Travis CI: For Rust 1.32.0, only do a compilation check

Since our `dev-dependencies` now require a newer Rust version, no
longer do a full test for Rust 1.32.0. It seems the `time` crate is
especially aggressive in its MSRV bumping; the `dev-dependencies`
should be investigated to see if they can be trimmed to lower the MSRV
for running the test suite.

.gitignore

@@ -7,3 +7,4 @@
 *.a
 /target/
 Cargo.lock
+/msrv-test/target/

.travis.yml

@@ -1,6 +1,5 @@
 language: rust
 rust:
-  - 1.32.0 # uniform paths
   - stable
   - beta
   - nightly
@@ -42,6 +41,10 @@ matrix:
         - cargo clean
         - cargo test --all --all-features
         - cargo test --all --release --all-features
+    - name: "MSRV (1.32.0) compile check"
+      rust: 1.32.0 # uniform paths
+      script:
+        - cd msrv-test && cargo build
     - stage: lint
       name: "Rust: rustfmt"
       rust: stable

README.md

@@ -34,6 +34,12 @@ draft API features, have a look at
 The current 0.9 release series requires `libzmq` 4.1 or newer. New
 release series of `zmq` may require newer `libzmq` versions.
 
+Regarding the minimum Rust version required, `zmq` is CI-tested on
+current stable, beta and nightly channels of Rust. Additionally, it is
+made sure that the code still compiles on Rust 1.32.0. However, no
+tests are run for that build, so use `zmq` on older Rust versions on
+your own risk. It is however likely that it will just work anyways.
+
 # Installation
 
 rust-zmq is available from [crates.io](https://crates.io). Users
@@ -123,7 +129,7 @@ need set `LIBZMQ_PREFIX` as described above.
 fn main() {
     let ctx = zmq::Context::new();
 
-    let mut socket = ctx.socket(zmq::REQ).unwrap();
+    let socket = ctx.socket(zmq::REQ).unwrap();
     socket.connect("tcp://127.0.0.1:1234").unwrap();
     socket.send("hello world!", 0).unwrap();
 }

msrv-test/Cargo.toml

@@ -0,0 +1,8 @@
+[package]
+name = "msrv-test"
+version = "0.0.1"
+authors = ["Andreas Rottmann <a.rottmann@gmx.at>"]
+edition = "2018"
+
+[dependencies]
+zmq = { path = ".." }

msrv-test/src/main.rs

@@ -0,0 +1,7 @@
+fn main() {
+    let ctx = zmq::Context::new();
+
+    let socket = ctx.socket(zmq::REQ).unwrap();
+    socket.connect("tcp://127.0.0.1:1234").unwrap();
+    socket.send("hello world!", 0).unwrap();
+}