ci: verify minimum supported Rust version

In the current state of our Rust code base we don't really have any
requirements for the minimum supported Rust version yet, as we don't use
any features introduced by a recent version of Rust. Consequently, we
have decided that we want to aim for a rather old version and edition of
Rust, where the hope is that using an old version will make alternatives
like gccrs viable earlier for compiling Git.

But while we specify the Rust edition, we don't yet specify a Rust
version. And even if we did, the Rust version would only be enforced for
our own code, but not for any of our dependencies.

We don't yet have any dependencies at the current point in time. But
let's add some safeguards by specifying the minimum supported Rust
version and using cargo-msrv(1) to verify that this version can be
satisfied for all of our dependencies.

Note that we fix the version of cargo-msrv(1) at v0.18.1. This is the
latest release supported by Ubuntu's Rust version.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Cargo.toml

@@ -2,6 +2,7 @@
 name = "gitcore"
 version = "0.1.0"
 edition = "2018"
+rust-version = "1.49.0"
 
 [lib]
 crate-type = ["staticlib"]

ci/install-dependencies.sh

@@ -10,6 +10,8 @@ begin_group "Install dependencies"
 P4WHENCE=https://cdist2.perforce.com/perforce/r23.2
 LFSWHENCE=https://github.com/github/git-lfs/releases/download/v$LINUX_GIT_LFS_VERSION
 JGITWHENCE=https://repo1.maven.org/maven2/org/eclipse/jgit/org.eclipse.jgit.pgm/6.8.0.202311291450-r/org.eclipse.jgit.pgm-6.8.0.202311291450-r.sh
+CARGO_MSRV_VERSION=0.18.4
+CARGO_MSRV_WHENCE=https://github.com/foresterre/cargo-msrv/releases/download/v$CARGO_MSRV_VERSION/cargo-msrv-x86_64-unknown-linux-musl-v$CARGO_MSRV_VERSION.tgz
 
 # Make sudo a no-op and execute the command directly when running as root.
 # While using sudo would be fine on most platforms when we are root already,
@@ -130,6 +132,12 @@ RustAnalysis)
 	sudo apt-get -q -y install rustup
 	rustup default stable
 	rustup component add clippy rustfmt
+
+	wget -q "$CARGO_MSRV_WHENCE" -O "cargo-msvc.tgz"
+	sudo mkdir -p "$CUSTOM_PATH"
+	sudo tar -xf "cargo-msvc.tgz" --strip-components=1 \
+		--directory "$CUSTOM_PATH" --wildcards "*/cargo-msrv"
+	sudo chmod a+x "$CUSTOM_PATH/cargo-msrv"
 	;;
 sparse)
 	sudo apt-get -q -y install libssl-dev libcurl4-openssl-dev \

ci/run-rust-checks.sh

@@ -14,4 +14,9 @@ then
 	RET=1
 fi
 
+if ! group "Check for minimum required Rust version" cargo msrv verify
+then
+	RET=1
+fi
+
 exit $RET