[aeneas] Preserve variable names (#3560)

In #3514, the LLBC does not
keep the variable names from the Rust source. This PR extracts the names
of variables from MIR and carries it over to the LLBC.

For instance, for `tests/expected/llbc/basic1` which has the following
Rust:
```rust
fn select(s: bool, x: i32, y: i32) -> i32 {
    if s { x } else { y }
}
```
without this PR, running Aeneas on the output LLBC produces:
```lean
def select (b : Bool) (i : I32) (i1 : I32) : Result I32 :=
  if b
  then Result.ok i
  else Result.ok i1
```
but with this PR, it produces:
```lean
def select (s : Bool) (x : I32) (y : I32) : Result I32 :=
  if s
  then Result.ok x
  else Result.ok y
```

This should not be merged before #3514, so keeping it as a draft for the
time being.

The actual diff on top of #3514 can be viewed here:
zhassan-aws/kani@llbc4...zhassan-aws:kani:llbc-names

By submitting this pull request, I confirm that my contribution is made
under the terms of the Apache 2.0 and MIT licenses.

kani-compiler/src/codegen_aeneas_llbc/mir_to_ullbc/mod.rs

@@ -16,7 +16,7 @@ use charon_lib::ast::Rvalue as CharonRvalue;
 use charon_lib::ast::Span as CharonSpan;
 use charon_lib::ast::meta::{AttrInfo, Loc, RawSpan};
 use charon_lib::ast::types::Ty as CharonTy;
-use charon_lib::ast::{AbortKind, Body as CharonBody, Var, VarId, make_locals_generator};
+use charon_lib::ast::{AbortKind, Body as CharonBody, Var, VarId};
 use charon_lib::ast::{
     AnyTransId, Assert, BodyId, BuiltinTy, Disambiguator, FileName, FunDecl, FunSig, GenericArgs,
     GenericParams, IntegerTy, ItemKind, ItemMeta, ItemOpacity, Literal, LiteralTy, Name, Opaque,
@@ -39,14 +39,16 @@ use charon_lib::ullbc_ast::{
     Terminator as CharonTerminator,
 };
 use charon_lib::{error_assert, error_or_panic};
+use rustc_data_structures::fx::FxHashMap;
 use rustc_errors::MultiSpan;
 use rustc_middle::ty::TyCtxt;
 use rustc_smir::rustc_internal;
 use rustc_span::def_id::DefId as InternalDefId;
 use stable_mir::abi::PassMode;
+use stable_mir::mir::VarDebugInfoContents;
 use stable_mir::mir::mono::Instance;
 use stable_mir::mir::{
-    BasicBlock, BinOp, Body, BorrowKind, CastKind, ConstOperand, Mutability, Operand, Place,
+    BasicBlock, BinOp, Body, BorrowKind, CastKind, ConstOperand, Local, Mutability, Operand, Place,
     ProjectionElem, Rvalue, Statement, StatementKind, SwitchTargets, Terminator, TerminatorKind,
 };
 use stable_mir::ty::{
@@ -64,6 +66,7 @@ pub struct Context<'a, 'tcx> {
     instance: Instance,
     translated: &'a mut TranslatedCrate,
     errors: &'a mut ErrorCtx<'tcx>,
+    local_names: FxHashMap<Local, String>,
 }
 
 impl<'a, 'tcx> Context<'a, 'tcx> {
@@ -75,7 +78,17 @@ impl<'a, 'tcx> Context<'a, 'tcx> {
         translated: &'a mut TranslatedCrate,
         errors: &'a mut ErrorCtx<'tcx>,
     ) -> Self {
-        Self { tcx, instance, translated, errors }
+        let mut local_names = FxHashMap::default();
+        // populate names of locals
+        for info in instance.body().unwrap().var_debug_info {
+            if let VarDebugInfoContents::Place(p) = info.value {
+                if p.projection.is_empty() {
+                    local_names.insert(p.local, info.name);
+                }
+            }
+        }
+
+        Self { tcx, instance, translated, errors, local_names }
     }
 
     fn tcx(&self) -> TyCtxt<'tcx> {
@@ -466,12 +479,11 @@ impl<'a, 'tcx> Context<'a, 'tcx> {
         // - the input arguments
         // - the remaining locals, used for the intermediate computations
         let mut locals = Vector::new();
-        {
-            let mut add_variable = make_locals_generator(&mut locals);
-            mir_body.local_decls().for_each(|(_, local)| {
-                add_variable(self.translate_ty(local.ty));
-            });
-        }
+        mir_body.local_decls().for_each(|(local, local_decl)| {
+            let ty = self.translate_ty(local_decl.ty);
+            let name = self.local_names.get(&local);
+            locals.push_with(|index| Var { index, name: name.cloned(), ty });
+        });
         locals
     }
 

tests/expected/llbc/basic0/expected

@@ -1,8 +1,8 @@
 fn test::is_zero(@1: i32) -> bool\
 {\
     let @0: bool; // return\
-    let @1: i32; // arg #1\
+    let i@1: i32; // arg #1\
 
-    @0 := copy (@1) == const (0 : i32)\
+    @0 := copy (i@1) == const (0 : i32)\
     return\
 }

tests/expected/llbc/basic1/expected

@@ -1,15 +1,15 @@
 fn test::select(@1: bool, @2: i32, @3: i32) -> i32
 {
     let @0: i32; // return
-    let @1: bool; // arg #1
-    let @2: i32; // arg #2
-    let @3: i32; // arg #3
+    let s@1: bool; // arg #1
+    let x@2: i32; // arg #2
+    let y@3: i32; // arg #3
 
-    if copy (@1) {
-        @0 := copy (@2)
+    if copy (s@1) {
+        @0 := copy (x@2)
     }
     else {
-        @0 := copy (@3)
+        @0 := copy (y@3)
     }
     return
 }