From d1d94ba026eda45a2b998d68524c14929604c748 Mon Sep 17 00:00:00 2001
From: Guillaume Gomez <guillaume1.gomez@gmail.com>
Date: Fri, 2 Oct 2020 13:52:03 +0200
Subject: [PATCH] Improve E0777 help message

---
 compiler/rustc_expand/src/proc_macro.rs | 17 +++++++++++++----
 src/test/ui/error-codes/E0777.rs        |  3 +++
 src/test/ui/error-codes/E0777.stderr    | 12 +++++++++++-
 3 files changed, 27 insertions(+), 5 deletions(-)

diff --git a/compiler/rustc_expand/src/proc_macro.rs b/compiler/rustc_expand/src/proc_macro.rs
index 320d8fdef4d..4c95f19b96d 100644
--- a/compiler/rustc_expand/src/proc_macro.rs
+++ b/compiler/rustc_expand/src/proc_macro.rs
@@ -6,6 +6,7 @@ use rustc_ast::tokenstream::{TokenStream, TokenTree};
 use rustc_ast::{self as ast, *};
 use rustc_data_structures::sync::Lrc;
 use rustc_errors::{struct_span_err, Applicability, ErrorReported};
+use rustc_lexer::is_ident;
 use rustc_parse::nt_to_tokenstream;
 use rustc_span::symbol::sym;
 use rustc_span::{Span, DUMMY_SP};
@@ -182,14 +183,22 @@ crate fn collect_derives(cx: &mut ExtCtxt<'_>, attrs: &mut Vec<ast::Attribute>)
             .filter_map(|nmi| match nmi {
                 NestedMetaItem::Literal(lit) => {
                     error_reported_filter_map = true;
-                    struct_span_err!(
+                    let mut err = struct_span_err!(
                         cx.sess,
                         lit.span,
                         E0777,
                         "expected path to a trait, found literal",
-                    )
-                    .help("for example, write `#[derive(Debug)]` for `Debug`")
-                    .emit();
+                    );
+                    let token = lit.token.to_string();
+                    if token.starts_with('"')
+                        && token.len() > 2
+                        && is_ident(&token[1..token.len() - 1])
+                    {
+                        err.help(&format!("try using `#[derive({})]`", &token[1..token.len() - 1]));
+                    } else {
+                        err.help("for example, write `#[derive(Debug)]` for `Debug`");
+                    }
+                    err.emit();
                     None
                 }
                 NestedMetaItem::MetaItem(mi) => Some(mi),
diff --git a/src/test/ui/error-codes/E0777.rs b/src/test/ui/error-codes/E0777.rs
index ab633e4f535..ff70f736866 100644
--- a/src/test/ui/error-codes/E0777.rs
+++ b/src/test/ui/error-codes/E0777.rs
@@ -1,4 +1,7 @@
 #[derive("Clone")] //~ ERROR E0777
+#[derive("Clone
+")]
+//~^^ ERROR E0777
 struct Foo;
 
 fn main() {}
diff --git a/src/test/ui/error-codes/E0777.stderr b/src/test/ui/error-codes/E0777.stderr
index b356ea508f4..ea73c58993e 100644
--- a/src/test/ui/error-codes/E0777.stderr
+++ b/src/test/ui/error-codes/E0777.stderr
@@ -4,8 +4,18 @@ error[E0777]: expected path to a trait, found literal
 LL | #[derive("Clone")]
    |          ^^^^^^^
    |
+   = help: try using `#[derive(Clone)]`
+
+error[E0777]: expected path to a trait, found literal
+  --> $DIR/E0777.rs:2:10
+   |
+LL |   #[derive("Clone
+   |  __________^
+LL | | ")]
+   | |_^
+   |
    = help: for example, write `#[derive(Debug)]` for `Debug`
 
-error: aborting due to previous error
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0777`.