enhance diagnostic emitted for E0617 in missing cast for variadic arg

Author: HTGAzureX1212

compiler/rustc_hir_analysis/src/structured_errors/missing_cast_for_variadic_arg.rs

@@ -1,5 +1,5 @@
 use crate::{errors, structured_errors::StructuredDiagnostic};
-use rustc_errors::{codes::*, DiagnosticBuilder, ErrCode};
+use rustc_errors::{codes::*, Applicability, DiagnosticBuilder, ErrCode};
 use rustc_middle::ty::{Ty, TypeVisitableExt};
 use rustc_session::Session;
 use rustc_span::Span;
@@ -41,6 +41,28 @@ impl<'tcx> StructuredDiagnostic<'tcx> for MissingCastForVariadicArg<'tcx, '_> {
             err.downgrade_to_delayed_bug();
         }
 
+        if let Ok(snippet) = self.sess.source_map().span_to_snippet(self.span) {
+            if self.ty.is_fn() {
+                err.span_suggestion(
+                    self.span,
+                    "cast the value into a function pointer",
+                    format!("{} as {}", snippet, self.cast_ty),
+                    Applicability::MachineApplicable,
+                )
+                .help("a function item is zero-sized and needs to be casted into a function pointer to be used in FFI")
+                .note("for more information on function items, visit https://doc.rust-lang.org/reference/types/function-item.html");
+            } else {
+                err.span_suggestion(
+                    self.span,
+                    &format!("cast the value to `{}`", self.cast_ty),
+                    format!("{} as {}", snippet, self.cast_ty),
+                    Applicability::MachineApplicable,
+                );
+            }
+        } else {
+            err.help(&format!("cast the value into `{}`", self.cast_ty));
+        }
+
         err
     }
 

compiler/rustc_middle/src/ty/print/pretty.rs

@@ -662,8 +662,7 @@ pub trait PrettyPrinter<'tcx>: Printer<'tcx> + fmt::Write {
                 if with_no_queries() {
                     p!(print_def_path(def_id, args));
                 } else {
-                    let sig = self.tcx().fn_sig(def_id).instantiate(self.tcx(), args);
-                    p!(print(sig), " {{", print_value_path(def_id, args), "}}");
+                    p!(print(def_id), " {{individual function type for", print_value_path(def_id, args), "}}");
                 }
             }
             ty::FnPtr(ref bare_fn) => p!(print(bare_fn)),