feat: stabilize -Cmetadata=no as default behavior

doc/book/src/reference/unstable.md

@@ -91,7 +91,7 @@ Each new feature described below should explain how to use it.
     * [checksum-freshness](#checksum-freshness) --- When passed, the decision as to whether a crate needs to be rebuilt is made using file checksums instead of the file mtime.
     * [panic-abort-tests](#panic-abort-tests) --- Allows running tests with the "abort" panic strategy.
     * [host-config](#host-config) --- Allows setting `[target]`-like configuration settings for host build targets.
-    * [embed-metadata](#embed-metadata) --- If set to `no`, cargo will pass `-Zembed-metadata=no` to the compiler, which avoid embedding metadata into rlib and dylib artifacts, to save disk space.
+    * [embed-metadata](#embed-metadata) --- If set to `yes`, cargo will not pass `-Cembed-metadata=no` to the compiler, so metadata is embedded into rlib and dylib artifacts.
     * [target-applies-to-host](#target-applies-to-host) --- Alters whether certain flags will be passed to host build targets.
     * [gc](#gc) --- Global cache garbage collection.
     * [open-namespaces](#open-namespaces) --- Allow multiple packages to participate in the same API namespace
@@ -1911,19 +1911,17 @@ whether documentations are required to re-generate. This can be combined with
 * Original Pull Request: [#15378](https://github.com/rust-lang/cargo/pull/15378)
 * Tracking Issue: [#15495](https://github.com/rust-lang/cargo/issues/15495)
 
-The default behavior of Rust is to embed crate metadata into `rlib` and `dylib` artifacts.
-Since Cargo also passes `--emit=metadata` to these intermediate artifacts to enable pipelined
-compilation, this means that a lot of metadata ends up being duplicated on disk, which wastes
-disk space in the target directory.
+By default, when rustc supports it, Cargo passes the `-Cembed-metadata=no` flag to the compiler,
+which instructs it not to embed metadata within rlib and dylib artifacts.
+In this case, the metadata will only be stored in `.rmeta` files.
 
-If you pass `-Zembed-metadata=no` to Cago, it will then pass the `-Zembed-metadata=no` flag to the compiler, which instructs it not to embed metadata within rlib and dylib artifacts. In this case, the metadata will only
-be stored in `.rmeta` files.
+To embed metadata anyway, pass `-Zembed-metadata=yes`:
 
 ```console
-cargo +nightly -Zembed-metadata=no build
+cargo +nightly -Zembed-metadata=yes build
 ```
 
-> Note that this flag is planned to be removed in the future, as the `no` behavior should become the default.
+> Note that this flag is planned to be removed in the future.
 
 ## `unstable-editions`
 

src/compiler/build_context/target_info.rs

@@ -354,19 +354,19 @@ impl TargetInfo {
                 }
             }
 
+            // TODO: Remove this and also `-Zembed-metadata` flag
+            // once Cargo' MSRV is at the version stabilizing `-Cembed-metadata`
+            // (possibly 1.101)
             let should_embed_metadata = match gctx.cli_unstable().embed_metadata {
                 Some(v) => v,
-                None => {
-                    let cargo_nightly = matches!(
-                        crate::version().release_channel.as_deref(),
-                        Some("nightly" | "dev")
-                    );
-                    let rustc_nightly = matches!(rustc.version.pre.as_str(), "dev" | "nightly");
-
-                    // Enable -Zembed-metadata=no by default if both cargo and rustc are nightly
-                    let is_nightly = cargo_nightly && rustc_nightly;
-                    !is_nightly
-                }
+                None => rustc
+                    .cached_output(
+                        crate_type_process
+                            .clone()
+                            .args(&["-C", "embed-metadata=no"]),
+                        extra_fingerprint,
+                    )
+                    .is_err(),
             };
 
             return Ok(TargetInfo {

src/compiler/mod.rs

@@ -1354,7 +1354,7 @@ fn build_base_args(
         .info(unit.kind)
         .should_embed_metadata()
     {
-        // Nightly rustc supports the -Zembed-metadata=no flag, which tells it to avoid including
+        // The -Cembed-metadata=no flag tells rustc to avoid including
         // full metadata in rlib/dylib artifacts, to save space on disk. In this case, metadata
         // will only be stored in .rmeta files.
         // When we use this flag, we should also pass --emit=metadata to all artifacts that
@@ -1365,12 +1365,12 @@ fn build_base_args(
         // files on disk.
         if unit.benefits_from_no_embed_metadata() {
             cmd.arg("--emit=dep-info,metadata,link");
-            cmd.args(&["-Z", "embed-metadata=no"]);
+            cmd.args(&["-C", "embed-metadata=no"]);
         } else {
             cmd.arg("--emit=dep-info,link");
         }
     } else {
-        // If we don't use -Zembed-metadata=no, we emit .rmeta files only for rlib outputs.
+        // If we don't use -Cembed-metadata=no, we emit .rmeta files only for rlib outputs.
         // This metadata may be used in this session for a pipelined compilation, or it may
         // be used in a future Cargo session as part of a pipelined compile.
         if !unit.requires_upstream_objects() {
@@ -1927,7 +1927,7 @@ pub fn extern_args(
                 if output.flavor == FileFlavor::Linkable {
                     pass(&output.path);
                 }
-                // If we use -Zembed-metadata=no, we also need to pass the path to the
+                // If we use -Cembed-metadata=no, we also need to pass the path to the
                 // corresponding .rmeta file to the linkable artifact, because the
                 // normal dependency (rlib) doesn't contain the full metadata.
                 else if no_embed_metadata && output.flavor == FileFlavor::Rmeta {

tests/testsuite/build.rs

@@ -6450,7 +6450,7 @@ fn renamed_uplifted_artifact_remains_unmodified_after_rebuild() {
     assert!(not_the_same, "renamed uplifted artifact must be unmodified");
 }
 
-#[cargo_test(nightly, reason = "-Zembed-metadata is nightly only")]
+#[cargo_test(nightly, reason = "-Cembed-metadata requires rustc 1.101")]
 fn embed_metadata_no() {
     let p = project()
         .file(
@@ -6478,11 +6478,8 @@ fn embed_metadata_no() {
         )
         .build();
 
-    p.cargo("build")
-        .arg("-Zembed-metadata=no")
-        .masquerade_as_nightly_cargo(&["-Z embed-metadata"])
-        .arg("-v")
-        .with_stderr_contains("[RUNNING] `[..]-Z embed-metadata=no[..]`")
+    p.cargo("build -v")
+        .with_stderr_contains("[RUNNING] `[..]-C embed-metadata=no[..]`")
         .with_stderr_contains(
 "[RUNNING] `[..]--extern bar=[ROOT]/foo/target/debug/build/bar/[HASH]/out/libbar-[HASH].rmeta[..]`",
         )
@@ -6491,7 +6488,7 @@ fn embed_metadata_no() {
 
 // Make sure that cargo passes --extern=<dep>.rmeta even if <dep>
 // is compiled as a dylib.
-#[cargo_test(nightly, reason = "-Zembed-metadata is nightly only")]
+#[cargo_test(nightly, reason = "-Cembed-metadata requires rustc 1.101")]
 fn embed_metadata_no_dylib_dep() {
     let p = project()
         .file(
@@ -6529,18 +6526,15 @@ fn embed_metadata_no_dylib_dep() {
         )
         .build();
 
-    p.cargo("build")
-        .arg("-Zembed-metadata=no")
-        .masquerade_as_nightly_cargo(&["-Z embed-metadata"])
-        .arg("-v")
-        .with_stderr_contains("[RUNNING] `[..]-Z embed-metadata=no[..]`")
+    p.cargo("build -v")
+        .with_stderr_contains("[RUNNING] `[..]-C embed-metadata=no[..]`")
         .with_stderr_contains(
     "[RUNNING] `[..]--extern bar=[ROOT]/foo/target/debug/build/bar/[HASH]/out/libbar.rmeta[..]`",
         )
         .run();
 }
 
-#[cargo_test(nightly, reason = "-Zembed-metadata is nightly only")]
+#[cargo_test(nightly, reason = "-Cembed-metadata requires rustc 1.101")]
 fn embed_metadata_no_invalidate() {
     // Invalidate all deps when -Zembed-metadata is toggled
     let p = project()
@@ -6570,8 +6564,6 @@ fn embed_metadata_no_invalidate() {
         .build();
 
     p.cargo("build")
-        .arg("-Zembed-metadata=no")
-        .masquerade_as_nightly_cargo(&["-Z embed-metadata"])
         .with_stderr_data(str![[r#"
 [LOCKING] 1 package to highest compatible version
 [COMPILING] bar v0.5.0 ([ROOT]/foo/bar)