Use --locked instead of checking Cargo.lock afterwards

The verification being done here is exactly what `--locked` is for.

Allowing the check to proceed even if it changes the dependencies
could have the benefit of showing more information. But it is not
obvious that the information it shows would be relevant to the
situation being investigated.

(If that information would usually be relevant, then
`cargo +nightly update -Zminimal-versions` may be too strong, and
`cargo +nightly update -Zdirect-minimal-versions` could be
considered. Even better than either of those *might* be, somehow,
to temporarily downgrade locked dependencies only as much as
necessary to make them all MSRV-compatible.)

In any case, `--locked` is more readable, and considerably simpler.

In addition to making complementary changes to the `msrv.yml`
workflow and to the `justfile` recipe it uses, this also changes
the `Makefile` rule corresponding to the `justfile` recipe, so they
continue to do the same thing if used in local experiments.

Author: EliahKagan

.github/workflows/msrv.yml

@@ -48,8 +48,5 @@ jobs:
       - name: Downgrade locked dependencies to lowest allowed versions
         run: |
           cargo +nightly update -Zminimal-versions  # TODO(msrv): Use non-`-Z` way when available.
-          git add Cargo.lock  # Stage for a later `git diff` check.
       - name: Run some `cargo check` commands on `gix`
         run: just ci-check-msrv
-      - name: Check `cargo` didn't have to change the versions
-        run: git diff --exit-code -- Cargo.lock

Makefile

@@ -126,8 +126,8 @@ bench-gix-config:
 
 check-msrv-on-ci: ## Check the minimal support rust version for currently installed Rust version
 	rustc --version
-	cargo check --package gix
-	cargo check --package gix --no-default-features --features async-network-client,max-performance
+	cargo check --locked --package gix
+	cargo check --locked --package gix --no-default-features --features async-network-client,max-performance
 
 ##@ Maintenance
 

justfile

@@ -246,8 +246,8 @@ check-size:
 # Check the MSRV, *if* the toolchain is set and `Cargo.lock` is downgraded (used on CI)
 ci-check-msrv:
     rustc --version
-    cargo check -p gix
-    cargo check -p gix --no-default-features --features async-network-client,max-performance
+    cargo check --locked -p gix
+    cargo check --locked -p gix --no-default-features --features async-network-client,max-performance
 
 # Enter a nix-shell able to build on macOS
 nix-shell-macos: