From 4d747128eb1e1c0da7432b071107d29721e19a04 Mon Sep 17 00:00:00 2001
From: Amanieu d'Antras <amanieu@gmail.com>
Date: Sun, 26 May 2024 00:11:32 +0100
Subject: [PATCH] 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.
---
 .../src/check/intrinsicck.rs                  | 24 +++++++++++++++----
 compiler/rustc_hir_typeck/src/lib.rs          |  9 ++++---
 2 files changed, 24 insertions(+), 9 deletions(-)

diff --git a/compiler/rustc_hir_analysis/src/check/intrinsicck.rs b/compiler/rustc_hir_analysis/src/check/intrinsicck.rs
index 2e965c59ebb..9286d3d83bf 100644
--- a/compiler/rustc_hir_analysis/src/check/intrinsicck.rs
+++ b/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 } => {
diff --git a/compiler/rustc_hir_typeck/src/lib.rs b/compiler/rustc_hir_typeck/src/lib.rs
index 2c793664509..ebb42e76670 100644
--- a/compiler/rustc_hir_typeck/src/lib.rs
+++ b/compiler/rustc_hir_typeck/src/lib.rs
@@ -264,11 +264,10 @@ 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,