optimization: switch fastbuild opt default to 1 from 0 (bazelbuild#1091)

* optimization: switch fastbuild opt default to 1 from 0

This will very slightly increase build times but applies basic
optimizations that will do things like make iterators _much_
faster. Since fastbuild is how people iterate most of the time, this
seems like a sensible default value.

* Regenerate documentation

Co-authored-by: Augie Fackler <augie@google.com>

docs/flatten.md

@@ -1167,7 +1167,7 @@ See @rules_rust//rust:repositories.bzl for examples of defining the @rust_cpuX r
 | <a id="rust_toolchain-default_edition"></a>default_edition |  The edition to use for rust_* rules that don't specify an edition.   | String | optional | "2018" |
 | <a id="rust_toolchain-dylib_ext"></a>dylib_ext |  The extension for dynamic libraries created from rustc.   | String | required |  |
 | <a id="rust_toolchain-exec_triple"></a>exec_triple |  The platform triple for the toolchains execution environment. For more details see: https://docs.bazel.build/versions/master/skylark/rules.html#configurations   | String | optional | "" |
-| <a id="rust_toolchain-opt_level"></a>opt_level |  Rustc optimization levels.   | <a href="https://bazel.build/docs/skylark/lib/dict.html">Dictionary: String -> String</a> | optional | {"dbg": "0", "fastbuild": "0", "opt": "3"} |
+| <a id="rust_toolchain-opt_level"></a>opt_level |  Rustc optimization levels.   | <a href="https://bazel.build/docs/skylark/lib/dict.html">Dictionary: String -> String</a> | optional | {"dbg": "0", "fastbuild": "1", "opt": "3"} |
 | <a id="rust_toolchain-os"></a>os |  The operating system for the current toolchain   | String | required |  |
 | <a id="rust_toolchain-rust_doc"></a>rust_doc |  The location of the <code>rustdoc</code> binary. Can be a direct source or a filegroup containing one item.   | <a href="https://bazel.build/docs/build-ref.html#labels">Label</a> | optional | None |
 | <a id="rust_toolchain-rust_lib"></a>rust_lib |  The rust standard library.   | <a href="https://bazel.build/docs/build-ref.html#labels">Label</a> | optional | None |

docs/rust_repositories.md

@@ -93,7 +93,7 @@ See @rules_rust//rust:repositories.bzl for examples of defining the @rust_cpuX r
 | <a id="rust_toolchain-default_edition"></a>default_edition |  The edition to use for rust_* rules that don't specify an edition.   | String | optional | "2018" |
 | <a id="rust_toolchain-dylib_ext"></a>dylib_ext |  The extension for dynamic libraries created from rustc.   | String | required |  |
 | <a id="rust_toolchain-exec_triple"></a>exec_triple |  The platform triple for the toolchains execution environment. For more details see: https://docs.bazel.build/versions/master/skylark/rules.html#configurations   | String | optional | "" |
-| <a id="rust_toolchain-opt_level"></a>opt_level |  Rustc optimization levels.   | <a href="https://bazel.build/docs/skylark/lib/dict.html">Dictionary: String -> String</a> | optional | {"dbg": "0", "fastbuild": "0", "opt": "3"} |
+| <a id="rust_toolchain-opt_level"></a>opt_level |  Rustc optimization levels.   | <a href="https://bazel.build/docs/skylark/lib/dict.html">Dictionary: String -> String</a> | optional | {"dbg": "0", "fastbuild": "1", "opt": "3"} |
 | <a id="rust_toolchain-os"></a>os |  The operating system for the current toolchain   | String | required |  |
 | <a id="rust_toolchain-rust_doc"></a>rust_doc |  The location of the <code>rustdoc</code> binary. Can be a direct source or a filegroup containing one item.   | <a href="https://bazel.build/docs/build-ref.html#labels">Label</a> | optional | None |
 | <a id="rust_toolchain-rust_lib"></a>rust_lib |  The rust standard library.   | <a href="https://bazel.build/docs/build-ref.html#labels">Label</a> | optional | None |

rust/toolchain.bzl

@@ -342,7 +342,7 @@ rust_toolchain = rule(
             doc = "Rustc optimization levels.",
             default = {
                 "dbg": "0",
-                "fastbuild": "0",
+                "fastbuild": "1",
                 "opt": "3",
             },
         ),