charon_lib/transform/normalize/
transform_dyn_trait_calls.rs

1//! Transform method calls on `&dyn Trait` to vtable function pointer calls.
2//!
3//! This pass converts direct method calls on trait objects into calls through vtable
4//! function pointers. For example:
5//!
6//! ```rust,ignore
7//! let x: &dyn Trait = &obj;
8//! x.method(args);
9//! ```
10//!
11//! is transformed from:
12//! ```text
13//! @0 := call <dyn Trait as Trait>::method(x, args)
14//! ```
15//! to:
16//! ```text
17//! @0 := (move (*@receiver.ptr_metadata).method_check)(move (@receiver), move (@args)) // Call through function pointer
18//! ```
19
20use super::super::ctx::UllbcPass;
21use crate::{
22    errors::Error,
23    formatter::IntoFormatter,
24    pretty::FmtWithCtx,
25    raise_error, register_error,
26    transform::{TransformCtx, ctx::UllbcStatementTransformCtx},
27    ullbc_ast::*,
28};
29
30/// Transform a call to a trait method on a dyn trait object
31fn transform_dyn_trait_call(
32    ctx: &mut UllbcStatementTransformCtx<'_>,
33    call: &mut Call,
34) -> Result<(), Error> {
35    let fmt_ctx = &ctx.ctx.into_fmt();
36
37    // Detect if this call should be transformed
38    let FnOperand::Regular(fn_ptr) = &call.func else {
39        return Ok(()); // Not a regular function call
40    };
41    let FnPtrKind::Trait(trait_ref, method_name, _) = &fn_ptr.kind else {
42        return Ok(()); // Not a trait method call
43    };
44    let TraitRefKind::Dyn = &trait_ref.kind else {
45        return Ok(()); // Not a dyn trait trait call
46    };
47
48    // Get the type of the vtable struct.
49    let vtable_decl_ref: TypeDeclRef = {
50        // Get the trait declaration by its ID
51        let Some(trait_decl) = ctx.ctx.translated.trait_decls.get(trait_ref.trait_id()) else {
52            return Ok(()); // Unknown trait
53        };
54        // Get vtable ref from definition for correct ID.
55        let Some(vtable_ty) = &trait_decl.vtable else {
56            raise_error!(
57                ctx.ctx,
58                ctx.span,
59                "Found a `dyn Trait` method call for non-dyn-compatible trait `{}`!",
60                trait_ref.trait_id().with_ctx(fmt_ctx)
61            );
62        };
63        vtable_ty.clone().substitute_with_tref(trait_ref)
64    };
65    let vtable_decl_id = *vtable_decl_ref.id.as_adt().unwrap();
66    let Some(vtable_decl) = ctx.ctx.translated.type_decls.get(vtable_decl_id) else {
67        return Ok(()); // Missing data
68    };
69    if matches!(vtable_decl.kind, TypeDeclKind::Opaque) {
70        return Ok(()); // Missing data
71    }
72
73    // Retreive the method field from the vtable struct definition.
74    let method_field_name = format!("method_{}", method_name);
75    let Some((method_field_id, method_field)) =
76        vtable_decl.get_field_by_name(None, &method_field_name)
77    else {
78        let vtable_name = vtable_decl_ref.id.with_ctx(fmt_ctx).to_string();
79        raise_error!(
80            ctx.ctx,
81            ctx.span,
82            "Could not determine method index for {} in vtable {}",
83            method_name,
84            vtable_name
85        );
86    };
87    let method_ty = method_field
88        .ty
89        .clone()
90        .substitute(&vtable_decl_ref.generics);
91
92    // Get the receiver (first argument).
93    if call.args.is_empty() {
94        raise_error!(ctx.ctx, ctx.span, "Dyn trait call has no arguments!");
95    }
96    let dyn_trait_place = match &call.args[0] {
97        Operand::Copy(place) | Operand::Move(place) => place,
98        Operand::Const(_) => {
99            panic!("Unexpected constant as receiver for dyn trait method call")
100        }
101    };
102
103    // Construct the `(*ptr.ptr_metadata).method_field` place.
104    let vtable_ty = TyKind::Adt(vtable_decl_ref).into_ty();
105    let ptr_to_vtable_ty = Ty::new(TyKind::RawPtr(vtable_ty.clone(), RefKind::Shared));
106    let method_field_place = dyn_trait_place
107        .clone()
108        .project(ProjectionElem::PtrMetadata, ptr_to_vtable_ty)
109        .project(ProjectionElem::Deref, vtable_ty)
110        .project(
111            ProjectionElem::Field(FieldProjKind::Adt(vtable_decl_id, None), method_field_id),
112            method_ty,
113        );
114
115    // Transform the original call to use the function pointer
116    call.func = FnOperand::Dynamic(Operand::Copy(method_field_place));
117
118    Ok(())
119}
120
121pub struct Transform;
122impl UllbcPass for Transform {
123    fn transform_function(&self, ctx: &mut TransformCtx, decl: &mut FunDecl) {
124        decl.transform_ullbc_terminators(ctx, |ctx, term| {
125            if let TerminatorKind::Call { call, .. } = &mut term.kind {
126                let _ = transform_dyn_trait_call(ctx, call);
127            }
128        });
129    }
130}
charon_lib/transform/normalize/transform_dyn_trait_calls.rs

charon_lib/transform/normalize/
transform_dyn_trait_calls.rs
