Auto merge of #14548 - shannmu:_cargo_test_names, r=epage

feat: Add custom completer for completing test names

### What does this PR try to resolve?
Tracking issue #14520

Add custom completer for `cargo test --test <TAB>`

Author: bors

src/cargo/util/command_prelude.rs

@@ -156,7 +156,11 @@ pub trait CommandExt: Sized {
     ) -> Self {
         self.arg_targets_lib_bin_example(lib, bin, bins, example, examples)
             ._arg(flag("tests", tests).help_heading(heading::TARGET_SELECTION))
-            ._arg(optional_multi_opt("test", "NAME", test).help_heading(heading::TARGET_SELECTION))
+            ._arg(
+                optional_multi_opt("test", "NAME", test)
+                    .help_heading(heading::TARGET_SELECTION)
+                    .add(clap_complete::ArgValueCandidates::new(get_test_candidates)),
+            )
             ._arg(flag("benches", benches).help_heading(heading::TARGET_SELECTION))
             ._arg(
                 optional_multi_opt("bench", "NAME", bench).help_heading(heading::TARGET_SELECTION),
@@ -1042,6 +1046,17 @@ pub fn lockfile_path(
     return Ok(Some(path));
 }
 
+fn get_test_candidates() -> Vec<clap_complete::CompletionCandidate> {
+    get_targets_from_metadata()
+        .unwrap_or_default()
+        .into_iter()
+        .filter_map(|target| match target.kind() {
+            TargetKind::Test => Some(clap_complete::CompletionCandidate::new(target.name())),
+            _ => None,
+        })
+        .collect::<Vec<_>>()
+}
+
 fn get_bin_candidates() -> Vec<clap_complete::CompletionCandidate> {
     get_targets_from_metadata()
         .unwrap_or_default()