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
+                },
+            });
         }
     }
 

src/tools/tidy/src/ui_tests.rs

@@ -116,7 +116,9 @@ pub fn check(path: &Path, bad: &mut bool) {
                 // must strip all of them.
                 let testname =
                     file_path.file_name().unwrap().to_str().unwrap().split_once('.').unwrap().0;
-                if !file_path.with_file_name(testname).with_extension("rs").exists() {
+                if !file_path.with_file_name(testname).with_extension("rs").exists()
+                    && !testname.contains("ignore-tidy")
+                {
                     tidy_error!(bad, "Stray file with UI testing output: {:?}", file_path);
                 }
 

tests/ui/command/need-crate-arg-ignore-tidy.x.rs

@@ -0,0 +1,2 @@
+// issue: 113981
+pub fn main() {}

tests/ui/command/need-crate-arg-ignore-tidy.x.stderr

@@ -0,0 +1,6 @@
+error: invalid character `'.'` in crate name: `need_crate_arg_ignore_tidy.x`
+   |
+   = help: you can either pass `--crate-name` on the command line or add `#![crate_name="…"]` to set the crate name
+
+error: aborting due to previous error
+