(c2rust-analyze) Add known perms for const refs before dataflow typeck.

Also, add skeleton code for checking that these perms are not modified after dataflow typeck,
but I'm not yet sure how to lookup the `PermissionSet` for a `PointerId`, so that part is `TODO`ed.

Author: kkysen

c2rust-analyze/src/dataflow/type_check.rs

@@ -5,10 +5,10 @@ use crate::util::{describe_rvalue, ty_callee, Callee, RvalueDesc};
 use assert_matches::assert_matches;
 use rustc_hir::def_id::DefId;
 use rustc_middle::mir::{
-    AggregateKind, BinOp, Body, Location, Mutability, Operand, Place, PlaceRef, ProjectionElem,
-    Rvalue, Statement, StatementKind, Terminator, TerminatorKind,
+    AggregateKind, BinOp, Body, ConstantKind, Location, Mutability, Operand, Place, PlaceRef,
+    ProjectionElem, Rvalue, Statement, StatementKind, Terminator, TerminatorKind,
 };
-use rustc_middle::ty::{SubstsRef, Ty, TyKind};
+use rustc_middle::ty::{self, SubstsRef, Ty, TyKind};
 
 /// Visitor that walks over the MIR, computing types of rvalues/operands/places and generating
 /// constraints as a side effect.
@@ -454,6 +454,21 @@ impl<'tcx> TypeChecker<'tcx, '_> {
     }
 }
 
+fn const_perms(constant: ConstantKind) -> PermissionSet {
+    let ref_ty = constant.ty();
+    let ty = match ref_ty.kind() {
+        ty::Ref(_, ty, _) => ty,
+        _ => panic!("expected only `Ref`s for constants: {ref_ty:?}"),
+    };
+    if ty.is_array() || ty.is_str() {
+        PermissionSet::READ | PermissionSet::OFFSET_ADD
+    } else if ty.is_primitive_ty() {
+        PermissionSet::READ
+    } else {
+        panic!("expected an array, str, or primitive type: {ty:?}");
+    }
+}
+
 pub fn visit<'tcx>(
     acx: &AnalysisCtxt<'_, 'tcx>,
     mir: &Body<'tcx>,
@@ -465,6 +480,11 @@ pub fn visit<'tcx>(
         equiv_constraints: Vec::new(),
     };
 
+    for (&constant, const_lty) in &acx.const_ref_tys {
+        tc.constraints
+            .add_all_perms(const_lty.label, const_perms(constant));
+    }
+
     for (bb, bb_data) in mir.basic_blocks().iter_enumerated() {
         for (i, stmt) in bb_data.statements.iter().enumerate() {
             tc.visit_statement(
@@ -484,5 +504,11 @@ pub fn visit<'tcx>(
         );
     }
 
+    for (&constant, const_lty) in &acx.const_ref_tys {
+        let _ptr_id = const_lty.label;
+        let _expected_perms = const_perms(constant);
+        // TODO: check that perms match the expected ones
+    }
+
     (tc.constraints, tc.equiv_constraints)
 }