doc: bump minimum Rust version to 1.20.0

This also clarifies our policy on increasing the minimum Rust version
required. In particular, we reserve the right to increase the minimum
Rust version in minor version releases of regexes, but never in patch
releases. We will default to a reasonably conservative interpretation
of this policy, and not bump the minimum required Rust version lightly.

If this policy turns out to be too aggressive, then we may alter it in
the future to state that the minimum Rust version is fixed for all of
regex 1.y.z, and can only be bumped on major regex version releases.

See rust-lang#457

Author: BurntSushi

Diff for: .travis.yml

@@ -2,7 +2,7 @@ dist: trusty
 sudo: false
 language: rust
 rust:
-- 1.12.0
+- 1.20.0
 - stable
 - beta
 - nightly

Diff for: README.md

@@ -11,6 +11,7 @@ by [RE2](https://github.com/google/re2).
 [![Build status](https://ci.appveyor.com/api/projects/status/github/rust-lang/regex?svg=true)](https://ci.appveyor.com/project/rust-lang-libs/regex)
 [![Coverage Status](https://coveralls.io/repos/github/rust-lang/regex/badge.svg?branch=master)](https://coveralls.io/github/rust-lang/regex?branch=master)
 [![](http://meritbadge.herokuapp.com/regex)](https://crates.io/crates/regex)
+[![Rust](https://img.shields.io/badge/rust-1.20%2B-blue.svg?maxAge=3600)](https://github.com/rust-lang/regex)
 
 ### Documentation
 
@@ -210,7 +211,22 @@ recommended for general use.
 
 [Documentation `regex-syntax`.](https://docs.rs/regex-syntax)
 
-# License
+
+### Minimum Rust version policy
+
+This crate's minimum supported `rustc` version is `1.20.0`.
+
+The current **tentative** policy is that the minimum Rust version required to
+use this crate can be increased in minor version updates. For example, if
+regex 1.0.0 requires Rust 1.20.0, then regex 1.0.z for all values of `z` will
+also require Rust 1.20.0 or newer. However, regex 1.y for `y > 0` may require
+a newer minimum version of Rust.
+
+In general, this crate will be conservative with respect to the minimum
+supported version of Rust.
+
+
+### License
 
 This project is licensed under either of
 

Diff for: ci/script.sh

@@ -8,15 +8,6 @@ set -ex
 cargo build --verbose
 cargo doc --verbose
 
-# If we're testing on an older version of Rust, then only check that we
-# can build the crate. This is because the dev dependencies might be updated
-# more frequently, and therefore might require a newer version of Rust.
-#
-# This isn't ideal. It's a compromise.
-if [ "$TRAVIS_RUST_VERSION" = "1.12.0" ]; then
-  exit
-fi
-
 # Run tests. If we have nightly, then enable our nightly features.
 if [ "$TRAVIS_RUST_VERSION" = "nightly" ]; then
   cargo test --verbose --features unstable

Diff for: src/vector/avx2.rs

@@ -54,21 +54,13 @@ impl AVX2VectorBuilder {
     }
 }
 
-// We define our union with a macro so that our code continues to compile on
-// Rust 1.12.
-macro_rules! defunion {
-    () => {
-        #[derive(Clone, Copy)]
-        #[allow(non_camel_case_types)]
-        pub union u8x32 {
-            vector: __m256i,
-            bytes: [u8; 32],
-        }
-    }
+#[derive(Clone, Copy)]
+#[allow(non_camel_case_types)]
+pub union u8x32 {
+    vector: __m256i,
+    bytes: [u8; 32],
 }
 
-defunion!();
-
 impl u8x32 {
     #[inline]
     unsafe fn splat(n: u8) -> u8x32 {

Diff for: src/vector/ssse3.rs

@@ -66,30 +66,22 @@ impl SSSE3VectorBuilder {
     }
 }
 
-// We define our union with a macro so that our code continues to compile on
-// Rust 1.12.
-macro_rules! defunion {
-    () => {
-        /// A u8x16 is a 128-bit vector with 16 single-byte lanes.
-        ///
-        /// It provides a safe API that uses only SSE2 or SSSE3 instructions.
-        /// The only way for callers to construct a value of this type is
-        /// through the SSSE3VectorBuilder type, and the only way to get a
-        /// SSSE3VectorBuilder is if the `ssse3` target feature is enabled.
-        ///
-        /// Note that generally speaking, all uses of this type should get
-        /// inlined, otherwise you probably have a performance bug.
-        #[derive(Clone, Copy)]
-        #[allow(non_camel_case_types)]
-        pub union u8x16 {
-            vector: __m128i,
-            bytes: [u8; 16],
-        }
-    }
+/// A u8x16 is a 128-bit vector with 16 single-byte lanes.
+///
+/// It provides a safe API that uses only SSE2 or SSSE3 instructions.
+/// The only way for callers to construct a value of this type is
+/// through the SSSE3VectorBuilder type, and the only way to get a
+/// SSSE3VectorBuilder is if the `ssse3` target feature is enabled.
+///
+/// Note that generally speaking, all uses of this type should get
+/// inlined, otherwise you probably have a performance bug.
+#[derive(Clone, Copy)]
+#[allow(non_camel_case_types)]
+pub union u8x16 {
+    vector: __m128i,
+    bytes: [u8; 16],
 }
 
-defunion!();
-
 impl u8x16 {
     #[inline]
     unsafe fn splat(n: u8) -> u8x16 {