Add bash/zsh shell completion

vintner completion bash|zsh prints a completion script meant to be
sourced (source <(vintner completion bash)); completes subcommands
(including short aliases), download's flags, and directory arguments
for install/env --bin. Mentioned in the top-level usage text and
documented in the README.

The Nivora package doesn't auto-install these system-wide yet - it'd
need Stapler's install-completion helper, whose calling convention
isn't documented anywhere in this repo or Nivora's other packages, so
guessing at it risked a broken package build for a nice-to-have.
source <(vintner completion bash) works today regardless of install
method (Nivora, prebuilt binary, or from source).

cmd/vintner/completion.go

@@ -0,0 +1,140 @@
+package main
+
+import "fmt"
+
+// runCompletion prints a shell completion script for shell ("bash" or
+// "zsh") to stdout, meant to be sourced directly:
+//
+//	source <(vintner completion bash)   # or add to ~/.bashrc
+//	source <(vintner completion zsh)    # or add to ~/.zshrc
+//
+// The flag lists below are hand-maintained alongside download.go/env.go's
+// flag.FlagSet definitions rather than generated from them - there's no
+// reflection-friendly registry to walk, and the flag set rarely changes.
+func runCompletion(args []string) int {
+	if len(args) != 1 {
+		fmt.Println("usage: vintner completion bash|zsh")
+		return 1
+	}
+	switch args[0] {
+	case "bash":
+		fmt.Print(bashCompletionScript)
+		return 0
+	case "zsh":
+		fmt.Print(zshCompletionScript)
+		return 0
+	default:
+		fmt.Printf("vintner completion: unsupported shell %q (want bash or zsh)\n", args[0])
+		return 1
+	}
+}
+
+const downloadFlags = "--dest --cache --major --preview --manifest --accept-license " +
+	"--msvc-version --sdk-version --host-arch --only-host --language " +
+	"--include-optional --skip-recommended --only-download --only-unpack " +
+	"--keep-unpack --skip-patch --list-workloads --list-components " +
+	"--print-deps-tree --with-wdk --architecture --ignore -h --help"
+
+var bashCompletionScript = `# vintner bash completion - eval "$(vintner completion bash)"
+_vintner_complete() {
+    local cur cmd
+    COMPREPLY=()
+    cur="${COMP_WORDS[COMP_CWORD]}"
+    cmd="${COMP_WORDS[1]}"
+
+    if [ "$COMP_CWORD" -eq 1 ]; then
+        COMPREPLY=($(compgen -W "download dl install i env e version v help h completion" -- "$cur"))
+        return 0
+    fi
+
+    case "$cmd" in
+        download|dl)
+            COMPREPLY=($(compgen -W "` + downloadFlags + `" -- "$cur"))
+            ;;
+        install|i)
+            COMPREPLY=($(compgen -d -- "$cur"))
+            ;;
+        env|e)
+            COMPREPLY=($(compgen -W "--bin -h --help" -- "$cur"))
+            ;;
+        completion)
+            COMPREPLY=($(compgen -W "bash zsh" -- "$cur"))
+            ;;
+    esac
+    return 0
+}
+complete -F _vintner_complete vintner
+`
+
+var zshCompletionScript = `#compdef vintner
+# vintner zsh completion - source <(vintner completion zsh)
+
+_vintner() {
+    local -a subcommands
+    subcommands=(
+        'download:fetch and unpack MSVC/WinSDK/WDK'
+        'dl:alias for download'
+        'install:wire up wrappers for a downloaded MSVC'
+        'i:alias for install'
+        'env:print INCLUDE/LIB for native clang-cl/lld-link use'
+        'e:alias for env'
+        'version:print the version'
+        'v:alias for version'
+        'help:print usage'
+        'h:alias for help'
+        'completion:print a shell completion script'
+    )
+
+    if (( CURRENT == 2 )); then
+        _describe 'command' subcommands
+        return
+    fi
+
+    case "${words[2]}" in
+        download|dl)
+            local -a flags
+            flags=(
+                '--dest[directory to install into]:directory:_files -/'
+                '--cache[persistent download cache directory]:directory:_files -/'
+                '--major[major VS version]:version:'
+                '--preview[use the preview/insiders channel]'
+                '--manifest[use a predownloaded installer manifest file]:file:_files'
+                '--accept-license[do not prompt for accepting the license]'
+                '--msvc-version[install a specific MSVC toolchain version]:version:'
+                '--sdk-version[install a specific Windows SDK version]:version:'
+                '--host-arch[host architecture]:arch:(x86 x64 arm64)'
+                '--only-host[only download packages matching the host architecture]'
+                '--language[preferred package language]:language:'
+                '--include-optional[include all optional dependencies]'
+                '--skip-recommended[skip recommended dependencies]'
+                '--only-download[stop after downloading package files]'
+                '--only-unpack[unpack without pruning to just the CLI tools]'
+                '--keep-unpack[keep the scratch unpack dir]'
+                '--skip-patch[do not apply the Wine compatibility patches]'
+                '--list-workloads[list available workloads and exit]'
+                '--list-components[list available components and exit]'
+                '--print-deps-tree[print the dependency tree and exit]'
+                '--with-wdk[also fetch the Windows Driver Kit]'
+                '--architecture[target architecture]:arch:(x86 x64 arm arm64 host)'
+                '--ignore[package id to skip]:package id:'
+                '-h[show help]'
+                '--help[show help]'
+            )
+            _arguments $flags
+            ;;
+        install|i)
+            _files -/
+            ;;
+        env|e)
+            _arguments \
+                '--bin[bin/<arch> directory produced by install]:directory:_files -/' \
+                '-h[show help]' '--help[show help]'
+            ;;
+        completion)
+            _values 'shell' bash zsh
+            ;;
+    esac
+}
+
+_vintner "$@"
+`

cmd/vintner/completion_test.go

@@ -0,0 +1,44 @@
+package main
+
+import (
+	"os/exec"
+	"strings"
+	"testing"
+)
+
+// TestCompletionScriptsAreSyntacticallyValid catches the easy way to break
+// these: a typo in the hand-maintained flag lists that produces invalid
+// shell syntax. It shells out to bash/zsh -n rather than parsing the script
+// itself, so it's testing exactly what a user's shell would see.
+func TestCompletionScriptsAreSyntacticallyValid(t *testing.T) {
+	for _, tc := range []struct {
+		shell  string
+		script string
+	}{
+		{"bash", bashCompletionScript},
+		{"zsh", zshCompletionScript},
+	} {
+		t.Run(tc.shell, func(t *testing.T) {
+			if _, err := exec.LookPath(tc.shell); err != nil {
+				t.Skipf("%s not installed", tc.shell)
+			}
+			cmd := exec.Command(tc.shell, "-n", "/dev/stdin")
+			cmd.Stdin = strings.NewReader(tc.script)
+			if out, err := cmd.CombinedOutput(); err != nil {
+				t.Fatalf("%s -n rejected the completion script: %v\n%s", tc.shell, err, out)
+			}
+		})
+	}
+}
+
+func TestRunCompletionUnknownShell(t *testing.T) {
+	if code := runCompletion([]string{"fish"}); code != 1 {
+		t.Errorf("runCompletion([\"fish\"]) = %d, want 1", code)
+	}
+	if code := runCompletion(nil); code != 1 {
+		t.Errorf("runCompletion(nil) = %d, want 1", code)
+	}
+	if code := runCompletion([]string{"bash", "extra"}); code != 1 {
+		t.Errorf("runCompletion with extra arg = %d, want 1", code)
+	}
+}

cmd/vintner/main.go

@@ -47,6 +47,8 @@ func runCLI(args []string) int {
 		return runInstall(args[1:])
 	case "env", "e":
 		return runEnv(args[1:])
+	case "completion":
+		return runCompletion(args[1:])
 	case "version", "v", "--version":
 		fmt.Println("vintner " + version)
 		return 0