templater: add helper method that unwraps Option<T> property

I'll add a few more optional property types, and I don't want to duplicate the
error message. Type names are capitalized for consistency.

cli/src/commit_templater.rs

@@ -38,7 +38,7 @@ use crate::template_builder::{
 use crate::template_parser::{self, FunctionCallNode, TemplateParseError, TemplateParseResult};
 use crate::templater::{
     self, PlainTextFormattedProperty, Template, TemplateFormatter, TemplateProperty,
-    TemplatePropertyError, TemplatePropertyExt as _,
+    TemplatePropertyExt as _,
 };
 use crate::{revset_util, text_util};
 
@@ -130,9 +130,7 @@ impl<'repo> TemplateLanguage<'repo> for CommitTemplateLanguage<'repo> {
                 let type_name = "Commit";
                 let table = &self.build_fn_table.commit_methods;
                 let build = template_parser::lookup_method(type_name, table, function)?;
-                let inner_property = property.and_then(|opt| {
-                    opt.ok_or_else(|| TemplatePropertyError("No commit available".into()))
-                });
+                let inner_property = property.try_unwrap(type_name);
                 build(self, build_ctx, Box::new(inner_property), function)
             }
             CommitTemplatePropertyKind::CommitList(property) => {
@@ -154,9 +152,7 @@ impl<'repo> TemplateLanguage<'repo> for CommitTemplateLanguage<'repo> {
                 let type_name = "RefName";
                 let table = &self.build_fn_table.ref_name_methods;
                 let build = template_parser::lookup_method(type_name, table, function)?;
-                let inner_property = property.and_then(|opt| {
-                    opt.ok_or_else(|| TemplatePropertyError("No RefName available".into()))
-                });
+                let inner_property = property.try_unwrap(type_name);
                 build(self, build_ctx, Box::new(inner_property), function)
             }
             CommitTemplatePropertyKind::RefNameList(property) => {

cli/src/templater.rs

@@ -349,6 +349,17 @@ pub trait TemplatePropertyExt: TemplateProperty {
         TemplateFunction::new(self, move |value| Ok(function(value)))
     }
 
+    /// Translates to a property that will unwrap an extracted `Option` value
+    /// of the specified `type_name`, mapping `None` to `Err`.
+    fn try_unwrap<O>(self, type_name: &str) -> impl TemplateProperty<Output = O>
+    where
+        Self: TemplateProperty<Output = Option<O>> + Sized,
+    {
+        self.and_then(move |opt| {
+            opt.ok_or_else(|| TemplatePropertyError(format!("No {type_name} available").into()))
+        })
+    }
+
     /// Converts this property into `Template`.
     fn into_template<'a>(self) -> Box<dyn Template + 'a>
     where

cli/tests/test_global_opts.rs

@@ -473,7 +473,7 @@ fn test_color_ui_messages() {
     );
     insta::assert_snapshot!(stdout, @r###"
     167f90e7600a50f85c4f909b53eaf546faa82879
-    <Error: No commit available>  (elided revisions)
+    <Error: No Commit available>  (elided revisions)
     0000000000000000000000000000000000000000
     "###);
 

cli/tests/test_tag_command.rs

@@ -100,7 +100,7 @@ fn test_tag_list() {
     [conflicted_tag]
     present: true
     conflict: true
-    normal_target: <Error: No commit available>
+    normal_target: <Error: No Commit available>
     removed_targets: commit1
     added_targets: commit2 commit3
     [test_tag]