Auto merge of #3841 - matklad:encourage-explicit-version, r=alexcrichton

Encourage tools writers to explicitly pin metadata version

We do support versioning of metadata, but let's encourage tool's writers to actually use it.

They might not realize that this flag exists at all, or they can be too lazy (like myself :( ) to use it.

We can also make this flag mandatory, but I think that's a little bit to far.

src/bin/metadata.rs

@@ -9,7 +9,7 @@ pub struct Options {
     flag_color: Option<String>,
     flag_features: Vec<String>,
     flag_all_features: bool,
-    flag_format_version: u32,
+    flag_format_version: Option<u32>,
     flag_manifest_path: Option<String>,
     flag_no_default_features: bool,
     flag_no_deps: bool,
@@ -34,7 +34,7 @@ Options:
     --no-deps                  Output information only about the root package
                                and don't fetch dependencies.
     --manifest-path PATH       Path to the manifest
-    --format-version VERSION   Format version [default: 1]
+    --format-version VERSION   Format version
                                Valid values: 1
     -v, --verbose ...          Use verbose output (-vv very verbose/build.rs output)
     -q, --quiet                No output printed to stdout
@@ -51,12 +51,17 @@ pub fn execute(options: Options, config: &Config) -> CliResult {
                      options.flag_locked)?;
     let manifest = find_root_manifest_for_wd(options.flag_manifest_path, config.cwd())?;
 
+    if options.flag_format_version.is_none() {
+        config.shell().warn("please specify `--format-version` flag explicitly to \
+                             avoid compatibility problems")?
+    }
+
     let options = OutputMetadataOptions {
         features: options.flag_features,
         all_features: options.flag_all_features,
         no_default_features: options.flag_no_default_features,
         no_deps: options.flag_no_deps,
-        version: options.flag_format_version,
+        version: options.flag_format_version.unwrap_or(1),
     };
 
     let ws = Workspace::new(&manifest, config)?;

tests/metadata.rs

@@ -53,6 +53,21 @@ fn cargo_metadata_simple() {
     }"#));
 }
 
+#[test]
+fn cargo_metadata_warns_on_implicit_version() {
+    let p = project("foo")
+        .file("src/foo.rs", "")
+        .file("Cargo.toml", &basic_bin_manifest("foo"));
+    p.build();
+
+    assert_that(p.cargo("metadata"),
+                execs().with_stderr("\
+[WARNING] please specify `--format-version` flag explicitly to avoid compatibility problems"));
+
+    assert_that(p.cargo("metadata").arg("--format-version").arg("1"),
+                execs().with_stderr(""));
+}
+
 #[test]
 fn library_with_several_crate_types() {
     let p = project("foo")
@@ -520,8 +535,8 @@ fn cargo_metadata_with_invalid_manifest() {
     let p = project("foo")
             .file("Cargo.toml", "");
 
-    assert_that(p.cargo_process("metadata"), execs().with_status(101)
-                                                    .with_stderr("\
+    assert_that(p.cargo_process("metadata").arg("--format-version").arg("1"),
+                execs().with_status(101).with_stderr("\
 [ERROR] failed to parse manifest at `[..]`
 
 Caused by: