Improve error

Author: rylev

compiler/rustc_ast_lowering/src/lib.rs

@@ -46,7 +46,7 @@ use rustc_ast_pretty::pprust;
 use rustc_data_structures::captures::Captures;
 use rustc_data_structures::fx::FxHashSet;
 use rustc_data_structures::sync::Lrc;
-use rustc_errors::struct_span_err;
+use rustc_errors::{struct_span_err, Applicability};
 use rustc_hir as hir;
 use rustc_hir::def::{DefKind, Namespace, PartialRes, PerNS, Res};
 use rustc_hir::def_id::{DefId, DefIdMap, LocalDefId, CRATE_DEF_ID};
@@ -2785,9 +2785,14 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                 )
             } else {
                 let msg = "trait objects must include the `dyn` keyword";
-                let label = "`dyn` keyword should be added before this trait";
+                let label = "add `dyn` keyword before this trait";
                 let mut err = struct_span_err!(self.sess, span, E0782, "{}", msg,);
-                err.span_label(span, label);
+                err.span_suggestion_verbose(
+                    span.shrink_to_lo(),
+                    label,
+                    String::from("dyn "),
+                    Applicability::MachineApplicable,
+                );
                 err.emit();
             }
         }

compiler/rustc_error_codes/src/error_codes/E0783.md

@@ -2,7 +2,7 @@ The range pattern `...` is no longer allowed.
 
 Erroneous code example:
 
-```edition2021,compile_fail,E782
+```edition2021,compile_fail,E783
 match 2u8 {
     0...9 => println!("Got a number less than 10"), // error!
     _ => println!("Got a number 10 or more"),

src/test/ui/dyn-keyword/dyn-2021-edition-error.stderr

@@ -2,19 +2,34 @@ error[E0782]: trait objects must include the `dyn` keyword
   --> $DIR/dyn-2021-edition-error.rs:6:14
    |
 LL |     let _x: &SomeTrait = todo!();
-   |              ^^^^^^^^^ `dyn` keyword should be added before this trait
+   |              ^^^^^^^^^
+   |
+help: add `dyn` keyword before this trait
+   |
+LL |     let _x: &dyn SomeTrait = todo!();
+   |              ^^^
 
 error[E0782]: trait objects must include the `dyn` keyword
   --> $DIR/dyn-2021-edition-error.rs:3:17
    |
 LL | fn function(x: &SomeTrait, y: Box<SomeTrait>) {
-   |                 ^^^^^^^^^ `dyn` keyword should be added before this trait
+   |                 ^^^^^^^^^
+   |
+help: add `dyn` keyword before this trait
+   |
+LL | fn function(x: &dyn SomeTrait, y: Box<SomeTrait>) {
+   |                 ^^^
 
 error[E0782]: trait objects must include the `dyn` keyword
   --> $DIR/dyn-2021-edition-error.rs:3:35
    |
 LL | fn function(x: &SomeTrait, y: Box<SomeTrait>) {
-   |                                   ^^^^^^^^^ `dyn` keyword should be added before this trait
+   |                                   ^^^^^^^^^
+   |
+help: add `dyn` keyword before this trait
+   |
+LL | fn function(x: &SomeTrait, y: Box<dyn SomeTrait>) {
+   |                                   ^^^
 
 error: aborting due to 3 previous errors