Create E0777 error code for "invalid literal in derive"

Author: GuillaumeGomez

compiler/rustc_error_codes/src/error_codes.rs

@@ -459,6 +459,7 @@ E0773: include_str!("./error_codes/E0773.md"),
 E0774: include_str!("./error_codes/E0774.md"),
 E0775: include_str!("./error_codes/E0775.md"),
 E0776: include_str!("./error_codes/E0776.md"),
+E0777: include_str!("./error_codes/E0777.md"),
 ;
 //  E0006, // merged with E0005
 //  E0008, // cannot bind by-move into a pattern guard

compiler/rustc_error_codes/src/error_codes/E0777.md

@@ -0,0 +1,19 @@
+A literal value was used inside `#[derive]`.
+
+Erroneous code example:
+
+```compile_fail,E0777
+#[derive("Clone")] // error!
+struct Foo;
+```
+
+Only paths to traits are allowed as argument inside `#[derive]`. You can find
+more information about the `#[derive]` attribute in the [Rust Book].
+
+
+```
+#[derive(Clone)] // ok!
+struct Foo;
+```
+
+[Rust Book]: https://doc.rust-lang.org/book/appendix-03-derivable-traits.html

compiler/rustc_expand/src/proc_macro.rs

@@ -5,7 +5,7 @@ use rustc_ast::token;
 use rustc_ast::tokenstream::{TokenStream, TokenTree};
 use rustc_ast::{self as ast, *};
 use rustc_data_structures::sync::Lrc;
-use rustc_errors::{Applicability, ErrorReported};
+use rustc_errors::{struct_span_err, Applicability, ErrorReported};
 use rustc_parse::nt_to_tokenstream;
 use rustc_span::symbol::sym;
 use rustc_span::{Span, DUMMY_SP};
@@ -182,9 +182,14 @@ 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;
-                    cx.struct_span_err(lit.span, "expected path to a trait, found literal")
-                        .help("for example, write `#[derive(Debug)]` for `Debug`")
-                        .emit();
+                    struct_span_err!(
+                        cx.sess,
+                        lit.span,
+                        E0777,
+                        "expected path to a trait, found literal",
+                    )
+                    .help("for example, write `#[derive(Debug)]` for `Debug`")
+                    .emit();
                     None
                 }
                 NestedMetaItem::MetaItem(mi) => Some(mi),

src/test/ui/error-codes/E0777.rs

@@ -0,0 +1,4 @@
+#[derive("Clone")] //~ ERROR E0777
+struct Foo;
+
+fn main() {}

src/test/ui/error-codes/E0777.stderr

@@ -0,0 +1,11 @@
+error[E0777]: expected path to a trait, found literal
+  --> $DIR/E0777.rs:1:10
+   |
+LL | #[derive("Clone")]
+   |          ^^^^^^^
+   |
+   = help: for example, write `#[derive(Debug)]` for `Debug`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0777`.