Check minimal version of Rust that compiles

Specify a minimum version of the Rust compiler that successfully
builds the crate. This allows us to be aware if the minimal version
increases and to avoid issues such as
rust-lang#370

The minimum version is a best-effort measured value and is different to
the MSRV which is a support guarantee. The minimum version can be
incremented by a PR in order to pass tests, as long as the MSRV holds.
When the minimum version increases, the next release should be a minor
version, to allow any affected users to pin to a previous minor version.

Author: jongiddy

.github/workflows/main.yml

@@ -7,8 +7,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        # I don't really understand the build matrix here...
-        build: [stable, beta, nightly, macos, windows, mingw]
+        build: [stable, beta, nightly, minimal, macos, windows, mingw]
         include:
           - build: stable
             os: ubuntu-latest
@@ -19,6 +18,9 @@ jobs:
           - build: nightly
             os: ubuntu-latest
             rust: nightly
+          - build: minimal
+            os: ubuntu-latest
+            rust: 1.56.1
           - build: macos
             os: macos-latest
             rust: stable
@@ -48,11 +50,11 @@ jobs:
     - run: cargo test --features cloudflare_zlib --no-default-features
       if: matrix.build != 'mingw'
     - run: |
-        if ! cargo check --no-default-features 2>&1 | grep "You need to choose"; then 
+        if ! cargo check --no-default-features 2>&1 | grep "You need to choose"; then
           echo "expected message stating a zlib backend must be chosen"
           exit 1
         fi
-      if: matrix.build == 'stable' 
+      if: matrix.build == 'stable'
 
   rustfmt:
     name: Rustfmt & Docs

Cargo.toml

@@ -5,6 +5,7 @@ version = "1.0.31"
 edition = "2018"
 license = "MIT OR Apache-2.0"
 readme = "README.md"
+rust-version = "1.56.1"
 keywords = ["gzip", "deflate", "zlib", "zlib-ng", "encoding"]
 categories = ["compression", "api-bindings"]
 repository = "https://github.com/rust-lang/flate2-rs"

README.md

@@ -23,10 +23,19 @@ flate2 = "1.0"
 
 ## MSRV (Minimum Supported Rust Version) Policy
 
-This crate supports the current stable and the last stable for the latest version.
+This crate supports the current stable and the last stable compiler for the latest version.
 For example, if the current stable is 1.64, this crate supports 1.64 and 1.63.
 Older stables may work, but we don't guarantee these will continue to work.
 
+The `Cargo.toml` file specifies a minimum version for which builds passed at some point.
+This value is indicative only, and may change at any time.
+
+The minimum version is a best-effort measured value and is different to the MSRV
+which is a support guarantee. The minimum version can be incremented by a PR in order
+to pass tests, as long as the MSRV holds. When the minimum version increases, the next
+release should be a minor version, to allow any affected users to pin to a previous
+minor version.
+
 ## Compression
 
 ```rust