Raise minimum supported Rust version to 1.40.0

1.38.0 or higher is required for Fuchsia support, which will be added in
a subsequent commit.

1.38.0 introduced cfg(doctest), but it was not stabilized until 1.40.0.
cfg(doctest) is used by remove_dir_all, which is a transitive dependency
via tempfile. This means we must raise MSRV to 1.40.0 to avoid unstable
feature errors.

Author: tamird

.cirrus.yml

@@ -116,7 +116,7 @@ task:
         TARGET: x86_64-unknown-linux-musl
         TOOLCHAIN: 1.40.0
   container:
-    image: rust:1.36
+    image: rust:1.40
   setup_script:
     - rustup toolchain install $TOOLCHAIN
     - rustup target add --toolchain $TOOLCHAIN $TARGET
@@ -161,7 +161,7 @@ task:
       env:
         TARGET: x86_64-unknown-netbsd
   container:
-    image: rust:1.36
+    image: rust:1.40
   setup_script:
     - rustup target add $TARGET
   script:

CHANGELOG.md

@@ -7,6 +7,9 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 ### Added
 - Added `mremap` (#[1306](https://github.com/nix-rust/nix/pull/1306))
 
+### Changed
+- Minimum supported Rust version is now 1.40.0.
+  ([#1285](https://github.com/nix-rust/nix/pull/1285))
 ### Fixed
 ### Changed