Tweak type inference for const operands in inline asm

Previously these would be treated like integer literals and default to
`i32` if a type could not be determined. To allow for
forward-compatibility with `str` constants in the future, this PR
changes type inference to use an unbound type variable instead.

The actual type checking is deferred until after typeck where we still
ensure that the final type for the `const` operand is an integer type.

Author: Amanieu

compiler/rustc_hir_analysis/src/check/intrinsicck.rs

@@ -455,10 +455,26 @@ impl<'a, 'tcx> InlineAsmCtxt<'a, 'tcx> {
                         );
                     }
                 }
-                // No special checking is needed for these:
-                // - Typeck has checked that Const operands are integers.
-                // - AST lowering guarantees that SymStatic points to a static.
-                hir::InlineAsmOperand::Const { .. } | hir::InlineAsmOperand::SymStatic { .. } => {}
+                hir::InlineAsmOperand::Const { anon_const } => {
+                    let ty = self.tcx.type_of(anon_const.def_id).instantiate_identity();
+                    match ty.kind() {
+                        ty::Error(_) => {}
+                        ty::Int(_) | ty::Uint(_) => {}
+                        _ => {
+                            self.tcx
+                                .dcx()
+                                .struct_span_err(*op_sp, "invalid type for `const` operand")
+                                .with_span_label(
+                                    self.tcx.def_span(anon_const.def_id),
+                                    format!("is {} `{}`", ty.kind().article(), ty),
+                                )
+                                .with_help("`const` operands must be of an integer type")
+                                .emit();
+                        }
+                    };
+                }
+                // AST lowering guarantees that SymStatic points to a static.
+                hir::InlineAsmOperand::SymStatic { .. } => {}
                 // Check that sym actually points to a function. Later passes
                 // depend on this.
                 hir::InlineAsmOperand::SymFn { anon_const } => {

compiler/rustc_hir_typeck/src/lib.rs

@@ -265,11 +265,10 @@ fn infer_type_if_missing<'tcx>(fcx: &FnCtxt<'_, 'tcx>, node: Node<'tcx>) -> Opti
             Node::Expr(&hir::Expr { kind: hir::ExprKind::InlineAsm(asm), span, .. })
             | Node::Item(&hir::Item { kind: hir::ItemKind::GlobalAsm(asm), span, .. }) => {
                 asm.operands.iter().find_map(|(op, _op_sp)| match op {
-                    hir::InlineAsmOperand::Const { anon_const } if anon_const.hir_id == id => {
-                        // Inline assembly constants must be integers.
-                        Some(fcx.next_int_var())
-                    }
-                    hir::InlineAsmOperand::SymFn { anon_const } if anon_const.hir_id == id => {
+                    hir::InlineAsmOperand::Const { anon_const }
+                    | hir::InlineAsmOperand::SymFn { anon_const }
+                        if anon_const.hir_id == id =>
+                    {
                         Some(fcx.next_ty_var(span))
                     }
                     _ => None,