteratorRandom::choose`]: ../rand/rand/seq/trait.IteratorRandom.html#method.choose
+[`IteratorRandom::choose_stable`]: ../rand/rand/seq/trait.IteratorRandom.html#method.choose_stable
+[`rand_distr::WeightedAliasIndex`]: ../rand/rand_distr/struct.WeightedAliasIndex.html
+[`rand_distr::InverseGaussian`]: ../rand/rand_distr/struct.InverseGaussian.html
+[`rand_distr::NormalInverseGaussian`]: ../rand/rand_distr/struct.NormalInverseGaussian.html
+[`rand_distr::Dirichlet`]: ../rand/rand_distr/struct.Dirichlet.html
+[`rand_distr::Poisson`]: ../rand/rand_distr/struct.Poisson.html
+[`rand_distr::Exp`]: ../rand/rand_distr/struct.Exp.html