Rollup merge of #117095 - klinvill:smir-fn-arg-count, r=oli-obk

Add way to differentiate argument locals from other locals in Stable MIR

This PR resolves rust-lang/project-stable-mir#47 which request a way to differentiate argument locals in a SMIR `Body` from other locals.

Specifically, this PR exposes the `arg_count` field from the MIR `Body`. However, I'm opening this as a draft PR because I think there are a few outstanding questions on how this information should be exposed and described. Namely:

- Is exposing `arg_count` the best way to surface this information to SMIR users? Would it be better to leave `arg_count` as a private field and add public methods (e.g. `fn arguments(&self) -> Iter<'_, LocalDecls>`) that may use the underlying `arg_count` info from the MIR body, but expose this information to users in a more convenient form? Or is it best to stick close to the current MIR convention?
- If the answer to the above point is to stick with the current MIR convention (`arg_count`), is it reasonable to also commit to sticking to the current MIR convention that the first local is always the return local, while the next `arg_count` locals are always the (in-order) argument locals?
- Should `Body` in SMIR only represent function bodies (as implied by the comment I added)? That seems to be the current case in MIR, but should this restriction always be the case for SMIR?

r? `@celinval`
r? `@oli-obk`

compiler/rustc_smir/src/rustc_smir/mod.rs

@@ -287,9 +287,8 @@ impl<'tcx> Stable<'tcx> for mir::Body<'tcx> {
     type T = stable_mir::mir::Body;
 
     fn stable(&self, tables: &mut Tables<'tcx>) -> Self::T {
-        stable_mir::mir::Body {
-            blocks: self
-                .basic_blocks
+        stable_mir::mir::Body::new(
+            self.basic_blocks
                 .iter()
                 .map(|block| stable_mir::mir::BasicBlock {
                     terminator: block.terminator().stable(tables),
@@ -300,15 +299,15 @@ impl<'tcx> Stable<'tcx> for mir::Body<'tcx> {
                         .collect(),
                 })
                 .collect(),
-            locals: self
-                .local_decls
+            self.local_decls
                 .iter()
                 .map(|decl| stable_mir::mir::LocalDecl {
                     ty: decl.ty.stable(tables),
                     span: decl.source_info.span.stable(tables),
                 })
                 .collect(),
-        }
+            self.arg_count,
+        )
     }
 }