C#: Implicit span conversion.

csharp/ql/lib/semmle/code/csharp/Conversion.qll

@@ -28,6 +28,7 @@ private module Cached {
    *
    * - Identity conversions
    * - Implicit numeric conversions
+   * - Implicit span conversions
    * - Implicit nullable conversions
    * - Implicit reference conversions
    * - Boxing conversions
@@ -38,6 +39,8 @@ private module Cached {
     or
     convNumeric(fromType, toType)
     or
+    convSpan(fromType, toType)
+    or
     convNullableType(fromType, toType)
     or
     convRefTypeNonNull(fromType, toType)
@@ -81,6 +84,7 @@ private predicate implicitConversionNonNull(Type fromType, Type toType) {
  *
  * - Identity conversions
  * - Implicit numeric conversions
+ * - Implicit span conversions
  * - Implicit nullable conversions
  * - Implicit reference conversions
  * - Boxing conversions
@@ -491,6 +495,53 @@ private predicate convNumericChar(SimpleType toType) {
 
 private predicate convNumericFloat(SimpleType toType) { toType instanceof DoubleType }
 
+private class SpanType extends GenericType {
+  SpanType() { this.getUnboundGeneric() instanceof SystemSpanStruct }
+
+  Type getElementType() { result = this.getTypeArgument(0) }
+}
+
+private class ReadOnlySpanType extends GenericType {
+  ReadOnlySpanType() { this.getUnboundGeneric() instanceof SystemReadOnlySpanStruct }
+
+  Type getElementType() { result = this.getTypeArgument(0) }
+}
+
+private class SimpleArrayType extends ArrayType {
+  SimpleArrayType() {
+    this.getRank() = 1 and
+    this.getDimension() = 1
+  }
+}
+
+/**
+ * INTERNAL: Do not use.
+ *
+ * Holds if there is an implicit span conversion from `fromType` to `toType`.
+ *
+ * 10.2.1: Implicit span conversions (added in C# 14).
+ * [Documentation](https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/proposals/csharp-14.0/first-class-span-types#span-conversions)
+ */
+predicate convSpan(Type fromType, Type toType) {
+  fromType.(SimpleArrayType).getElementType() = toType.(SpanType).getElementType()
+  or
+  exists(Type fromElementType, Type toElementType |
+    (
+      fromElementType = fromType.(SimpleArrayType).getElementType() or
+      fromElementType = fromType.(SpanType).getElementType() or
+      fromElementType = fromType.(ReadOnlySpanType).getElementType()
+    ) and
+    toElementType = toType.(ReadOnlySpanType).getElementType()
+  |
+    convIdentity(fromElementType, toElementType)
+    or
+    convCovariance(fromElementType, toElementType)
+  )
+  or
+  fromType instanceof SystemStringClass and
+  toType.(ReadOnlySpanType).getElementType() instanceof CharType
+}
+
 /**
  * INTERNAL: Do not use.
  *
@@ -784,8 +835,8 @@ predicate convConversionOperator(Type fromType, Type toType) {
   )
 }
 
-/** 13.1.3.2: Variance conversion. */
-private predicate convVariance(GenericType fromType, GenericType toType) {
+pragma[nomagic]
+private predicate convVarianceAux(UnboundGenericType ugt, GenericType fromType, GenericType toType) {
   // Semantically equivalent with
   // ```ql
   // ugt = fromType.getUnboundGeneric()
@@ -805,10 +856,23 @@ private predicate convVariance(GenericType fromType, GenericType toType) {
   // ```
   // but performance is improved by explicitly evaluating the `i`th argument
   // only when all preceding arguments are convertible.
-  Variance::convVarianceSingle(_, fromType, toType)
+  Variance::convVarianceSingle(ugt, fromType, toType)
   or
+  Variance::convVarianceMultiple(ugt, fromType, toType, ugt.getNumberOfTypeParameters() - 1)
+}
+
+/** 13.1.3.2: Variance conversion. */
+private predicate convVariance(GenericType fromType, GenericType toType) {
+  convVarianceAux(_, fromType, toType)
+}
+
+/**
+ * Holds, if `fromType` is covariance convertible to `toType`.
+ */
+private predicate convCovariance(GenericType fromType, GenericType toType) {
   exists(UnboundGenericType ugt |
-    Variance::convVarianceMultiple(ugt, fromType, toType, ugt.getNumberOfTypeParameters() - 1)
+    convVarianceAux(ugt, fromType, toType) and
+    forall(TypeParameter tp | tp = ugt.getATypeParameter() | tp.isOut())
   )
 }