Rust: Associated types by paldepind · Pull Request #19214 · github/codeql

paldepind · 2025-04-04T08:22:25Z

This PR improves calls to methods in trait blocks (not methods in impl blocks) that refer to associated types.

One approach to implementing this would be: When a method call resolves to a method inside a trait we find the impl block that makes the self argument implement the trait. This impl block contains the definition of the associated type, and when we read off the declared type of the method we convert the associated type to the specific definition of the type.

However, this change would have to be done inside getDeclaredType which right now doesn't depend on the call site at all. Hence, instead of doing that, this PR handles associated types by treating them as type parameters. I.e. a trait like this:

trait MyTrait {
  type AssociatedType;
  // ...
}

is treated as-if it was

trait MyTrait<AssociatedType> {
  // ...
}

One downside of this approach is that it's not immediately clear to me how to handle associated types with generics like this:

trait MyTraitAssoc2 {
    type GenericAssociatedType<AssociatedParam>;

    fn put<A>(&self, a: A) -> Self::GenericAssociatedType<A>;
}

Here Self::GenericAssociatedType refers to a type parameter of put. So what should we do for the A passed to it?The GenericAssociatedType type parameter would have to be handled as a sort of higher-kinded type.

My impression is that associated types with generics is a feature that is not used very much. So I suggest that it's ok to not handle that feature, unless there's some straightforward way to do it.

paldepind · 2025-04-04T09:37:08Z

+        println!("{:?}", x4);
+
+        let x5 = S2;
+        println!("{:?}", x5.m1()); // $ method=m1 MISSING: type=x5.m1():A.S2


This missing type is probably worth fixing. But since it's not for a method in a trait block and not a regression I didn't look into it for this PR.

This got fixed when I merged main. Did you do something to fix this @hvitved? 😁

Wouldn't that be your very own #19146? :-) Because now the Self::AssociatedType return type of m1 inside impl MyTrait for S2 can resolve to Wrapper<S2>

hvitved

Looks great, a few suggestions.

hvitved · 2025-04-07T11:31:23Z

  TArrayType() or // todo: add size?
  TRefType() or // todo: add mut?
  TTypeParamTypeParameter(TypeParam t) or
+  TAssociatedTypeTypeParameter(TypeAlias t) { any(TraitItemNode trait).getADescendant() = t } or


I have a slight preference for getAnAssocItem over getADescendant, but it should do that same.

hvitved · 2025-04-07T11:32:38Z

  }
 }

+/** Gets type alias that is the `i`th type parameter of `trait`. */


Gets type -> Gets the type.

Perhaps add a comment that type aliases are numbered arbitrarily but consecutively, starting from the index following the last ordinary type parameter.

hvitved · 2025-04-07T11:34:27Z

+ *   // ...
+ * }
+ * ```
+ * is treated as if it where


where -> was

hvitved · 2025-04-07T11:34:54Z

+  TypeAlias getTypeAlias() { result = typeAlias }
+
+  /** Gets the trait that contains this associated type declaration. */
+  TraitItemNode getTrait() { result.getADescendant() = typeAlias }


Again, I prefer getAnAssocItem.

hvitved · 2025-04-07T11:35:41Z

+  /** Gets the trait that contains this associated type declaration. */
+  TraitItemNode getTrait() { result.getADescendant() = typeAlias }
+
+  int getIndex() { traitAliasIndex(this.getTrait(), result, typeAlias) }


this.getTrait() -> _

hvitved · 2025-04-07T11:39:05Z

+    exists(ImplItemNode impl, AssociatedTypeTypeParameter param, TypeAlias alias |
+      this = impl.getTraitPath() and
+      param.getTrait() = resolvePath(this) and
+      alias = impl.getASuccessor(param.getTypeAlias().getName().getText()) and
+      result = alias.getTypeRepr() and
+      param.getIndex() = i
+    )


It took me a while to see why type arguments are not mixed up when a trait has multiple associated types. It would be a nice to have a test for that.

hvitved · 2025-04-07T11:44:06Z

  override Type resolveType() { result = TTypeParamTypeParameter(this) }
 }

+// Used to represent implicit associated type type arguments in traits.


type type -> type

It's not the easiest thing to read but the repeated type is intentional as it's supposed to be "associated type" type arguments. I've reworded it so that the two "type"s don't occur back-to-back.

hvitved · 2025-04-07T11:47:11Z

+  override TypeReprMention getTypeArgument(int i) { none() }
+
+  override Type resolveType() { result = TAssociatedTypeTypeParameter(this) }


Not all TypeAliases should be type mentions, so perhaps

private Type t; TypeAliasMention() { t = TAssociatedTypeTypeParameter(this) } override TypeReprMention getTypeArgument(int i) { none() } override Type resolveType() { result = t }

Yes, good point!

Rust: Expand on type inference tests for associated types

f9ff92a

github-actions Bot added the Rust Pull requests that update Rust code label Apr 4, 2025

Rust: Handle associated types in trait methods

77e1b23

paldepind force-pushed the rust-ti-associated branch from 4d8454c to 77e1b23 Compare April 4, 2025 08:25

github-advanced-security AI found potential problems Apr 4, 2025

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Comment thread rust/ql/lib/codeql/rust/internal/TypeMention.qll Fixed

paldepind changed the title ~~Rust ti associated~~ Rust: Associated types Apr 4, 2025

paldepind commented Apr 4, 2025

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paldepind marked this pull request as ready for review April 4, 2025 09:37

paldepind requested a review from hvitved April 4, 2025 09:37

hvitved requested changes Apr 7, 2025

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paldepind added 3 commits April 7, 2025 16:46

Rust: Minor changes based on PR review

8e76bb1

Rust: Add type inference test for trait with multiple associated types

602e617

Merge branch 'main' into rust-ti-associated

48e5b0a

hvitved approved these changes Apr 7, 2025

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paldepind merged commit 9dc008b into github:main Apr 8, 2025
17 checks passed

paldepind deleted the rust-ti-associated branch April 8, 2025 11:46

		override TypeReprMention getTypeArgument(int i) { none() }

		override Type resolveType() { result = TAssociatedTypeTypeParameter(this) }

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paldepind commented Apr 4, 2025 •

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