Rollup merge of rust-lang#138536 - makai410:mut-mir-visitor, r=celinval

stable_mir: Add `MutMirVisitor`

Resolves: [rust-lang/project-stable-mir#81](rust-lang/project-stable-mir#81).

I am unsure if we should add a `MutableBody` like Kani did.
Currently, I use `&mut Body` in `MutMirVisitor::visit_body()`.

r? ``````@celinval``````

Author: matthiaskrgr

compiler/stable_mir/src/mir.rs

@@ -5,4 +5,4 @@ pub mod pretty;
 pub mod visit;
 
 pub use body::*;
-pub use visit::MirVisitor;
+pub use visit::{MirVisitor, MutMirVisitor};

compiler/stable_mir/src/mir/body.rs

@@ -77,6 +77,22 @@ impl Body {
         &self.locals[self.arg_count + 1..]
     }
 
+    /// Returns a mutable reference to the local that holds this function's return value.
+    pub(crate) fn ret_local_mut(&mut self) -> &mut LocalDecl {
+        &mut self.locals[RETURN_LOCAL]
+    }
+
+    /// Returns a mutable slice of locals corresponding to this function's arguments.
+    pub(crate) fn arg_locals_mut(&mut self) -> &mut [LocalDecl] {
+        &mut self.locals[1..][..self.arg_count]
+    }
+
+    /// Returns a mutable slice of inner locals for this function.
+    /// Inner locals are those that are neither the return local nor the argument locals.
+    pub(crate) fn inner_locals_mut(&mut self) -> &mut [LocalDecl] {
+        &mut self.locals[self.arg_count + 1..]
+    }
+
     /// Convenience function to get all the locals in this function.
     ///
     /// Locals are typically accessed via the more specific methods `ret_local`,