fix: improve message for inactive weak optional feature with edition2024 through unused dep collection

src/cargo/core/summary.rs

@@ -30,6 +30,35 @@ struct Inner {
     rust_version: Option<RustVersion>,
 }
 
+/// Indicates the dependency inferred from the `dep` syntax that should exist,
+/// but missing on the resolved dependencies tables.
+#[derive(Debug)]
+pub struct MissingDependencyError {
+    pub dep_name: InternedString,
+    pub feature: InternedString,
+    pub feature_value: FeatureValue,
+    /// Indicates the dependency inferred from the `dep?` syntax that is weak optional
+    pub weak_optional: bool,
+}
+
+impl std::error::Error for MissingDependencyError {}
+
+impl fmt::Display for MissingDependencyError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let Self {
+            dep_name,
+            feature,
+            feature_value: fv,
+            ..
+        } = self;
+
+        write!(
+            f,
+            "feature `{feature}` includes `{fv}`, but `{dep_name}` is not a dependency",
+        )
+    }
+}
+
 impl Summary {
     #[tracing::instrument(skip_all)]
     pub fn new(
@@ -274,7 +303,12 @@ fn build_feature_map(
 
                     // Validation of the feature name will be performed in the resolver.
                     if !is_any_dep {
-                        bail!("feature `{feature}` includes `{fv}`, but `{dep_name}` is not a dependency");
+                        bail!(MissingDependencyError {
+                            feature: *feature,
+                            feature_value: (*fv).clone(),
+                            dep_name: *dep_name,
+                            weak_optional: *weak,
+                        })
                     }
                     if *weak && !is_optional_dep {
                         bail!(

src/cargo/util/context/mod.rs

@@ -2031,7 +2031,7 @@ impl ConfigError {
     }
 
     fn is_missing_field(&self) -> bool {
-        self.error.downcast_ref::<MissingField>().is_some()
+        self.error.downcast_ref::<MissingFieldError>().is_some()
     }
 
     fn missing(key: &ConfigKey) -> ConfigError {
@@ -2067,15 +2067,15 @@ impl fmt::Display for ConfigError {
 }
 
 #[derive(Debug)]
-struct MissingField(String);
+struct MissingFieldError(String);
 
-impl fmt::Display for MissingField {
+impl fmt::Display for MissingFieldError {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         write!(f, "missing field `{}`", self.0)
     }
 }
 
-impl std::error::Error for MissingField {}
+impl std::error::Error for MissingFieldError {}
 
 impl serde::de::Error for ConfigError {
     fn custom<T: fmt::Display>(msg: T) -> Self {
@@ -2087,7 +2087,7 @@ impl serde::de::Error for ConfigError {
 
     fn missing_field(field: &'static str) -> Self {
         ConfigError {
-            error: anyhow::Error::new(MissingField(field.to_string())),
+            error: anyhow::Error::new(MissingFieldError(field.to_string())),
             definition: None,
         }
     }

src/cargo/util/lints.rs

@@ -199,7 +199,11 @@ fn verify_feature_enabled(
     Ok(())
 }
 
-fn get_span(document: &ImDocument<String>, path: &[&str], get_value: bool) -> Option<Range<usize>> {
+pub fn get_span(
+    document: &ImDocument<String>,
+    path: &[&str],
+    get_value: bool,
+) -> Option<Range<usize>> {
     let mut table = document.as_item().as_table_like()?;
     let mut iter = path.into_iter().peekable();
     while let Some(key) = iter.next() {
@@ -240,7 +244,7 @@ fn get_span(document: &ImDocument<String>, path: &[&str], get_value: bool) -> Op
 
 /// Gets the relative path to a manifest from the current working directory, or
 /// the absolute path of the manifest if a relative path cannot be constructed
-fn rel_cwd_manifest_path(path: &Path, gctx: &GlobalContext) -> String {
+pub fn rel_cwd_manifest_path(path: &Path, gctx: &GlobalContext) -> String {
     diff_paths(path, gctx.cwd())
         .unwrap_or_else(|| path.to_path_buf())
         .display()

src/cargo/util/toml/mod.rs

@@ -5,6 +5,7 @@ use std::path::{Path, PathBuf};
 use std::rc::Rc;
 use std::str::{self, FromStr};
 
+use crate::core::summary::MissingDependencyError;
 use crate::AlreadyPrintedError;
 use anyhow::{anyhow, bail, Context as _};
 use cargo_platform::Platform;
@@ -14,6 +15,7 @@ use cargo_util_schemas::manifest::{RustVersion, StringOrBool};
 use itertools::Itertools;
 use lazycell::LazyCell;
 use pathdiff::diff_paths;
+use toml_edit::ImDocument;
 use url::Url;
 
 use crate::core::compiler::{CompileKind, CompileTarget};
@@ -28,6 +30,7 @@ use crate::core::{GitReference, PackageIdSpec, SourceId, WorkspaceConfig, Worksp
 use crate::sources::{CRATES_IO_INDEX, CRATES_IO_REGISTRY};
 use crate::util::errors::{CargoResult, ManifestError};
 use crate::util::interning::InternedString;
+use crate::util::lints::{get_span, rel_cwd_manifest_path};
 use crate::util::{self, context::ConfigRelativePath, GlobalContext, IntoUrl, OptVersionReq};
 
 mod embedded;
@@ -1435,24 +1438,42 @@ fn to_real_manifest(
             .unwrap_or_else(|| semver::Version::new(0, 0, 0)),
         source_id,
     );
-    let summary = Summary::new(
-        pkgid,
-        deps,
-        &resolved_toml
-            .features
-            .as_ref()
-            .unwrap_or(&Default::default())
-            .iter()
-            .map(|(k, v)| {
-                (
-                    InternedString::new(k),
-                    v.iter().map(InternedString::from).collect(),
-                )
-            })
-            .collect(),
-        resolved_package.links.as_deref(),
-        rust_version.clone(),
-    )?;
+    let summary = {
+        let summary = Summary::new(
+            pkgid,
+            deps,
+            &resolved_toml
+                .features
+                .as_ref()
+                .unwrap_or(&Default::default())
+                .iter()
+                .map(|(k, v)| {
+                    (
+                        InternedString::new(k),
+                        v.iter().map(InternedString::from).collect(),
+                    )
+                })
+                .collect(),
+            resolved_package.links.as_deref(),
+            rust_version.clone(),
+        );
+        // editon2024 stops exposing implicit features, which will strip weak optional dependencies from `dependencies`,
+        // need to check whether `dep_name` is stripped as unused dependency
+        if let Err(ref err) = summary {
+            if let Some(missing_dep) = err.downcast_ref::<MissingDependencyError>() {
+                missing_dep_diagnostic(
+                    missing_dep,
+                    &original_toml,
+                    &document,
+                    &contents,
+                    manifest_file,
+                    gctx,
+                )?;
+            }
+        }
+        summary?
+    };
+
     if summary.features().contains_key("default-features") {
         warnings.push(
             "`default-features = [\"..\"]` was found in [features]. \
@@ -1558,6 +1579,85 @@ fn to_real_manifest(
     Ok(manifest)
 }
 
+fn missing_dep_diagnostic(
+    missing_dep: &MissingDependencyError,
+    orig_toml: &TomlManifest,
+    document: &ImDocument<String>,
+    contents: &str,
+    manifest_file: &Path,
+    gctx: &GlobalContext,
+) -> CargoResult<()> {
+    let dep_name = missing_dep.dep_name;
+    let manifest_path = rel_cwd_manifest_path(manifest_file, gctx);
+    let feature_value_span =
+        get_span(&document, &["features", missing_dep.feature.as_str()], true).unwrap();
+
+    let title = format!(
+        "feature `{}` includes `{}`, but `{}` is not a dependency",
+        missing_dep.feature, missing_dep.feature_value, &dep_name
+    );
+    let help = format!("enable the dependency with `dep:{dep_name}`");
+    let info_label = format!(
+        "`{}` is an unused optional dependency since no feature enables it",
+        &dep_name
+    );
+    let message = Level::Error.title(&title);
+    let snippet = Snippet::source(&contents)
+        .origin(&manifest_path)
+        .fold(true)
+        .annotation(Level::Error.span(feature_value_span.start..feature_value_span.end));
+    let message = if missing_dep.weak_optional {
+        let mut orig_deps = vec![
+            (
+                orig_toml.dependencies.as_ref(),
+                vec![DepKind::Normal.kind_table()],
+            ),
+            (
+                orig_toml.build_dependencies.as_ref(),
+                vec![DepKind::Build.kind_table()],
+            ),
+        ];
+        for (name, platform) in orig_toml.target.iter().flatten() {
+            orig_deps.push((
+                platform.dependencies.as_ref(),
+                vec!["target", name, DepKind::Normal.kind_table()],
+            ));
+            orig_deps.push((
+                platform.build_dependencies.as_ref(),
+                vec!["target", name, DepKind::Normal.kind_table()],
+            ));
+        }
+
+        if let Some((_, toml_path)) = orig_deps.iter().find(|(deps, _)| {
+            if let Some(deps) = deps {
+                deps.keys().any(|p| *p.as_str() == *dep_name)
+            } else {
+                false
+            }
+        }) {
+            let toml_path = toml_path
+                .iter()
+                .map(|s| *s)
+                .chain(std::iter::once(dep_name.as_str()))
+                .collect::<Vec<_>>();
+            let dep_span = get_span(&document, &toml_path, false).unwrap();
+
+            message
+                .snippet(snippet.annotation(Level::Warning.span(dep_span).label(&info_label)))
+                .footer(Level::Help.title(&help))
+        } else {
+            message.snippet(snippet)
+        }
+    } else {
+        message.snippet(snippet)
+    };
+
+    if let Err(err) = gctx.shell().print_message(message) {
+        return Err(err.into());
+    }
+    Err(AlreadyPrintedError::new(anyhow!("").into()).into())
+}
+
 fn to_virtual_manifest(
     contents: String,
     document: toml_edit::ImDocument<String>,

tests/testsuite/features.rs

@@ -256,11 +256,14 @@ fn invalid6() {
     p.cargo("check --features foo")
         .with_status(101)
         .with_stderr_data(str![[r#"
+[ERROR] feature `foo` includes `bar/baz`, but `bar` is not a dependency
+ --> Cargo.toml:9:23
+  |
+9 |                 foo = ["bar/baz"]
+  |                       ^^^^^^^^^^^
+  |
 [ERROR] failed to parse manifest at `[ROOT]/foo/Cargo.toml`
 
-Caused by:
-  feature `foo` includes `bar/baz`, but `bar` is not a dependency
-
 "#]])
         .run();
 }
@@ -288,11 +291,14 @@ fn invalid7() {
     p.cargo("check --features foo")
         .with_status(101)
         .with_stderr_data(str![[r#"
+[ERROR] feature `foo` includes `bar/baz`, but `bar` is not a dependency
+ --> Cargo.toml:9:23
+  |
+9 |                 foo = ["bar/baz"]
+  |                       ^^^^^^^^^^^
+  |
 [ERROR] failed to parse manifest at `[ROOT]/foo/Cargo.toml`
 
-Caused by:
-  feature `foo` includes `bar/baz`, but `bar` is not a dependency
-
 "#]])
         .run();
 }