Add help for crate arg when crate name is invalid

compiler/rustc_session/messages.ftl

@@ -45,6 +45,7 @@ session_int_literal_too_large = integer literal is too large
     .note = value exceeds limit of `{$limit}`
 
 session_invalid_character_in_create_name = invalid character `{$character}` in crate name: `{$crate_name}`
+session_invalid_character_in_create_name_help = you can either pass `--crate-name` on the command line or add `#![crate_name="…"]` to set the crate name
 
 session_invalid_float_literal_suffix = invalid suffix `{$suffix}` for float literal
     .label = invalid suffix `{$suffix}`

compiler/rustc_session/src/errors.rs

@@ -199,6 +199,14 @@ pub struct InvalidCharacterInCrateName {
     pub span: Option<Span>,
     pub character: char,
     pub crate_name: Symbol,
+    #[subdiagnostic]
+    pub crate_name_help: Option<InvalidCrateNameHelp>,
+}
+
+#[derive(Subdiagnostic)]
+pub enum InvalidCrateNameHelp {
+    #[help(session_invalid_character_in_create_name_help)]
+    AddCrateName,
 }
 
 #[derive(Subdiagnostic)]

compiler/rustc_session/src/output.rs

@@ -2,7 +2,7 @@
 use crate::config::{CrateType, Input, OutFileName, OutputFilenames, OutputType};
 use crate::errors::{
     CrateNameDoesNotMatch, CrateNameEmpty, CrateNameInvalid, FileIsNotWriteable,
-    InvalidCharacterInCrateName,
+    InvalidCharacterInCrateName, InvalidCrateNameHelp,
 };
 use crate::Session;
 use rustc_ast::{self as ast, attr};
@@ -101,7 +101,16 @@ pub fn validate_crate_name(sess: &Session, s: Symbol, sp: Option<Span>) {
                 continue;
             }
             err_count += 1;
-            sess.emit_err(InvalidCharacterInCrateName { span: sp, character: c, crate_name: s });
+            sess.emit_err(InvalidCharacterInCrateName {
+                span: sp,
+                character: c,
+                crate_name: s,
+                crate_name_help: if sp.is_none() {
+                    Some(InvalidCrateNameHelp::AddCrateName)
+                } else {
+                    None
+                },
+            });
         }
     }