fix check-msrv ci job by resolving to latest compatible dependencies (#4972)

* fix `check-msrv` ci job by resolving to latest compatible version

using `resolver.incompatible-rust-versions = "fallback"`

* Do the same for examples

* Set `rust-version` in example projects

* set msrv in examples, simplify noxfile

* don't need toml

* repeat for `pyo3-ffi` examples

---------

Co-authored-by: David Hewitt <mail@davidhewitt.dev>

Author: bschoenmaeckers

examples/decorator/Cargo.toml

@@ -2,6 +2,7 @@
 name = "decorator"
 version = "0.1.0"
 edition = "2021"
+rust-version = "1.63"
 
 [lib]
 name = "decorator"

examples/getitem/Cargo.toml

@@ -2,6 +2,7 @@
 name = "getitem"
 version = "0.1.0"
 edition = "2021"
+rust-version = "1.63"
 
 [lib]
 name = "getitem"

examples/maturin-starter/Cargo.toml

@@ -2,6 +2,7 @@
 name = "maturin-starter"
 version = "0.1.0"
 edition = "2021"
+rust-version = "1.63"
 
 [lib]
 name = "maturin_starter"

examples/plugin/Cargo.toml

@@ -2,7 +2,7 @@
 name = "plugin_example"
 version = "0.1.0"
 edition = "2021"
-
+rust-version = "1.63"
 
 [dependencies]
 pyo3={path="../../", features=["macros"]}

examples/setuptools-rust-starter/Cargo.toml

@@ -2,6 +2,7 @@
 name = "setuptools-rust-starter"
 version = "0.1.0"
 edition = "2021"
+rust-version = "1.63"
 
 [lib]
 name = "setuptools_rust_starter"

examples/word-count/Cargo.toml

@@ -2,6 +2,7 @@
 name = "word-count"
 version = "0.1.0"
 edition = "2021"
+rust-version = "1.63"
 
 [lib]
 name = "word_count"

noxfile.py

@@ -624,21 +624,13 @@ def check_changelog(session: nox.Session):
 def set_msrv_package_versions(session: nox.Session):
     from collections import defaultdict
 
-    if toml is None:
-        session.error("requires Python 3.11 or `toml` to be installed")
-
     projects = (
-        None,
-        "examples/decorator",
-        "examples/maturin-starter",
-        "examples/setuptools-rust-starter",
-        "examples/word-count",
+        PYO3_DIR,
+        *(Path(p).parent for p in glob("examples/*/Cargo.toml")),
+        *(Path(p).parent for p in glob("pyo3-ffi/examples/*/Cargo.toml")),
     )
     min_pkg_versions = {
-        "regex": "1.9.6",
-        "proptest": "1.2.0",
         "trybuild": "1.0.89",
-        "eyre": "0.6.8",
         "allocator-api2": "0.2.10",
         "indexmap": "2.5.0",  # to be compatible with hashbrown 0.14
         "hashbrown": "0.14.5",  # https://github.com/rust-lang/hashbrown/issues/574
@@ -647,13 +639,15 @@ def set_msrv_package_versions(session: nox.Session):
     # run cargo update first to ensure that everything is at highest
     # possible version, so that this matches what CI will resolve to.
     for project in projects:
-        if project is None:
-            _run_cargo(session, "update")
-        else:
-            _run_cargo(session, "update", f"--manifest-path={project}/Cargo.toml")
+        _run_cargo(
+            session,
+            "+stable",
+            "update",
+            f"--manifest-path={project}/Cargo.toml",
+            env=os.environ | {"CARGO_RESOLVER_INCOMPATIBLE_RUST_VERSIONS": "fallback"},
+        )
 
-    for project in projects:
-        lock_file = Path(project or "") / "Cargo.lock"
+        lock_file = project / "Cargo.lock"
 
         def load_pkg_versions():
             cargo_lock = toml.loads(lock_file.read_text())
@@ -679,19 +673,15 @@ def load_pkg_versions():
                     # and re-read `Cargo.lock`
                     pkg_versions = load_pkg_versions()
 
-    # As a smoke test, cargo metadata solves all dependencies, so
-    # will break if any crates rely on cargo features not
-    # supported on MSRV
-    for project in projects:
-        if project is None:
-            _run_cargo(session, "metadata", silent=True)
-        else:
-            _run_cargo(
-                session,
-                "metadata",
-                f"--manifest-path={project}/Cargo.toml",
-                silent=True,
-            )
+        # As a smoke test, cargo metadata solves all dependencies, so
+        # will break if any crates rely on cargo features not
+        # supported on MSRV
+        _run_cargo(
+            session,
+            "metadata",
+            f"--manifest-path={project}/Cargo.toml",
+            silent=True,
+        )
 
 
 @nox.session(name="ffi-check")

pyo3-ffi/examples/sequential/Cargo.toml

@@ -2,6 +2,7 @@
 name = "sequential"
 version = "0.1.0"
 edition = "2021"
+rust-version = "1.63"
 
 [lib]
 name = "sequential"

pyo3-ffi/examples/string-sum/Cargo.toml

@@ -2,6 +2,7 @@
 name = "string_sum"
 version = "0.1.0"
 edition = "2021"
+rust-version = "1.63"
 
 [lib]
 name = "string_sum"