Rollup merge of #83916 - Amanieu:asm_anonconst, r=petrochenkov

Use AnonConst for asm! constants

This replaces the old system which used explicit promotion. See #83169 for more background.

The syntax for `const` operands is still the same as before: `const <expr>`.

Fixes #83169

Because the implementation is heavily based on inline consts, we suffer from the same issues:
- We lose the ability to use expressions derived from generics. See the deleted tests in `src/test/ui/asm/const.rs`.
- We are hitting the same ICEs as inline consts, for example #78174. It is unlikely that we will be able to stabilize this before inline consts are stabilized.

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

@@ -355,9 +355,11 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                                 }),
                             }
                         }
-                        thir::InlineAsmOperand::Const { expr } => mir::InlineAsmOperand::Const {
-                            value: unpack!(block = this.as_local_operand(block, expr)),
-                        },
+                        thir::InlineAsmOperand::Const { value, span } => {
+                            mir::InlineAsmOperand::Const {
+                                value: box Constant { span, user_ty: None, literal: value.into() },
+                            }
+                        }
                         thir::InlineAsmOperand::SymFn { expr } => {
                             mir::InlineAsmOperand::SymFn { value: box this.as_constant(expr) }
                         }

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

@@ -503,8 +503,12 @@ impl<'thir, 'tcx> Cx<'thir, 'tcx> {
                             in_expr: self.mirror_expr(in_expr),
                             out_expr: out_expr.as_ref().map(|expr| self.mirror_expr(expr)),
                         },
-                        hir::InlineAsmOperand::Const { ref expr } => {
-                            InlineAsmOperand::Const { expr: self.mirror_expr(expr) }
+                        hir::InlineAsmOperand::Const { 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);
+                            let span = self.tcx.hir().span(anon_const.hir_id);
+
+                            InlineAsmOperand::Const { value, span }
                         }
                         hir::InlineAsmOperand::Sym { ref expr } => {
                             let qpath = match expr.kind {

compiler/rustc_mir_build/src/thir/mod.rs

@@ -374,7 +374,8 @@ pub enum InlineAsmOperand<'thir, 'tcx> {
         out_expr: Option<&'thir Expr<'thir, 'tcx>>,
     },
     Const {
-        expr: &'thir Expr<'thir, 'tcx>,
+        value: &'tcx Const<'tcx>,
+        span: Span,
     },
     SymFn {
         expr: &'thir Expr<'thir, 'tcx>,