Rust: Model String -> str implicit conversion in type inference

Author: hvitved

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

@@ -1153,12 +1153,27 @@ private module MethodCall {
 
     pragma[nomagic]
     override Type getTypeAt(TypePath path) {
-      exists(TypePath path0 | result = inferType(super.getReceiver(), path0) |
-        path0.isCons(TRefTypeParameter(), path)
+      exists(TypePath path0, Type t0 |
+        t0 = inferType(super.getReceiver(), path0) and
+        (
+          path0.isCons(TRefTypeParameter(), path)
+          or
+          not path0.isCons(TRefTypeParameter(), _) and
+          not (path0.isEmpty() and result = TRefType()) and
+          path = path0
+        )
+      |
+        result = t0
         or
-        not path0.isCons(TRefTypeParameter(), _) and
-        not (path0.isEmpty() and result = TRefType()) and
-        path = path0
+        // We do not yet model the `Deref` trait, so we hard-code the fact that
+        // `String` dereferences to `str` here. This allows us e.g. to resolve
+        // `x.parse::<usize>()` to the function `<core::str>::parse` when `x` has
+        // type `String`.
+        //
+        // See also https://doc.rust-lang.org/reference/expressions/method-call-expr.html#r-expr.method.autoref-deref
+        path.isEmpty() and
+        t0.(StructType).asItemNode().(Struct).getCanonicalPath() = "alloc::string::String" and
+        result.(StructType).asItemNode() instanceof Builtins::Str
       )
     }
   }

rust/ql/test/library-tests/type-inference/main.rs

@@ -1111,7 +1111,7 @@ mod method_call_type_conversion {
         let x9 : String = "Hello".to_string(); // $ type=x9:String
         // Implicit `String` -> `str` conversion happense via the `Deref` trait:
         // https://doc.rust-lang.org/std/string/struct.String.html#deref.
-        let u = x9.parse::<u32>(); // $ MISSING: method=parse type=u:T.u32
+        let u = x9.parse::<u32>(); // $ method=parse type=u:T.u32
     }
 }
 

rust/ql/test/library-tests/type-inference/type-inference.expected

@@ -1390,9 +1390,16 @@ inferType
 | main.rs:1109:26:1109:27 | x7 | T | file://:0:0:0:0 | & |
 | main.rs:1109:26:1109:27 | x7 | T.&T | main.rs:1060:5:1061:14 | S2 |
 | main.rs:1111:13:1111:14 | x9 |  | {EXTERNAL LOCATION} | String |
+| main.rs:1111:13:1111:14 | x9 |  | {EXTERNAL LOCATION} | str |
 | main.rs:1111:27:1111:33 | "Hello" |  | {EXTERNAL LOCATION} | str |
 | main.rs:1111:27:1111:45 | "Hello".to_string() |  | {EXTERNAL LOCATION} | String |
+| main.rs:1111:27:1111:45 | "Hello".to_string() |  | {EXTERNAL LOCATION} | str |
+| main.rs:1114:13:1114:13 | u |  | {EXTERNAL LOCATION} | Result |
+| main.rs:1114:13:1114:13 | u | T | {EXTERNAL LOCATION} | u32 |
 | main.rs:1114:17:1114:18 | x9 |  | {EXTERNAL LOCATION} | String |
+| main.rs:1114:17:1114:18 | x9 |  | {EXTERNAL LOCATION} | str |
+| main.rs:1114:17:1114:33 | x9.parse() |  | {EXTERNAL LOCATION} | Result |
+| main.rs:1114:17:1114:33 | x9.parse() | T | {EXTERNAL LOCATION} | u32 |
 | main.rs:1121:16:1121:20 | SelfParam |  | file://:0:0:0:0 | & |
 | main.rs:1121:16:1121:20 | SelfParam | &T | main.rs:1119:5:1127:5 | Self [trait MyTrait] |
 | main.rs:1124:16:1124:20 | SelfParam |  | file://:0:0:0:0 | & |