Merge #1279

1279: Implement type checking for the `IfLet` expression. r=philberty a=antego

Addresses #1177.

This change implements the type check for the `IfLet` expression. This is the adapted type checking code from the `MatchExpr`.

Tested on this code
```
enum E {
    X(u8),
}

fn main() -> i32 {
    let mut res = 0;
    let v = E::X(4);
    if let E::X(n) = v {
        res = n;
    }

    0
}
```
Compilation finishes without errors.

Next is the implementation of the code generation but I'll need help to find where it's located.

Co-authored-by: antego <antego@users.noreply.github.com>

2 files changed, 36 insertions, 0 deletions

diff --git a/gcc/rust/hir/tree/rust-hir-expr.h b/gcc/rust/hir/tree/rust-hir-expr.h
index f6f035e..789ad78 100644
--- a/gcc/rust/hir/tree/rust-hir-expr.h
+++ b/gcc/rust/hir/tree/rust-hir-expr.h
@@ -3477,6 +3477,19 @@ public:
   void accept_vis (HIRFullVisitor &vis) override;
   void accept_vis (HIRExpressionVisitor &vis) override;
 
+  std::unique_ptr<Expr> &get_scrutinee_expr ()
+  {
+    rust_assert (value != nullptr);
+    return value;
+  }
+
+  std::vector<std::unique_ptr<Pattern> > &get_patterns ()
+  {
+    return match_arm_patterns;
+  }
+
+  BlockExpr *get_if_block () { return if_block.get (); }
+
   ExprType get_expression_type () const final override
   {
     return ExprType::IfLet;
diff --git a/gcc/rust/typecheck/rust-hir-type-check-expr.h b/gcc/rust/typecheck/rust-hir-type-check-expr.h
index b20a048..411d45d 100644
--- a/gcc/rust/typecheck/rust-hir-type-check-expr.h
+++ b/gcc/rust/typecheck/rust-hir-type-check-expr.h
@@ -703,6 +703,29 @@ public:
       infered = if_blk_resolved->unify (else_blk_resolved);
   }
 
+  void visit (HIR::IfLetExpr &expr) override
+  {
+    // this needs to perform a least upper bound coercion on the blocks and then
+    // unify the scruintee and arms
+    TyTy::BaseType *scrutinee_tyty
+      = TypeCheckExpr::Resolve (expr.get_scrutinee_expr ().get (), false);
+
+    for (auto &pattern : expr.get_patterns ())
+      {
+	TyTy::BaseType *kase_arm_ty
+	  = TypeCheckPattern::Resolve (pattern.get (), scrutinee_tyty);
+
+	TyTy::BaseType *checked_kase = scrutinee_tyty->unify (kase_arm_ty);
+	if (checked_kase->get_kind () == TyTy::TypeKind::ERROR)
+	  return;
+      }
+
+    TypeCheckExpr::Resolve (expr.get_if_block (), inside_loop);
+
+    infered
+      = TyTy::TupleType::get_unit_type (expr.get_mappings ().get_hirid ());
+  }
+
   void visit (HIR::BlockExpr &expr) override;
 
   void visit (HIR::UnsafeBlockExpr &expr) override