Pin Rust version to 1.46

Specify the minimum Rust version in cargo.toml, exercise this version in CI,
and document it in the manual.

We require 1.46 for bitflags, which is a dependency of clap.

Fixes #51

.github/workflows/test.yml

@@ -11,7 +11,7 @@ jobs:
       - uses: actions-rs/toolchain@v1
         with:
           profile: minimal
-          toolchain: stable
+          toolchain: 1.46.0
           override: true
       - uses: actions-rs/cargo@v1
         with:
@@ -25,7 +25,7 @@ jobs:
       - uses: actions-rs/toolchain@v1
         with:
           profile: minimal
-          toolchain: stable
+          toolchain: 1.46.0
           override: true
       - uses: actions-rs/cargo@v1
         with:
@@ -39,11 +39,12 @@ jobs:
       - uses: actions-rs/toolchain@v1
         with:
           profile: minimal
-          toolchain: stable
+          toolchain: 1.46.0
           override: true
       - uses: actions-rs/cargo@v1
         with:
           command: package
+          args: --allow-dirty
 
   fmt:
     name: Rustfmt
@@ -53,7 +54,7 @@ jobs:
       - uses: actions-rs/toolchain@v1
         with:
           profile: minimal
-          toolchain: stable
+          toolchain: 1.46.0
           override: true
       - run: rustup component add rustfmt
       - uses: actions-rs/cargo@v1

Cargo.toml

@@ -9,6 +9,7 @@ authors = ["Wilfred Hughes <me@wilfred.me.uk>"]
 keywords = ["diff", "syntax"]
 categories = ["development-tools", "command-line-utilities", "parser-implementations"]
 edition = "2018"
+rust-version = "1.46.0"
 include = [
         "/build.rs",
         "/src/",

manual/src/getting_started.md

@@ -4,7 +4,7 @@
 
 The latest version of difftastic is [available on
 crates.io](https://crates.io/crates/difftastic). You can install it
-with Cargo.
+with Cargo. Difftastic requires Rust 1.46 or later.
 
 ```
 $ cargo install difftastic