feat(errors): Report available tags blocks

Fixes #183

liquid-compiler/Cargo.toml

@@ -15,6 +15,7 @@ travis-ci = { repository = "cobalt-org/liquid-rust" }
 appveyor  = { repository = "johannhof/liquid-rust" }
 
 [dependencies]
+itertools = "0.7.0"
 pest = "2.0"
 pest_derive = "2.0"
 

liquid-compiler/src/lib.rs

@@ -1,7 +1,7 @@
 extern crate liquid_error;
 extern crate liquid_interpreter;
 extern crate liquid_value;
-
+extern crate itertools;
 extern crate pest;
 #[macro_use]
 extern crate pest_derive;

liquid-compiler/src/parser.rs

@@ -427,7 +427,17 @@ impl<'a> Tag<'a> {
                 },
                 position,
             );
-            Err(convert_pest_error(pest_error))
+            let mut all_tags: Vec<_> = options.tags.plugin_names().collect();
+            all_tags.sort_unstable();
+            let all_tags = itertools::join(all_tags, ", ");
+            let mut all_blocks: Vec<_> = options.blocks.plugin_names().collect();
+            all_blocks.sort_unstable();
+            let all_blocks = itertools::join(all_blocks, ", ");
+            let error = convert_pest_error(pest_error)
+                .context("requested", name.to_owned())
+                .context("available tags", all_tags)
+                .context("available blocks", all_blocks);
+            Err(error)
         }
     }
 }

liquid-interpreter/src/context.rs

@@ -122,7 +122,9 @@ impl<'g> Context<'g> {
                 f
             })
             .ok_or_else(|| {
-                let available = itertools::join(self.filters.plugin_names(), ", ");
+                let mut available: Vec<_> = self.filters.plugin_names().collect();
+                available.sort_unstable();
+                let available = itertools::join(available, ", ");
                 Error::with_msg("Unknown filter")
                     .context("requested filter", name.to_owned())
                     .context("available filters", available)