clippy: warn on disallowed_methods for std::env::{var,var_os}

In 11588 we want to avoid reading from environment variables,
11727 did that well. I wonder if we could leverage tools to help
with this. Thankfully, clippy has a `disallowed_methods`[1] lint,
helping us enforce the rule.

[1]: https://rust-lang.github.io/rust-clippy/stable/index.html#disallowed_methods

Author: weihanglo

clippy.toml

@@ -0,0 +1,6 @@
+disallowed-methods = [
+    { path = "std::env::var", reason = "Use `Config::get_env` instead. See rust-lang/cargo#11588" },
+    { path = "std::env::var_os", reason = "Use `Config::get_env_os` instead. See rust-lang/cargo#11588" },
+    { path = "std::env::vars", reason = "Not recommended to use in Cargo. See rust-lang/cargo#11588" },
+    { path = "std::env::vars_os", reason = "Not recommended to use in Cargo. See rust-lang/cargo#11588" },
+]

src/cargo/core/compiler/fingerprint/mod.rs

@@ -1658,14 +1658,14 @@ fn local_fingerprints_deps(
         local.push(LocalFingerprint::RerunIfChanged { output, paths });
     }
 
-    local.extend(
-        deps.rerun_if_env_changed
-            .iter()
-            .map(|var| LocalFingerprint::RerunIfEnvChanged {
-                var: var.clone(),
-                val: env::var(var).ok(),
-            }),
-    );
+    local.extend(deps.rerun_if_env_changed.iter().map(|var| {
+        #[allow(clippy::disallowed_methods)]
+        let val = env::var(var).ok();
+        LocalFingerprint::RerunIfEnvChanged {
+            var: var.clone(),
+            val,
+        }
+    }));
 
     local
 }

src/cargo/core/features.rs

@@ -946,11 +946,13 @@ impl CliUnstable {
             self.add(flag, &mut warnings)?;
         }
 
-        if self.gitoxide.is_none()
-            && std::env::var_os("__CARGO_USE_GITOXIDE_INSTEAD_OF_GIT2")
+        if self.gitoxide.is_none() {
+            #[allow(clippy::disallowed_methods)]
+            if std::env::var_os("__CARGO_USE_GITOXIDE_INSTEAD_OF_GIT2")
                 .map_or(false, |value| value == "1")
-        {
-            self.gitoxide = GitoxideFeatures::safe().into();
+            {
+                self.gitoxide = GitoxideFeatures::safe().into();
+            }
         }
         Ok(warnings)
     }

src/cargo/lib.rs

@@ -4,6 +4,7 @@
 // Due to some of the default clippy lints being somewhat subjective and not
 // necessarily an improvement, we prefer to not use them at this time.
 #![allow(clippy::all)]
+#![warn(clippy::disallowed_methods)]
 #![warn(clippy::self_named_module_files)]
 #![allow(rustdoc::private_intra_doc_links)]