Better error message for unsized pointers

compiler/rustc_hir_analysis/src/check/intrinsicck.rs

@@ -27,6 +27,7 @@ enum NonAsmTypeReason<'tcx> {
     UnevaluatedSIMDArrayLength(DefId, ty::Const<'tcx>),
     Invalid(Ty<'tcx>),
     InvalidElement(DefId, Ty<'tcx>),
+    NotSizedPtr(Ty<'tcx>),
 }
 
 impl<'a, 'tcx> InlineAsmCtxt<'a, 'tcx> {
@@ -81,7 +82,13 @@ impl<'a, 'tcx> InlineAsmCtxt<'a, 'tcx> {
             ty::Float(FloatTy::F64) => Ok(InlineAsmType::F64),
             ty::Float(FloatTy::F128) => Ok(InlineAsmType::F128),
             ty::FnPtr(..) => Ok(asm_ty_isize),
-            ty::RawPtr(ty, _) if self.is_thin_ptr_ty(ty) => Ok(asm_ty_isize),
+            ty::RawPtr(elem_ty, _) => {
+                if self.is_thin_ptr_ty(elem_ty) {
+                    Ok(asm_ty_isize)
+                } else {
+                    Err(NonAsmTypeReason::NotSizedPtr(ty))
+                }
+            }
             ty::Adt(adt, args) if adt.repr().simd() => {
                 let fields = &adt.non_enum_variant().fields;
                 let field = &fields[FieldIdx::ZERO];
@@ -187,6 +194,16 @@ impl<'a, 'tcx> InlineAsmCtxt<'a, 'tcx> {
                             can be used as arguments for inline assembly",
                         ).emit();
                     }
+                    NonAsmTypeReason::NotSizedPtr(ty) => {
+                        let msg = format!(
+                            "cannot use value of unsized pointer type `{ty}` for inline assembly"
+                        );
+                        self.tcx
+                            .dcx()
+                            .struct_span_err(expr.span, msg)
+                            .with_note("only sized pointers can be used in inline assembly")
+                            .emit();
+                    }
                     NonAsmTypeReason::InvalidElement(did, ty) => {
                         let msg = format!(
                             "cannot use SIMD vector with element type `{ty}` for inline assembly"

tests/ui/asm/conditionally-sized-ptr-fail.rs

@@ -0,0 +1,19 @@
+//@ needs-asm-support
+
+use std::arch::asm;
+
+fn _f<T: ?Sized>(p: *mut T) {
+    unsafe {
+        asm!("/* {} */", in(reg) p);
+        //~^ ERROR cannot use value of unsized pointer type `*mut T` for inline assembly
+    }
+}
+
+fn _g(p: *mut [u8]) {
+    unsafe {
+        asm!("/* {} */", in(reg) p);
+        //~^ ERROR cannot use value of unsized pointer type `*mut [u8]` for inline assembly
+    }
+}
+
+fn main() {}

tests/ui/asm/conditionally-sized-ptr-fail.stderr

@@ -0,0 +1,18 @@
+error: cannot use value of unsized pointer type `*mut T` for inline assembly
+  --> $DIR/conditionally-sized-ptr-fail.rs:7:34
+   |
+LL |         asm!("/* {} */", in(reg) p);
+   |                                  ^
+   |
+   = note: only sized pointers can be used in inline assembly
+
+error: cannot use value of unsized pointer type `*mut [u8]` for inline assembly
+  --> $DIR/conditionally-sized-ptr-fail.rs:14:34
+   |
+LL |         asm!("/* {} */", in(reg) p);
+   |                                  ^
+   |
+   = note: only sized pointers can be used in inline assembly
+
+error: aborting due to 2 previous errors
+