Add feature_not_enabled_by_default lint.

It'll catch features that used to be enabled by default, and then stop being defaults.

Adds the lint requested by obi1kenobi/cargo-semver-checks-action#91

src/lints/feature_not_enabled_by_default.ron

@@ -0,0 +1,60 @@
+SemverQuery(
+    id: "feature_not_enabled_by_default",
+    human_readable_name: "package feature is not enabled by default",
+    description: "A feature has been removed from this package's set of default features.",
+    required_update: Major,
+    lint_level: Deny,
+    reference_link: Some("https://doc.rust-lang.org/cargo/reference/semver.html#cargo-feature-remove-another"),
+    query: r#"
+    {
+        CrateDiff {
+            baseline {
+                default_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 {
+                default_feature @fold @transform(op: "count") @filter(op: "=", value: ["$zero"]) {
+                    name @filter(op: "=", value: ["%name"])
+                }
+
+                # Ensure the feature still exists, just isn't enabled by default.
+                # If the feature is missing entirely, we want to report it
+                # in the `feature_missing` lint, not here.
+                feature @fold @transform(op: "count") @filter(op: ">", value: ["$zero"]) {
+                    name @filter(op: "=", value: ["%name"])
+                }
+            }
+        }
+    }"#,
+    arguments: {
+        "zero": 0,
+        "unstable_feature_pattern": "^(?:unstable|nightly|bench)(?:[-_].*)?$",
+        "underscore": "_",
+    },
+    error_message: "A feature is no longer enabled by default for this package. This will break downstream crates which rely on the package's default features and require the functionality of 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

@@ -1150,6 +1150,7 @@ add_lints!(
     enum_variant_missing,
     exported_function_changed_abi,
     feature_missing,
+    feature_not_enabled_by_default,
     function_abi_no_longer_unwind,
     function_changed_abi,
     function_const_removed,

test_crates/feature_not_enabled_by_default/new/Cargo.toml

@@ -0,0 +1,45 @@
+[package]
+publish = false
+name = "feature_not_enabled_by_default"
+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 from the defaults is a breaking change.
+rand = { version = "*", optional = true }
+
+[features]
+default = ["still_present", "transitive", "multi_root_transitive"]
+still_present = []
+becoming_non_default = []
+
+# The `transitive` feature used to depend on `indirect_feature`.
+# `transitive` is enabled by default, but removing `indirect_feature` from `transitive`
+# means that `indirect_feature` is *no longer enabled by default*.
+# This is breaking and should be reported.
+transitive = []
+indirect_feature = []
+
+# The `root` feature is depended on in multiple ways in the default features.
+# Breaking some of those paths, but not all of them, still means that `root` is enabled by default.
+# There's no breaking change here, and no lints should trigger.
+multi_root_transitive = ["path_a", "path_b"]
+path_a = ["root"]
+path_b = []
+root = []
+
+# We ignore unstable-looking feature names.
+# All of the following will be removed from the default features,
+# 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/feature_not_enabled_by_default/old/Cargo.toml

@@ -0,0 +1,46 @@
+[package]
+publish = false
+name = "feature_not_enabled_by_default"
+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 from the defaults is a breaking change.
+rand = { version = "*", optional = true }
+
+[features]
+default = ["becoming_non_default", "still_present", "missing_entirely", "rand", "transitive", "multi_root_transitive", "unstable", "nightly", "bench", "unstable", "unstable_underscore", "nightly", "nightly_underscore", "bench", "bench_underscore", "_underscore_prefix"]
+still_present = []         # No breakage here.
+becoming_non_default = []  # Should be reported.
+missing_entirely = []      # Flagged only by `feature_missing`, even though it was default too.
+
+# The `transitive` feature depends on `indirect_feature`.
+# `transitive` is enabled by default, but removing `indirect_feature` from `transitive`
+# means that `indirect_feature` is *no longer enabled by default*.
+# This is breaking and should be reported.
+transitive = ["indirect_feature"]
+indirect_feature = []
+
+# The `root` feature is depended on in multiple ways in the default features.
+# Breaking some of those paths, but not all of them, still means that `root` is enabled by default.
+# There's no breaking change here, and no lints should trigger.
+multi_root_transitive = ["path_a", "path_b"]
+path_a = ["root"]
+path_b = ["root"]
+root = []
+
+# We ignore unstable-looking feature names.
+# All of the following will be removed from the default features,
+# 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/features_simple/new/Cargo.toml

@@ -4,6 +4,11 @@ name = "features_simple"
 version = "0.1.0"
 edition = "2021"
 
+[package.metadata.cargo-semver-checks.lints]
+# This package is for testing that feature selection in the CLI works correctly,
+# so we aren't looking for breaking changes in the features themselves.
+feature_not_enabled_by_default = "allow"
+
 [dependencies]
 
 [features]

test_outputs/query_execution/feature_missing.snap

@@ -1,6 +1,7 @@
 ---
 source: src/query.rs
 expression: "&query_execution_results"
+snapshot_kind: text
 ---
 {
   "./test_crates/feature_missing/": [
@@ -17,6 +18,12 @@ expression: "&query_execution_results"
       "ordering_key": String("rand_pcg"),
     },
   ],
+  "./test_crates/feature_not_enabled_by_default/": [
+    {
+      "name": String("missing_entirely"),
+      "ordering_key": String("missing_entirely"),
+    },
+  ],
   "./test_crates/function_feature_changed/": [
     {
       "name": String("feature_to_be_removed"),

test_outputs/query_execution/feature_not_enabled_by_default.snap

@@ -0,0 +1,27 @@
+---
+source: src/query.rs
+expression: "&query_execution_results"
+snapshot_kind: text
+---
+{
+  "./test_crates/feature_not_enabled_by_default/": [
+    {
+      "name": String("becoming_non_default"),
+      "ordering_key": String("becoming_non_default"),
+    },
+    {
+      "name": String("indirect_feature"),
+      "ordering_key": String("indirect_feature"),
+    },
+    {
+      "name": String("rand"),
+      "ordering_key": String("rand"),
+    },
+  ],
+  "./test_crates/features_simple/": [
+    {
+      "name": String("foo"),
+      "ordering_key": String("foo"),
+    },
+  ],
+}