Implement feature_missing manifest lint to prove manifest lints work!

src/lints/feature_missing.ron

@@ -0,0 +1,53 @@
+SemverQuery(
+    id: "feature_missing",
+    human_readable_name: "package feature removed or renamed",
+    description: "A feature has been removed from this package's Cargo.toml.",
+    required_update: Major,
+    lint_level: Deny,
+    reference_link: Some("https://doc.rust-lang.org/cargo/reference/semver.html#cargo-feature-remove"),
+    query: r#"
+    {
+        CrateDiff {
+            baseline {
+                feature {
+                    # Until cargo ships with support for private and/or unstable feature names,
+                    # we'll rely on feature names to detect whether to flag feature removals.
+                    #
+                    # This lint will ignore features that match any of the following:
+                    # - start with an underscore (`_`) character
+                    # - are named `unstable`, `nightly`, or `bench`
+                    # - have a prefix of `unstable`, `nightly`, or `bench` followed by
+                    #   a dash (`-`) or underscore (`_`) character.
+                    #
+                    # Cargo tracking issues:
+                    # - unstable/nightly features: https://github.com/rust-lang/cargo/issues/10881
+                    # - private/hidden features: https://github.com/rust-lang/cargo/issues/10882
+                    name @tag
+                         @filter(op: "not_regex", value: ["$unstable_feature_pattern"])
+                         @filter(op: "not_has_prefix", value: ["$underscore"])
+                         @output
+
+                    # An explicit ordering key is needed since we don't have span information,
+                    # which what we usually use to order results in tests.
+                    name @output(name: "ordering_key")
+                }
+            }
+            current {
+                feature @fold @transform(op: "count") @filter(op: "=", value: ["$zero"]) {
+                    name @filter(op: "=", value: ["%name"])
+                }
+            }
+        }
+    }"#,
+    arguments: {
+        "zero": 0,
+        "unstable_feature_pattern": "^(?:unstable|nightly|bench)(?:[-_].*)?$",
+        "underscore": "_",
+    },
+    error_message: "A feature has been removed from this package's Cargo.toml. This will break downstream crates that enable this feature.",
+    per_result_error_template: Some("feature {{name}} in the package's Cargo.toml"),
+    // TODO: It's currently not possible to write witnesses for manifest lints,
+    //       since we'd need to generate a *Cargo.toml* witness instead of a Rust code witness.
+    //       Issue: https://github.com/obi1kenobi/cargo-semver-checks/issues/1008
+    witness: None,
+)

src/query.rs

@@ -329,7 +329,7 @@ mod tests {
     use std::borrow::Cow;
     use std::collections::BTreeSet;
     use std::path::PathBuf;
-    use std::sync::OnceLock;
+    use std::sync::{Arc, OnceLock};
     use std::{collections::BTreeMap, path::Path};
 
     use anyhow::Context;
@@ -631,14 +631,31 @@ mod tests {
         // Reorder vector of results into a deterministic order that will compensate for
         // nondeterminism in how the results are ordered.
         let key_func = |elem: &BTreeMap<String, FieldValue>| {
-            let filename = elem.get("span_filename").and_then(|value| value.as_str());
-            let line = elem.get("span_begin_line");
-
-            match (filename, line) {
-                (Some(filename), Some(line)) => (filename.to_owned(), line.as_usize()),
-                (Some(_filename), None) => panic!("A valid query must output `span_filename`. See https://github.com/obi1kenobi/cargo-semver-checks/blob/main/CONTRIBUTING.md for details."),
-                (None, Some(_line)) => panic!("A valid query must output `span_begin_line`. See https://github.com/obi1kenobi/cargo-semver-checks/blob/main/CONTRIBUTING.md for details."),
-                (None, None) => panic!("A valid query must output both `span_filename` and `span_begin_line`. See https://github.com/obi1kenobi/cargo-semver-checks/blob/main/CONTRIBUTING.md for details."),
+            // Queries should either:
+            // - define an explicit `ordering_key` string value sufficient to establish
+            //   a total order of results for each crate, or
+            // - define `span_filename` and `span_begin_line` values where the lint is being raised,
+            //   which will then define a total order of results for that query on that crate.
+            let ordering_key = elem
+                .get("ordering_key")
+                .and_then(|value| value.as_arc_str());
+            if let Some(key) = ordering_key {
+                (Arc::clone(key), 0)
+            } else {
+                let filename = elem.get("span_filename").map(|value| {
+                    value
+                        .as_arc_str()
+                        .expect("`span_filename` was not a string")
+                });
+                let line = elem
+                    .get("span_begin_line")
+                    .map(|value: &FieldValue| value.as_usize().expect("begin line was not an int"));
+                match (filename, line) {
+                    (Some(filename), Some(line)) => (Arc::clone(filename), line),
+                    (Some(_filename), None) => panic!("A valid query must output `span_filename`. See https://github.com/obi1kenobi/cargo-semver-checks/blob/main/CONTRIBUTING.md for details."),
+                    (None, Some(_line)) => panic!("A valid query must output `span_begin_line`. See https://github.com/obi1kenobi/cargo-semver-checks/blob/main/CONTRIBUTING.md for details."),
+                    (None, None) => panic!("A valid query must output both `span_filename` and `span_begin_line`. See https://github.com/obi1kenobi/cargo-semver-checks/blob/main/CONTRIBUTING.md for details."),
+                }
             }
         };
         for value in query_execution_results.values_mut() {
@@ -1037,6 +1054,7 @@ add_lints!(
     enum_variant_marked_non_exhaustive,
     enum_variant_missing,
     exported_function_changed_abi,
+    feature_missing,
     function_abi_no_longer_unwind,
     function_changed_abi,
     function_const_removed,

test_crates/feature_missing/new/Cargo.toml

@@ -0,0 +1,13 @@
+[package]
+publish = false
+name = "feature_missing"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
+
+[features]
+still_present = []
+# Explicitly-added feature, to replace the implicit feature defined by
+# the `optional = true` dependency in the previous crate version.
+rand_core = []

test_crates/feature_missing/old/Cargo.toml

@@ -0,0 +1,31 @@
+[package]
+publish = false
+name = "feature_missing"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
+# Since `rand` isn't used in a feature with `dep:rand` syntax,
+# it defines an implicit feature by that name.
+# Removing that implicit feature is a breaking change.
+rand = { version = "*", optional = true }
+# However, re-adding an explicit feature after removing the implicit one
+# will avoid the breakage.
+rand_core = { version = "*", optional = true}
+
+[features]
+still_present = []
+going_missing = []
+
+# We ignore unstable-looking feature names.
+# All of the following will be removed, and none of them should be flagged.
+unstable = []
+nightly = []
+bench = []
+unstable-dash = []
+unstable_underscore = []
+nightly-dash = []
+nightly_underscore = []
+bench-dash = []
+bench_underscore = []
+_underscore_prefix = []

test_crates/function_feature_changed/new/Cargo.toml

@@ -4,6 +4,19 @@ name = "function_feature_changed"
 version = "0.1.0"
 edition = "2021"
 
+# The purpose of this test crate is to ensure that cargo-semver-checks' feature selection
+# is robust to changes in which features exist and are enabled.
+#
+# We want cargo-semver-checks runs to succeed and fail on the basis of
+# what they find inside the crate, not just always fail because a feature has been deleted.
+# Hence, we disable this lint.
+#
+# This *only* affects directly running cargo-semver-checks against this crate.
+# It does not affect the lint query runs against all crates, since those run over
+# pre-generated rustdoc JSON and `cargo metadata` output and ignore this configuration.
+[package.metadata.cargo-semver-checks.lints]
+feature_missing = "allow"
+
 [dependencies]
 
 [features]

test_outputs/query_execution/feature_missing.snap

@@ -0,0 +1,22 @@
+---
+source: src/query.rs
+expression: "&query_execution_results"
+---
+{
+  "./test_crates/feature_missing/": [
+    {
+      "name": String("going_missing"),
+      "ordering_key": String("going_missing"),
+    },
+    {
+      "name": String("rand"),
+      "ordering_key": String("rand"),
+    },
+  ],
+  "./test_crates/function_feature_changed/": [
+    {
+      "name": String("feature_to_be_removed"),
+      "ordering_key": String("feature_to_be_removed"),
+    },
+  ],
+}