improve diagnostic for raw pointer field access with ->

Author: Kivooeo

compiler/rustc_hir_typeck/src/expr.rs

@@ -3278,21 +3278,24 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     }
 
     fn suggest_first_deref_field(&self, err: &mut Diag<'_>, base: &hir::Expr<'_>, field: Ident) {
-        err.span_label(field.span, "unknown field");
-        let val = if let Ok(base) = self.tcx.sess.source_map().span_to_snippet(base.span)
-            && base.len() < 20
-        {
-            format!("`{base}`")
-        } else {
-            "the value".to_string()
-        };
-        err.multipart_suggestion(
-            format!("{val} is a raw pointer; try dereferencing it"),
-            vec![
-                (base.span.shrink_to_lo(), "(*".to_string()),
-                (base.span.shrink_to_hi(), ")".to_string()),
-            ],
-            Applicability::MaybeIncorrect,
+        let full_span = base.span.to(field.span);
+
+        let base_snippet = self
+            .tcx
+            .sess
+            .source_map()
+            .span_to_snippet(base.span)
+            .unwrap_or_else(|_| "ptr".to_string());
+
+        let replacement = format!("(*{}).{}", base_snippet, field.name);
+
+        err.span_label(full_span, "unknown field access via raw pointer");
+
+        err.span_suggestion_verbose(
+            full_span,
+            "`->` is not valid in Rust; use `.` on a dereferenced raw pointer instead",
+            replacement,
+            Applicability::MachineApplicable,
         );
     }
 

compiler/rustc_parse/messages.ftl

@@ -248,7 +248,7 @@ parse_expected_trait_in_trait_impl_found_type = expected a trait, found type
 
 parse_expr_rarrow_call = `->` used for field access or method call
     .suggestion = try using `.` instead
-    .help = the `.` operator will dereference the value if needed
+    .help = the `.` operator will dereference the value if needed (if value is raw pointer you have to derefernce it by yourself)
 
 parse_extern_crate_name_with_dashes = crate name using dashes are not valid in `extern crate` statements
     .label = dash-separated idents are not valid