Add compatibility/arrow-functions-feature lint

crates/linter/src/plugin/compatibility/mod.rs

@@ -1,4 +1,5 @@
 use crate::definition::PluginDefinition;
+use crate::plugin::compatibility::rules::php74::arrow_functions_feature::ArrowFunctionsFeatureRule;
 use crate::plugin::compatibility::rules::php74::null_coalesce_assignment_feature::NullCoalesceAssignmentFeatureRule;
 use crate::plugin::compatibility::rules::php80::named_arguments_feature::NamedArgumentsFeatureRule;
 use crate::plugin::compatibility::rules::php80::promoted_properties_feature::PromotedPropertiesFeatureRule;
@@ -26,6 +27,7 @@ impl Plugin for CompatibilityPlugin {
     fn get_rules(&self) -> Vec<Box<dyn Rule>> {
         vec![
             // PHP 7.4
+            Box::new(ArrowFunctionsFeatureRule),
             Box::new(NullCoalesceAssignmentFeatureRule),
             // PHP 8.0
             Box::new(NamedArgumentsFeatureRule),

crates/linter/src/plugin/compatibility/rules/mod.rs

@@ -1,4 +1,5 @@
 pub mod php74 {
+    pub mod arrow_functions_feature;
     pub mod null_coalesce_assignment_feature;
 }
 

crates/linter/src/plugin/compatibility/rules/php74/arrow_functions_feature.rs

@@ -0,0 +1,65 @@
+use indoc::indoc;
+
+use mago_ast::ast::*;
+use mago_php_version::PHPVersion;
+use mago_reporting::*;
+use mago_span::*;
+use mago_walker::Walker;
+
+use crate::context::LintContext;
+use crate::definition::RuleDefinition;
+use crate::definition::RuleUsageExample;
+use crate::rule::Rule;
+
+#[derive(Clone, Copy, Debug)]
+pub struct ArrowFunctionsFeatureRule;
+
+impl Rule for ArrowFunctionsFeatureRule {
+    fn get_definition(&self) -> RuleDefinition {
+        RuleDefinition::enabled("Arrow Functions Feature", Level::Error)
+            .with_maximum_supported_php_version(PHPVersion::PHP73)
+            .with_description(indoc! {"
+                Flags any usage of the `fn` keyword for arrow functions, which was introduced in PHP 7.4.
+
+                In environments running older versions of PHP, you can use the `use` keyword to import variables instead.
+            "})
+            .with_example(RuleUsageExample::valid(
+                "Using the `use` keyword for arrow functions",
+                indoc! {r#"
+                    <?php
+
+                    $y = 2;
+
+                    // Works in all PHP versions (pre-7.4 included):
+                    $fn = function ($x) use ($y) {
+                        return $x + $y;
+                    };
+                "#},
+            ))
+            .with_example(RuleUsageExample::invalid(
+                "Using the `fn` keyword for arrow functions",
+                indoc! {r#"
+                    <?php
+
+                    $y = 2;
+
+                    // Only valid in PHP 7.4+:
+                    $fn = fn ($x) => $x + $y;
+                "#},
+            ))
+    }
+}
+
+impl<'a> Walker<LintContext<'a>> for ArrowFunctionsFeatureRule {
+    fn walk_in_arrow_function<'ast>(&self, arrow_function: &'ast ArrowFunction, context: &mut LintContext<'a>) {
+        let issue = Issue::new(
+            context.level(),
+            "The `fn` keyword for arrow functions is only available in PHP 7.4 and later.",
+        )
+        .with_annotation(
+            Annotation::primary(arrow_function.span()).with_message("Arrow function uses `fn` keyword."),
+        );
+
+        context.report(issue);
+    }
+}

crates/php-version/src/feature.rs

@@ -63,4 +63,5 @@ pub enum Feature {
     PropertyHooks,
     JsonValidate,
     ClosureInConstantExpressions,
+    ArrowFunctions,
 }

crates/php-version/src/lib.rs

@@ -184,7 +184,8 @@ impl PHPVersion {
             Feature::NullCoalesceAssign
             | Feature::ParameterContravariance
             | Feature::ReturnCovariance
-            | Feature::PregUnmatchedAsNull => self.0 >= 0x070400,
+            | Feature::PregUnmatchedAsNull
+            | Feature::ArrowFunctions => self.0 >= 0x070400,
             Feature::NonCapturingCatches
             | Feature::NativeUnionTypes
             | Feature::LessOverridenParametersWithVariadic