Implement and test monomorphization

compiler/rustc_smir/src/stable_mir/fold.rs

@@ -3,8 +3,8 @@ use std::ops::ControlFlow;
 use crate::rustc_internal::Opaque;
 
 use super::ty::{
-    Allocation, Binder, Const, ConstDef, ExistentialPredicate, FnSig, GenericArgKind, GenericArgs,
-    Promoted, RigidTy, TermKind, Ty, UnevaluatedConst,
+    Allocation, Binder, Const, ConstDef, ConstantKind, ExistentialPredicate, FnSig, GenericArgKind,
+    GenericArgs, Promoted, RigidTy, TermKind, Ty, TyKind, UnevaluatedConst,
 };
 
 pub trait Folder: Sized {
@@ -205,3 +205,26 @@ impl Foldable for FnSig {
         })
     }
 }
+
+pub enum Never {}
+
+/// In order to instantiate a `Foldable`'s generic parameters with specific arguments,
+/// `GenericArgs` can be used as a `Folder` that replaces all mentions of generic params
+/// with the entries in its list.
+impl Folder for GenericArgs {
+    type Break = Never;
+
+    fn visit_ty(&mut self, ty: &Ty) -> ControlFlow<Self::Break, Ty> {
+        ControlFlow::Continue(match ty.kind() {
+            TyKind::Param(p) => self[p],
+            _ => *ty,
+        })
+    }
+
+    fn fold_const(&mut self, c: &Const) -> ControlFlow<Self::Break, Const> {
+        ControlFlow::Continue(match &c.literal {
+            ConstantKind::Param(p) => self[p.clone()].clone(),
+            _ => c.clone(),
+        })
+    }
+}

compiler/rustc_smir/src/stable_mir/ty.rs

@@ -138,6 +138,22 @@ pub struct ImplDef(pub(crate) DefId);
 #[derive(Clone, Debug)]
 pub struct GenericArgs(pub Vec<GenericArgKind>);
 
+impl std::ops::Index<ParamTy> for GenericArgs {
+    type Output = Ty;
+
+    fn index(&self, index: ParamTy) -> &Self::Output {
+        self.0[index.index as usize].expect_ty()
+    }
+}
+
+impl std::ops::Index<ParamConst> for GenericArgs {
+    type Output = Const;
+
+    fn index(&self, index: ParamConst) -> &Self::Output {
+        self.0[index.index as usize].expect_const()
+    }
+}
+
 #[derive(Clone, Debug)]
 pub enum GenericArgKind {
     Lifetime(Region),
@@ -145,6 +161,28 @@ pub enum GenericArgKind {
     Const(Const),
 }
 
+impl GenericArgKind {
+    /// Panic if this generic argument is not a type, otherwise
+    /// return the type.
+    #[track_caller]
+    pub fn expect_ty(&self) -> &Ty {
+        match self {
+            GenericArgKind::Type(ty) => ty,
+            _ => panic!("{self:?}"),
+        }
+    }
+
+    /// Panic if this generic argument is not a const, otherwise
+    /// return the const.
+    #[track_caller]
+    pub fn expect_const(&self) -> &Const {
+        match self {
+            GenericArgKind::Const(c) => c,
+            _ => panic!("{self:?}"),
+        }
+    }
+}
+
 #[derive(Clone, Debug)]
 pub enum TermKind {
     Type(Ty),