cli: improve discoverability of template-aliases

When user types `jj log -T` without passing a template name we can list all of them

cli/src/command_error.rs

@@ -35,6 +35,7 @@ use jj_lib::working_copy::{ResetError, SnapshotError, WorkingCopyStateError};
 use jj_lib::workspace::WorkspaceInitError;
 use thiserror::Error;
 
+use crate::cli_util::WorkspaceCommandHelper;
 use crate::merge_tools::{
     ConflictResolveError, DiffEditError, DiffGenerateError, MergeToolConfigError,
 };
@@ -76,6 +77,24 @@ impl ErrorWithMessage {
     }
 }
 
+pub fn user_error_missing_template(workspace_command: &WorkspaceCommandHelper) -> CommandError {
+    let mut template_names = workspace_command
+        .template_aliases_map()
+        .symbol_aliases_keys();
+    template_names.sort_unstable();
+    let mut hint = String::new();
+    let padding = "   ";
+    for name in template_names {
+        hint.push('\n');
+        hint.push_str(padding);
+        hint.push_str(name);
+    }
+    user_error_with_hint(
+        String::from("Template is required"),
+        format!("The following template-aliases are defined:{}", hint),
+    )
+}
+
 pub fn user_error(err: impl Into<Box<dyn error::Error + Send + Sync>>) -> CommandError {
     user_error_with_hint_opt(err, None)
 }

cli/src/commands/log.rs

@@ -22,7 +22,7 @@ use jj_lib::revset_graph::{
 use tracing::instrument;
 
 use crate::cli_util::{CommandHelper, LogContentFormat, RevisionArg};
-use crate::command_error::CommandError;
+use crate::command_error::{user_error_missing_template, CommandError};
 use crate::diff_util::{self, DiffFormatArgs};
 use crate::graphlog::{get_graphlog, Edge};
 use crate::ui::Ui;
@@ -52,7 +52,7 @@ pub(crate) struct LogArgs {
     ///
     /// For the syntax, see https://github.com/martinvonz/jj/blob/main/docs/templates.md
     #[arg(long, short = 'T')]
-    template: Option<String>,
+    template: Option<Option<String>>,
     /// Show patch
     #[arg(long, short = 'p')]
     patch: bool,
@@ -101,7 +101,8 @@ pub(crate) fn cmd_log(
         diff_util::diff_formats_for_log(command.settings(), &args.diff_format, args.patch)?;
 
     let template_string = match &args.template {
-        Some(value) => value.to_string(),
+        Some(Some(value)) => value.to_string(),
+        Some(None) => return Err(user_error_missing_template(&workspace_command)),
         None => command.settings().config().get_string("templates.log")?,
     };
     let use_elided_nodes = command

cli/src/template_parser.rs

@@ -561,6 +561,10 @@ impl TemplateAliasesMap {
         Self::default()
     }
 
+    pub fn symbol_aliases_keys(&self) -> Vec<&str> {
+        self.symbol_aliases.keys().map(|s| s.as_str()).collect_vec()
+    }
+
     /// Adds new substitution rule `decl = defn`.
     ///
     /// Returns error if `decl` is invalid. The `defn` part isn't checked. A bad

cli/tests/test_log_command.rs

@@ -179,7 +179,7 @@ fn test_log_with_or_without_diff() {
     insta::assert_snapshot!(stderr, @r###"
     error: the argument '--git' cannot be used with '--color-words'
 
-    Usage: jj log --template <TEMPLATE> --no-graph --patch --git [PATHS]...
+    Usage: jj log --template [<TEMPLATE>] --no-graph --patch --git [PATHS]...
 
     For more information, try '--help'.
     "###);