cli: warn explicit paths not exist in either of diff trees

Maybe we can optimize it to check paths during diffing, but I think it's okay
to add extra lookup cost at the end. The size of the path arguments is usually
small.

Closes #505

cli/src/cli_util.rs

@@ -1722,6 +1722,32 @@ Discard the conflicting changes with `jj restore --from {}`.",
     Ok(())
 }
 
+/// Prints warning about explicit paths that don't match any of the tree
+/// entries.
+pub fn print_unmatched_explicit_paths<'a>(
+    ui: &Ui,
+    workspace_command: &WorkspaceCommandHelper,
+    expression: &FilesetExpression,
+    trees: impl IntoIterator<Item = &'a MergedTree>,
+) -> io::Result<()> {
+    let mut explicit_paths = expression.explicit_paths().collect_vec();
+    for tree in trees {
+        explicit_paths.retain(|&path| tree.path_value(path).is_absent());
+        if explicit_paths.is_empty() {
+            return Ok(());
+        }
+    }
+    let ui_paths = explicit_paths
+        .iter()
+        .map(|&path| workspace_command.format_file_path(path))
+        .join(", ");
+    writeln!(
+        ui.warning_default(),
+        "No matching entries for paths: {ui_paths}"
+    )?;
+    Ok(())
+}
+
 pub fn print_trackable_remote_branches(ui: &Ui, view: &View) -> io::Result<()> {
     let remote_branch_names = view
         .branches()

cli/src/commands/diff.rs

@@ -15,7 +15,7 @@
 use jj_lib::rewrite::merge_commit_trees;
 use tracing::instrument;
 
-use crate::cli_util::{CommandHelper, RevisionArg};
+use crate::cli_util::{print_unmatched_explicit_paths, CommandHelper, RevisionArg};
 use crate::command_error::CommandError;
 use crate::diff_util::{diff_formats_for, show_diff, DiffFormatArgs};
 use crate::ui::Ui;
@@ -76,9 +76,8 @@ pub(crate) fn cmd_diff(
         from_tree = merge_commit_trees(workspace_command.repo().as_ref(), &parents)?;
         to_tree = commit.tree()?
     }
-    let matcher = workspace_command
-        .parse_file_patterns(&args.paths)?
-        .to_matcher();
+    let fileset_expression = workspace_command.parse_file_patterns(&args.paths)?;
+    let matcher = fileset_expression.to_matcher();
     let diff_formats = diff_formats_for(command.settings(), &args.format)?;
     ui.request_pager();
     show_diff(
@@ -90,5 +89,11 @@ pub(crate) fn cmd_diff(
         matcher.as_ref(),
         &diff_formats,
     )?;
+    print_unmatched_explicit_paths(
+        ui,
+        &workspace_command,
+        &fileset_expression,
+        [&from_tree, &to_tree],
+    )?;
     Ok(())
 }

cli/tests/test_diff_command.rs

@@ -149,6 +149,33 @@ fn test_diff_basic() {
     file3 | 1 +
     3 files changed, 2 insertions(+), 1 deletion(-)
     "###);
+
+    // Unmatched paths should generate warnings
+    let (stdout, stderr) = test_env.jj_cmd_ok(
+        test_env.env_root(),
+        &[
+            "diff",
+            "-Rrepo",
+            "-s",
+            "repo",       // matches directory
+            "repo/file1", // deleted in to_tree, but exists in from_tree
+            "repo/x",
+            "repo/y/z",
+        ],
+    );
+    insta::assert_snapshot!(stdout.replace('\\', "/"), @r###"
+    D repo/file1
+    M repo/file2
+    A repo/file3
+    "###);
+    insta::assert_snapshot!(stderr.replace('\\', "/"), @r###"
+    Warning: No matching entries for paths: repo/x, repo/y/z
+    "###);
+
+    // Unmodified paths shouldn't generate warnings
+    let (stdout, stderr) = test_env.jj_cmd_ok(&repo_path, &["diff", "-s", "--from=@", "file2"]);
+    insta::assert_snapshot!(stdout, @"");
+    insta::assert_snapshot!(stderr, @"");
 }
 
 #[test]