Add MsgValidEscapers to show list of configured escapers

rinja_derive/src/generator.rs

@@ -15,7 +15,7 @@ use quote::quote;
 use crate::config::WhitespaceHandling;
 use crate::heritage::{Context, Heritage};
 use crate::input::{Source, TemplateInput};
-use crate::{CompileError, CRATE};
+use crate::{CompileError, MsgValidEscapers, CRATE};
 
 pub(crate) struct Generator<'a> {
     // The template input state: original struct AST and attributes
@@ -1427,10 +1427,15 @@ impl<'a> Generator<'a> {
                         .contains(&Cow::Borrowed(name))
                         .then_some(path.as_ref())
                 })
-                .ok_or_else(|| ctx.generate_error(
-                    &format!("invalid escaper '{name}' for `escape` filter"),
-                    node,
-                ))?,
+                .ok_or_else(|| {
+                    ctx.generate_error(
+                        &format!(
+                            "invalid escaper '{name}' for `escape` filter. {}",
+                            MsgValidEscapers(&self.input.config.escapers),
+                        ),
+                        node,
+                    )
+                })?,
             None => self.input.escaper,
         };
         buf.write(format_args!("{CRATE}::filters::escape("));

rinja_derive/src/input.rs

@@ -10,7 +10,7 @@ use quote::ToTokens;
 use syn::punctuated::Punctuated;
 
 use crate::config::{get_template_source, Config};
-use crate::CompileError;
+use crate::{CompileError, MsgValidEscapers};
 
 pub(crate) struct TemplateInput<'a> {
     pub(crate) ast: &'a syn::DeriveInput,
@@ -87,7 +87,10 @@ impl TemplateInput<'_> {
                     .then_some(path.as_ref())
             })
             .ok_or_else(|| {
-                CompileError::no_file_info(format!("no escaper defined for extension '{escaping}'"))
+                CompileError::no_file_info(format!(
+                    "no escaper defined for extension '{escaping}'. {}",
+                    MsgValidEscapers(&config.escapers),
+                ))
             })?;
 
         let mime_type =

rinja_derive/src/lib.rs

@@ -8,7 +8,8 @@ mod input;
 #[cfg(test)]
 mod tests;
 
-use std::collections::HashMap;
+use std::borrow::Cow;
+use std::collections::{BTreeSet, HashMap};
 use std::fmt;
 use std::path::Path;
 
@@ -231,6 +232,30 @@ impl fmt::Display for FileInfo<'_> {
     }
 }
 
+struct MsgValidEscapers<'a>(&'a [(Vec<Cow<'a, str>>, Cow<'a, str>)]);
+
+impl fmt::Display for MsgValidEscapers<'_> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let exts: BTreeSet<&str> = self
+            .0
+            .iter()
+            .flat_map(|(exts, _)| exts)
+            .map(|x| x.as_ref())
+            .collect();
+        f.write_str("The available extensions are:")?;
+        for (i, mut e) in exts.iter().copied().enumerate() {
+            if e.is_empty() {
+                e = r#""" <empty string>"#;
+            }
+            match i {
+                0 => write!(f, " {e}")?,
+                _ => write!(f, ", {e}")?,
+            }
+        }
+        Ok(())
+    }
+}
+
 // This is used by the code generator to decide whether a named filter is part of
 // Rinja or should refer to a local `filters` module. It should contain all the
 // filters shipped with Rinja, even the optional ones (since optional inclusion

testing/tests/ui/no-such-escaper.stderr

@@ -1,4 +1,4 @@
-error: invalid escaper 'latex' for `escape` filter
+error: invalid escaper 'latex' for `escape` filter. The available extensions are: "" <empty string>, htm, html, j2, jinja, jinja2, md, none, svg, txt, xml, yml
  --> LocalEscaper.html:1:38
        "text|escape(\"latex\")}}`."
  --> tests/ui/no-such-escaper.rs:3:10
@@ -8,7 +8,7 @@ error: invalid escaper 'latex' for `escape` filter
   |
   = note: this error originates in the derive macro `Template` (in Nightly builds, run with -Z macro-backtrace for more info)
 
-error: no escaper defined for extension 'tex'
+error: no escaper defined for extension 'tex'. The available extensions are: "" <empty string>, htm, html, j2, jinja, jinja2, md, none, svg, txt, xml, yml
   --> tests/ui/no-such-escaper.rs:12:10
    |
 12 | #[derive(Template)]