Add arg_count field to Body in Stable MIR

This field allows SMIR consumers to identify which locals correspond to
argument locals. It simply exposes the arg_count field from the MIR
representation.

Author: klinvill

compiler/rustc_smir/src/rustc_smir/mod.rs

@@ -302,6 +302,7 @@ impl<'tcx> Stable<'tcx> for mir::Body<'tcx> {
                     span: decl.source_info.span.stable(tables),
                 })
                 .collect(),
+            arg_count: self.arg_count,
         }
     }
 }

compiler/stable_mir/src/mir/body.rs

@@ -2,10 +2,20 @@ use crate::ty::{AdtDef, ClosureDef, Const, CoroutineDef, GenericArgs, Movability
 use crate::Opaque;
 use crate::{ty::Ty, Span};
 
+/// The SMIR representation of a single function.
 #[derive(Clone, Debug)]
 pub struct Body {
     pub blocks: Vec<BasicBlock>,
+
+    /// Declarations of locals.
+    ///
+    /// The first local is the return value pointer, followed by `arg_count`
+    /// locals for the function arguments, followed by any user-declared
+    /// variables and temporaries.
     pub locals: LocalDecls,
+
+    /// The number of arguments this function takes.
+    pub arg_count: usize,
 }
 
 type LocalDecls = Vec<LocalDecl>;