Auto merge of #77124 - spastorino:const-exprs-rfc-2920, r=oli-obk

Implement const expressions and patterns (RFC 2920)

cc `@ecstatic-morse` `@lcnr` `@oli-obk` `@petrochenkov`

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

@@ -33,6 +33,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 Constant { span, user_ty, literal }
             }
             ExprKind::StaticRef { literal, .. } => Constant { span, user_ty: None, literal },
+            ExprKind::ConstBlock { value } => Constant { span, user_ty: None, literal: value },
             _ => span_bug!(span, "expression is not a valid constant {:?}", kind),
         }
     }

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

@@ -254,6 +254,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             | ExprKind::Continue { .. }
             | ExprKind::Return { .. }
             | ExprKind::Literal { .. }
+            | ExprKind::ConstBlock { .. }
             | ExprKind::StaticRef { .. }
             | ExprKind::InlineAsm { .. }
             | ExprKind::LlvmInlineAsm { .. }

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

@@ -234,6 +234,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             }
             ExprKind::Yield { .. }
             | ExprKind::Literal { .. }
+            | ExprKind::ConstBlock { .. }
             | ExprKind::StaticRef { .. }
             | ExprKind::Block { .. }
             | ExprKind::Match { .. }

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

@@ -68,7 +68,9 @@ impl Category {
             | ExprKind::ThreadLocalRef(_)
             | ExprKind::LlvmInlineAsm { .. } => Some(Category::Rvalue(RvalueFunc::AsRvalue)),
 
-            ExprKind::Literal { .. } | ExprKind::StaticRef { .. } => Some(Category::Constant),
+            ExprKind::ConstBlock { .. } | ExprKind::Literal { .. } | ExprKind::StaticRef { .. } => {
+                Some(Category::Constant)
+            }
 
             ExprKind::Loop { .. }
             | ExprKind::Block { .. }

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

@@ -454,6 +454,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             | ExprKind::Array { .. }
             | ExprKind::Tuple { .. }
             | ExprKind::Closure { .. }
+            | ExprKind::ConstBlock { .. }
             | ExprKind::Literal { .. }
             | ExprKind::ThreadLocalRef(_)
             | ExprKind::StaticRef { .. } => {

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

@@ -511,6 +511,12 @@ fn make_mirror_unadjusted<'a, 'tcx>(
             inputs: asm.inputs_exprs.to_ref(),
         },
 
+        hir::ExprKind::ConstBlock(ref anon_const) => {
+            let anon_const_def_id = cx.tcx.hir().local_def_id(anon_const.hir_id);
+            let value = ty::Const::from_anon_const(cx.tcx, anon_const_def_id);
+
+            ExprKind::ConstBlock { value }
+        }
         // Now comes the rote stuff:
         hir::ExprKind::Repeat(ref v, ref count) => {
             let count_def_id = cx.tcx.hir().local_def_id(count.hir_id);

compiler/rustc_mir_build/src/thir/mod.rs

@@ -232,6 +232,9 @@ crate enum ExprKind<'tcx> {
     Return {
         value: Option<ExprRef<'tcx>>,
     },
+    ConstBlock {
+        value: &'tcx Const<'tcx>,
+    },
     Repeat {
         value: ExprRef<'tcx>,
         count: &'tcx Const<'tcx>,

compiler/rustc_mir_build/src/thir/pattern/mod.rs

@@ -856,6 +856,11 @@ impl<'a, 'tcx> PatCtxt<'a, 'tcx> {
             *self.lower_path(qpath, expr.hir_id, expr.span).kind
         } else {
             let (lit, neg) = match expr.kind {
+                hir::ExprKind::ConstBlock(ref anon_const) => {
+                    let anon_const_def_id = self.tcx.hir().local_def_id(anon_const.hir_id);
+                    let value = ty::Const::from_anon_const(self.tcx, anon_const_def_id);
+                    return *self.const_to_pat(value, expr.hir_id, expr.span, false).kind;
+                }
                 hir::ExprKind::Lit(ref lit) => (lit, false),
                 hir::ExprKind::Unary(hir::UnOp::UnNeg, ref expr) => {
                     let lit = match expr.kind {