thir: Add Become expression kind

Note that this doesn't implement `become` correctly, there is still no
early drop or tail calls involved. Moreother, it seems like MIR simply
can not represent tail calls in its current form so I'll probably need
to add new statement kinds or smh. to it in order to represent them...

Author: WaffleLapkin

compiler/rustc_middle/src/thir.rs

@@ -410,6 +410,10 @@ pub enum ExprKind<'tcx> {
     Return {
         value: Option<ExprId>,
     },
+    /// A `become` expression.
+    Become {
+        value: ExprId,
+    },
     /// An inline `const` block, e.g. `const {}`.
     ConstBlock {
         did: DefId,

compiler/rustc_middle/src/thir/visit.rs

@@ -100,6 +100,7 @@ pub fn walk_expr<'a, 'tcx: 'a, V: Visitor<'a, 'tcx>>(visitor: &mut V, expr: &Exp
                 visitor.visit_expr(&visitor.thir()[value])
             }
         }
+        Become { value } => visitor.visit_expr(&visitor.thir()[value]),
         ConstBlock { did: _, substs: _ } => {}
         Repeat { value, count: _ } => {
             visitor.visit_expr(&visitor.thir()[value]);

compiler/rustc_mir_build/src/build/expr/as_place.rs

@@ -549,6 +549,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             | ExprKind::Break { .. }
             | ExprKind::Continue { .. }
             | ExprKind::Return { .. }
+            | ExprKind::Become { .. }
             | ExprKind::Literal { .. }
             | ExprKind::NamedConst { .. }
             | ExprKind::NonHirLiteral { .. }

compiler/rustc_mir_build/src/build/expr/as_rvalue.rs

@@ -532,6 +532,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             | ExprKind::Break { .. }
             | ExprKind::Continue { .. }
             | ExprKind::Return { .. }
+            | ExprKind::Become { .. }
             | ExprKind::InlineAsm { .. }
             | ExprKind::PlaceTypeAscription { .. }
             | ExprKind::ValueTypeAscription { .. } => {

compiler/rustc_mir_build/src/build/expr/category.rs

@@ -82,7 +82,8 @@ impl Category {
             | ExprKind::Block { .. }
             | ExprKind::Break { .. }
             | ExprKind::Continue { .. }
-            | ExprKind::Return { .. } =>
+            | ExprKind::Return { .. }
+            | ExprKind::Become { .. } =>
             // FIXME(#27840) these probably want their own
             // category, like "nonterminating"
             {

compiler/rustc_mir_build/src/build/expr/into.rs

@@ -489,7 +489,10 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 block.unit()
             }
 
-            ExprKind::Continue { .. } | ExprKind::Break { .. } | ExprKind::Return { .. } => {
+            ExprKind::Continue { .. }
+            | ExprKind::Break { .. }
+            | ExprKind::Return { .. }
+            | ExprKind::Become { .. } => {
                 unpack!(block = this.stmt_expr(block, expr, None));
                 // No assign, as these have type `!`.
                 block.unit()

compiler/rustc_mir_build/src/build/expr/stmt.rs

@@ -99,6 +99,12 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 BreakableTarget::Return,
                 source_info,
             ),
+            ExprKind::Become { value } => this.break_scope(
+                block,
+                Some(&this.thir[value]),
+                BreakableTarget::Become,
+                source_info,
+            ),
             _ => {
                 assert!(
                     statement_scope.is_some(),

compiler/rustc_mir_build/src/build/scope.rs

@@ -182,6 +182,7 @@ pub(crate) enum BreakableTarget {
     Continue(region::Scope),
     Break(region::Scope),
     Return,
+    Become,
 }
 
 rustc_index::newtype_index! {
@@ -627,10 +628,10 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 .unwrap_or_else(|| span_bug!(span, "no enclosing breakable scope found"))
         };
         let (break_index, destination) = match target {
-            BreakableTarget::Return => {
+            BreakableTarget::Return | BreakableTarget::Become => {
                 let scope = &self.scopes.breakable_scopes[0];
                 if scope.break_destination != Place::return_place() {
-                    span_bug!(span, "`return` in item with no return scope");
+                    span_bug!(span, "`return` or `become` in item with no return scope");
                 }
                 (0, Some(scope.break_destination))
             }
@@ -668,6 +669,8 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             (None, None) => {}
         }
 
+        // FIXME(explicit_tail_calls): this should drop stuff early for `become`
+
         let region_scope = self.scopes.breakable_scopes[break_index].region_scope;
         let scope_index = self.scopes.scope_index(region_scope, span);
         let drops = if destination.is_some() {

compiler/rustc_mir_build/src/check_unsafety.rs

@@ -316,6 +316,7 @@ impl<'a, 'tcx> Visitor<'a, 'tcx> for UnsafetyVisitor<'a, 'tcx> {
             | ExprKind::Closure { .. }
             | ExprKind::Continue { .. }
             | ExprKind::Return { .. }
+            | ExprKind::Become { .. }
             | ExprKind::Yield { .. }
             | ExprKind::Loop { .. }
             | ExprKind::Let { .. }

compiler/rustc_mir_build/src/thir/cx/expr.rs

@@ -694,8 +694,8 @@ impl<'tcx> Cx<'tcx> {
 
                 ExprKind::Repeat { value: self.mirror_expr(v), count: *count }
             }
-            hir::ExprKind::Ret(ref v) => ExprKind::Return { value: v.map(|v| self.mirror_expr(v)) },
-            hir::ExprKind::Become(_call) => unimplemented!("lol (lmao)"),
+            hir::ExprKind::Ret(v) => ExprKind::Return { value: v.map(|v| self.mirror_expr(v)) },
+            hir::ExprKind::Become(call) => ExprKind::Become { value: self.mirror_expr(call) },
             hir::ExprKind::Break(dest, ref value) => match dest.target_id {
                 Ok(target_id) => ExprKind::Break {
                     label: region::Scope { id: target_id.local_id, data: region::ScopeData::Node },

compiler/rustc_mir_build/src/thir/print.rs

@@ -421,6 +421,12 @@ impl<'a, 'tcx> ThirPrinter<'a, 'tcx> {
 
                 print_indented!(self, "}", depth_lvl);
             }
+            Become { value } => {
+                print_indented!(self, "Return {", depth_lvl);
+                print_indented!(self, "value:", depth_lvl + 1);
+                self.print_expr(*value, depth_lvl + 2);
+                print_indented!(self, "}", depth_lvl);
+            }
             ConstBlock { did, substs } => {
                 print_indented!(self, "ConstBlock {", depth_lvl);
                 print_indented!(self, format!("did: {:?}", did), depth_lvl + 1);

compiler/rustc_ty_utils/src/consts.rs

@@ -240,7 +240,8 @@ fn recurse_build<'tcx>(
         ExprKind::Assign { .. } | ExprKind::AssignOp { .. } => {
             error(GenericConstantTooComplexSub::AssignNotSupported(node.span))?
         }
-        ExprKind::Closure { .. } | ExprKind::Return { .. } => {
+        // FIXME(explicit_tail_calls): maybe get `become` a new error
+        ExprKind::Closure { .. } | ExprKind::Return { .. } | ExprKind::Become { .. } => {
             error(GenericConstantTooComplexSub::ClosureAndReturnNotSupported(node.span))?
         }
         // let expressions imply control flow
@@ -336,6 +337,7 @@ impl<'a, 'tcx> IsThirPolymorphic<'a, 'tcx> {
             | thir::ExprKind::Break { .. }
             | thir::ExprKind::Continue { .. }
             | thir::ExprKind::Return { .. }
+            | thir::ExprKind::Become { .. }
             | thir::ExprKind::Array { .. }
             | thir::ExprKind::Tuple { .. }
             | thir::ExprKind::Adt(_)