shorthands: Support cooperation b/w custom and built-in (#335)

Instead of stopping expansion at the first match,
continue to expand the first argument of the result.
This allows user-defined shorthands to refer to built-in shorthands.

doc/src/cli/shorthand.md

@@ -82,3 +82,11 @@ For example:
 {gray}# Replace the "branch restack" shorthand{reset}
 {green}${reset} git config --global spice.shorthand.br "branch rename"
 ```
+
+If the result of a user-defined shorthand refers to a built-in shorthand,
+both will be expanded.
+
+```freeze language="terminal"
+{green}${reset} git config --global spice.shorthand.bb bco
+{gray}# bb will expand to bco, which will expand to "branch checkout"{reset}
+```

internal/cli/shorthand/expand.go

@@ -16,13 +16,34 @@ type Source interface {
 }
 
 // Sources is a list of shorthand sources composed together.
-// These are tried in order until one works.
+// These are tried in order repeatedly until there's nothing left to expand.
 type Sources []Source
 
 var _ Source = Sources(nil)
 
 // ExpandShorthand expands the given shorthand command.
-func (ss Sources) ExpandShorthand(cmd string) ([]string, bool) {
+func (ss Sources) ExpandShorthand(orig string) ([]string, bool) {
+	seen := make(map[string]struct{}) // to prevent infinite loops
+	result := []string{orig}
+	for len(result) > 0 {
+		cmd := result[0]
+		if _, done := seen[cmd]; done {
+			break
+		}
+		seen[cmd] = struct{}{}
+
+		next, ok := ss.expandOnce(cmd)
+		if !ok {
+			break
+		}
+
+		result = slices.Replace(result, 0, 1, next...)
+	}
+
+	return result, len(result) > 0 && result[0] != orig
+}
+
+func (ss Sources) expandOnce(cmd string) ([]string, bool) {
 	for _, s := range ss {
 		if args, ok := s.ExpandShorthand(cmd); ok {
 			return args, true

internal/cli/shorthand/expand_test.go

@@ -61,6 +61,53 @@ func TestExpand(t *testing.T) {
 			args: []string{"foo"},
 			want: []string{"qux", "baz"},
 		},
+		{
+			name: "Sources/Cooperative",
+			src: shorthand.Sources{
+				shorthandMap{"can": {"ca", "--no-edit"}},
+				shorthandMap{"ca": {"c", "--amend"}},
+				shorthandMap{"c": {"commit"}},
+			},
+			args: []string{"can", "--all"},
+			want: []string{"commit", "--amend", "--no-edit", "--all"},
+		},
+		{
+			name: "Sources/CooperativeReverse",
+			src: shorthand.Sources{
+				shorthandMap{"c": {"commit"}},
+				shorthandMap{"ca": {"c", "--amend"}},
+				shorthandMap{"can": {"ca", "--no-edit"}},
+			},
+			args: []string{"can", "--all"},
+			want: []string{"commit", "--amend", "--no-edit", "--all"},
+		},
+		{
+			name: "Sources/Delete",
+			src: shorthand.Sources{
+				shorthandMap{"foo": {"bar", "baz"}},
+				shorthandMap{"bar": {}},
+			},
+			args: []string{"foo"},
+			want: []string{"baz"},
+		},
+		{
+			name: "Sources/NoMatch",
+			src: shorthand.Sources{
+				shorthandMap{"foo": {"bar", "baz"}},
+				shorthandMap{"bar": {"qux"}},
+			},
+			args: []string{"qux"},
+			want: []string{"qux"},
+		},
+		{
+			name: "Sources/InfiniteLoop",
+			src: shorthand.Sources{
+				shorthandMap{"foo": {"bar"}},
+				shorthandMap{"bar": {"foo"}},
+			},
+			args: []string{"foo"},
+			want: []string{"foo"},
+		},
 	}
 
 	for _, tt := range tests {