Warn on unused patches.

src/cargo/ops/resolve.rs

@@ -10,6 +10,12 @@ use crate::sources::PathSource;
 use crate::util::errors::{CargoResult, CargoResultExt};
 use crate::util::profile;
 
+const UNUSED_PATCH_WARNING: &str = "\
+Check that the patched package version and available features are compatible
+with the dependency requirements. If the patch has a different version from
+what is locked in the Cargo.lock file, run `cargo update` to use the new
+version. This may also occur with an optional dependency that is not enabled.";
+
 /// Resolve all dependencies for the workspace using the previous
 /// lockfile as a guide if present.
 ///
@@ -334,6 +340,24 @@ pub fn resolve_with_previous<'cfg>(
         warn,
     )?;
     resolved.register_used_patches(registry.patches());
+    if warn {
+        // It would be good if this warning was more targeted and helpful
+        // (such as showing close candidates that failed to match). However,
+        // that's not terribly easy to do, so just show a general help
+        // message.
+        let warnings: Vec<String> = resolved
+            .unused_patches()
+            .iter()
+            .map(|pkgid| format!("Patch `{}` was not used in the crate graph.", pkgid))
+            .collect();
+        if !warnings.is_empty() {
+            ws.config().shell().warn(format!(
+                "{}\n{}",
+                warnings.join("\n"),
+                UNUSED_PATCH_WARNING
+            ))?;
+        }
+    }
     if let Some(previous) = previous {
         resolved.merge_from(previous)?;
     }

tests/testsuite/patch.rs

@@ -232,6 +232,11 @@ fn unused() {
         .with_stderr(
             "\
 [UPDATING] `[ROOT][..]` index
+[WARNING] Patch `bar v0.2.0 ([CWD]/bar)` was not used in the crate graph.
+[..]
+[..]
+[..]
+[..]
 [DOWNLOADING] crates ...
 [DOWNLOADED] bar v0.1.0 [..]
 [COMPILING] bar v0.1.0
@@ -240,7 +245,18 @@ fn unused() {
 ",
         )
         .run();
-    p.cargo("build").with_stderr("[FINISHED] [..]").run();
+    p.cargo("build")
+        .with_stderr(
+            "\
+[WARNING] Patch `bar v0.2.0 ([CWD]/bar)` was not used in the crate graph.
+[..]
+[..]
+[..]
+[..]
+[FINISHED] [..]
+",
+        )
+        .run();
 
     // unused patch should be in the lock file
     let mut lock = String::new();
@@ -293,6 +309,11 @@ fn unused_git() {
             "\
 [UPDATING] git repository `file://[..]`
 [UPDATING] `[ROOT][..]` index
+[WARNING] Patch `bar v0.2.0 ([..])` was not used in the crate graph.
+[..]
+[..]
+[..]
+[..]
 [DOWNLOADING] crates ...
 [DOWNLOADED] bar v0.1.0 [..]
 [COMPILING] bar v0.1.0
@@ -301,7 +322,18 @@ fn unused_git() {
 ",
         )
         .run();
-    p.cargo("build").with_stderr("[FINISHED] [..]").run();
+    p.cargo("build")
+        .with_stderr(
+            "\
+[WARNING] Patch `bar v0.2.0 ([..])` was not used in the crate graph.
+[..]
+[..]
+[..]
+[..]
+[FINISHED] [..]
+",
+        )
+        .run();
 }
 
 #[test]
@@ -419,9 +451,38 @@ fn add_ignored_patch() {
     ));
 
     p.cargo("build")
-        .with_stderr("[FINISHED] dev [unoptimized + debuginfo] target(s) in [..]")
+        .with_stderr(
+            "\
+[WARNING] Patch `bar v0.1.1 ([CWD]/bar)` was not used in the crate graph.
+[..]
+[..]
+[..]
+[..]
+[FINISHED] dev [unoptimized + debuginfo] target(s) in [..]",
+        )
+        .run();
+    p.cargo("build")
+        .with_stderr(
+            "\
+[WARNING] Patch `bar v0.1.1 ([CWD]/bar)` was not used in the crate graph.
+[..]
+[..]
+[..]
+[..]
+[FINISHED] [..]",
+        )
+        .run();
+
+    p.cargo("update").run();
+    p.cargo("build")
+        .with_stderr(
+            "\
+[COMPILING] bar v0.1.1 ([CWD]/bar)
+[COMPILING] foo v0.0.1 ([CWD])
+[FINISHED] dev [..]
+",
+        )
         .run();
-    p.cargo("build").with_stderr("[FINISHED] [..]").run();
 }
 
 #[test]