//A/B/C is a valid name for //A/B/C:C

> Prefer using the short name when referring to an eponymous target (`//x` instead of `//x:x`). If
> you are in the same package, prefer the local reference (`:x` instead of `//x`).

See: https://bazel.build/build/style-guide#target-naming

This syntax is supported on the command-line, but not in BUCK files. Unfortunately, the only(?)
Starlark formatter (buildifier: https://github.com/bazelbuild/buildtools) automatically abbreviates
targets on the assumption that this syntax is valid, which causes errors when using Buck2:

    ...
    4: Error coercing "//src/Foo"
    5: Invalid absolute target pattern `//src/Foo` is not allowed
    6: Expected a `:`, a trailing `/...` or the literal `...`.

We can adjust the parsing rules to allow this syntax in more places.

Author: 9999years

app/buck2_core/src/pattern/pattern.rs

@@ -302,7 +302,7 @@ impl<T: PatternType> ParsedPattern<T> {
             cell_alias_resolver,
             TargetParsingOptions {
                 relative,
-                infer_target: false,
+                infer_target: true,
                 strip_package_trailing_slash: false,
             },
             pattern,
@@ -1511,7 +1511,8 @@ mod tests {
             CellRelativePath::unchecked_new("package").to_owned(),
         );
 
-        assert_matches!(
+        assert_eq!(
+            mk_target("root", "package/path", "path"),
             ParsedPattern::<TargetPatternExtra>::parse_not_relaxed(
                 "path",
                 TargetParsingRel::AllowRelative(
@@ -1520,12 +1521,7 @@ mod tests {
                 ),
                 &resolver(),
                 &alias_resolver(),
-            ),
-            Err(e) => {
-                assert!(
-                    format!("{:?}", e).contains(&format!("{}", TargetPatternParseError::UnexpectedFormat))
-                );
-            }
+            )?
         );
 
         assert_eq!(
@@ -1693,7 +1689,7 @@ mod tests {
             ),
             Err(e) => {
                 assert!(
-                    format!("{:?}", e).contains(&format!("{}", TargetPatternParseError::UnexpectedFormat))
+                    format!("{:?}", e).contains(&format!("{}", TargetPatternParseError::AbsoluteRequired))
                 );
             }
         );