fix(toml): Improve error on missing package and workspace

That the error was confused was brought up in #13904

src/cargo/util/toml/mod.rs

@@ -106,7 +106,7 @@ pub fn read_manifest(
                 &mut errors,
             )
             .map(EitherManifest::Real)
-        } else {
+        } else if resolved_toml.workspace.is_some() {
             to_virtual_manifest(
                 contents,
                 document,
@@ -121,6 +121,8 @@ pub fn read_manifest(
                 &mut errors,
             )
             .map(EitherManifest::Virtual)
+        } else {
+            anyhow::bail!("manifest is missing either a `[package]` or a `[workspace]`")
         }
     })()
     .map_err(|err| {

tests/testsuite/build.rs

@@ -398,7 +398,7 @@ fn cargo_compile_with_invalid_manifest() {
 [ERROR] failed to parse manifest at `[ROOT]/foo/Cargo.toml`
 
 Caused by:
-  virtual manifests must be configured with [workspace]
+  manifest is missing either a `[package]` or a `[workspace]`
 
 "#]])
         .run();

tests/testsuite/metadata.rs

@@ -1876,7 +1876,7 @@ fn cargo_metadata_with_invalid_manifest() {
 [ERROR] failed to parse manifest at `[ROOT]/foo/Cargo.toml`
 
 Caused by:
-  virtual manifests must be configured with [workspace]
+  manifest is missing either a `[package]` or a `[workspace]`
 
 "#]])
         .run();