Display help message when fluent arg was referenced incorrectly

The fluent argument syntax is a little special and easy to get wrong, so
we emit a small help message when someone gets it wrong.

Example:
```
parser_mismatched_closing_delimiter = mismatched closing delimiter: `${delimiter}`
```
panics with
```
thread 'rustc' panicked at 'Encountered errors while formatting message for `parser_mismatched_closing_delimiter`
help: Argument `delimiter` exists but was not referenced correctly. Try using `{$delimiter}` instead
attr: `None`
args: `FluentArgs([("delimiter", String("}"))])`
errors: `[ResolverError(Reference(Message { id: "delimiter", attribute: None }))]`', compiler/rustc_errors/src/translation.rs:123:21
```

compiler/rustc_error_messages/src/lib.rs

@@ -30,7 +30,8 @@ use intl_memoizer::concurrent::IntlLangMemoizer;
 #[cfg(not(parallel_compiler))]
 use intl_memoizer::IntlLangMemoizer;
 
-pub use fluent_bundle::{FluentArgs, FluentError, FluentValue};
+pub use fluent_bundle::{self, FluentArgs, FluentError, FluentValue};
+
 pub use unic_langid::{langid, LanguageIdentifier};
 
 // Generates `DEFAULT_LOCALE_RESOURCES` static and `fluent_generated` module.

compiler/rustc_errors/src/translation.rs

@@ -1,7 +1,10 @@
 use crate::snippet::Style;
 use crate::{DiagnosticArg, DiagnosticMessage, FluentBundle};
 use rustc_data_structures::sync::Lrc;
-use rustc_error_messages::FluentArgs;
+use rustc_error_messages::{
+    fluent_bundle::resolver::errors::{ReferenceKind, ResolverError},
+    FluentArgs, FluentError,
+};
 use std::borrow::Cow;
 
 /// Convert diagnostic arguments (a rustc internal type that exists to implement
@@ -102,14 +105,31 @@ pub trait Translate {
             .or_else(|| translate_with_bundle(self.fallback_fluent_bundle()))
             .map(|(translated, errs)| {
                 // Always bail out for errors with the fallback bundle.
-                assert!(
-                    errs.is_empty(),
-                    "identifier: {:?}, attr: {:?}, args: {:?}, errors: {:?}",
-                    identifier,
-                    attr,
-                    args,
-                    errs
-                );
+
+                let mut help_messages = vec![];
+
+                if !errs.is_empty() {
+                    for error in &errs {
+                        match error {
+                            FluentError::ResolverError(ResolverError::Reference(
+                                ReferenceKind::Message { id, .. },
+                            )) if args.iter().any(|(arg_id, _)| arg_id == id) => {
+                                help_messages.push(format!("Argument `{id}` exists but was not referenced correctly. Try using `{{${id}}}` instead"));
+                            }
+                            _ => {}
+                        }
+                    }
+
+                    panic!(
+                        "Encountered errors while formatting message for `{identifier}`\n\
+                        help: {}\n\
+                        attr: `{attr:?}`\n\
+                        args: `{args:?}`\n\
+                        errors: `{errs:?}`",
+                        help_messages.join("\nhelp: ")
+                    );
+                }
+
                 translated
             })
             .expect("failed to find message in primary or fallback fluent bundles")