Rust: Simple type inference for index expressions

Author: hvitved

rust/ql/lib/codeql/rust/internal/TypeInference.qll

@@ -1010,6 +1010,24 @@ private StructType inferLiteralType(LiteralExpr le) {
   )
 }
 
+private class Vec extends Struct {
+  Vec() { this.getCanonicalPath() = "alloc::vec::Vec" }
+
+  TypeParamTypeParameter getElementTypeParameter() {
+    result.getTypeParam() = this.getGenericParamList().getTypeParam(0)
+  }
+}
+
+pragma[nomagic]
+private Type inferIndexExprType(IndexExpr ie, TypePath path) {
+  // TODO: Should be implemented as method resolution, using the special
+  // `std::ops::Index` trait.
+  exists(TypePath exprPath |
+    result = inferType(ie.getBase(), exprPath) and
+    exprPath.isCons(any(Vec v).getElementTypeParameter(), path)
+  )
+}
+
 private module MethodCall {
   /** An expression that calls a method. */
   abstract private class MethodCallImpl extends Expr {
@@ -1347,6 +1365,8 @@ private module Cached {
     or
     result = inferLiteralType(n) and
     path.isEmpty()
+    or
+    result = inferIndexExprType(n, path)
   }
 }