Define MSRV to be 1.31.0 and unconditionally use const-fn

.travis.yml

@@ -22,6 +22,26 @@ matrix:
       rust: stable
       if: (branch = staging OR branch = trying) OR (type = pull_request AND branch = master)
 
+    # MSRV
+    - env: TARGET=thumbv6m-none-eabi
+      rust: 1.31.0
+      if: (branch = staging OR branch = trying) OR (type = pull_request AND branch = master)
+
+    # MSRV
+    - env: TARGET=thumbv7m-none-eabi
+      rust: 1.31.0
+      if: (branch = staging OR branch = trying) OR (type = pull_request AND branch = master)
+
+    # MSRV
+    - env: TARGET=thumbv7em-none-eabi
+      rust: 1.31.0
+      if: (branch = staging OR branch = trying) OR (type = pull_request AND branch = master)
+
+    # MSRV
+    - env: TARGET=thumbv7em-none-eabihf
+      rust: 1.31.0
+      if: (branch = staging OR branch = trying) OR (type = pull_request AND branch = master)
+
     - env: TARGET=thumbv6m-none-eabi
       rust: nightly
       if: (branch = staging OR branch = trying) OR (type = pull_request AND branch = master)

Cargo.toml

@@ -16,10 +16,10 @@ links = "cortex-m"  # prevent multiple versions of this crate to be linked toget
 
 [dependencies]
 aligned = "0.3.1"
-bare-metal = "0.2.0"
+bare-metal = { version = "0.2.0", features = ["const-fn"] }
 volatile-register = "0.2.0"
 
 [features]
+const-fn = []
 cm7-r0p1 = []
-const-fn = ["bare-metal/const-fn"]
 inline-asm = []

ci/script.sh

@@ -8,15 +8,15 @@ main() {
     cargo check --target $TARGET
 
     if [ $TRAVIS_RUST_VERSION = nightly ]; then
-        cargo check --target $TARGET --features 'const-fn inline-asm'
+        cargo check --target $TARGET --features 'inline-asm'
     fi
 
     case $TARGET in
         thumbv7em-none-eabi*)
             cargo check --target $TARGET --features cm7-r0p1
 
             if [ $TRAVIS_RUST_VERSION = nightly ]; then
-                cargo check --target $TARGET --features 'cm7-r0p1 const-fn inline-asm'
+                cargo check --target $TARGET --features 'cm7-r0p1 inline-asm'
             fi
             ;;
 

src/lib.rs

@@ -24,11 +24,10 @@
 //!
 //! The disadvantage is that `inline-asm` requires a nightly toolchain.
 //!
-//! ## `const-fn`
+//! # Minimum Supported Rust Version (MSRV)
 //!
-//! Enabling this feature turns the `Mutex.new` constructor into a `const fn`.
-//!
-//! This feature requires a nightly toolchain.
+//! This crate is guaranteed to compile on stable Rust 1.31 and up. It *might*
+//! compile with older versions but that may change in any new patch release.
 
 #![cfg_attr(feature = "inline-asm", feature(asm))]
 #![deny(missing_docs)]