Add validation for example target paths with clearer error messages

src/cargo/util/toml/targets.rs

@@ -445,13 +445,17 @@ fn to_example_targets(
     let mut result = Vec::new();
     for toml in targets {
         let path = package_root.join(&toml.path.as_ref().expect("previously normalized").0);
+        let target_name = name_or_panic(&toml);
+
+        validate_target_path_as_source_file(&path, target_name, TARGET_KIND_HUMAN_EXAMPLE)?;
+
         let crate_types = match toml.crate_types() {
             Some(kinds) => kinds.iter().map(|s| s.into()).collect(),
             None => Vec::new(),
         };
 
         let mut target = Target::example_target(
-            name_or_panic(&toml),
+            target_name,
             crate_types,
             path,
             toml.required_features.clone(),
@@ -928,6 +932,54 @@ fn validate_unique_build_scripts(scripts: &[String]) -> CargoResult<()> {
     Ok(())
 }
 
+// Will check if `path` specified for a target is a valid source file
+fn validate_target_path_as_source_file(
+    path: &Path,
+    target_name: &str,
+    target_kind: &str,
+) -> CargoResult<()> {
+    if !path.exists() {
+        anyhow::bail!(
+            "can't find {} `{}` at path `{}`",
+            target_kind,
+            target_name,
+            path.display()
+        );
+    }
+
+    // If the path is likely a crate, then suggest setting the path to the entrypoint
+    if path.is_dir() {
+        anyhow::bail!(
+            "path `{}` for {} `{}` is a directory, but a source file was expected.\n\
+Consider setting the path to the intended entrypoint file instead: `{}/.../main.rs`",
+            path.display(),
+            target_kind,
+            target_name,
+            path.display(),
+        );
+    }
+
+    // Validate if the path has a .rs extension
+    if let Some(extension) = path.extension() {
+        if extension != "rs" {
+            anyhow::bail!(
+                "{} `{}` has path `{}` which does not have a `.rs` extension",
+                target_kind,
+                target_name,
+                path.display()
+            );
+        }
+    } else {
+        anyhow::bail!(
+            "{} `{}` has path `{}` which has no file extension (expected `.rs`)",
+            target_kind,
+            target_name,
+            path.display()
+        );
+    }
+    Ok(())
+}
+
 fn configure(
     toml: &TomlTarget,
     target: &mut Target,

tests/testsuite/bad_config.rs

@@ -3755,3 +3755,42 @@ please set bin.path in Cargo.toml
 "#]])
         .run();
 }
+
+#[cargo_test]
+fn example_target_path_validation() {
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+[package]
+name = "foo"
+version = "0.1.0"
+edition = "2024"
+
+[[example]]
+name = "fuzz"
+path = "examples/fuzz"
+"#,
+        )
+        .file("src/main.rs", "")
+        .file("examples/fuzz/src/main.rs", "")
+        .file(
+            "examples/fuzz/Cargo.toml",
+            r#"
+          [package]
+          name = "fuzz"
+          version = "0.1.0"
+          edition = "2024"
+          "#,
+        )
+        .build();
+    p.cargo("check --example fuzz")
+        .with_status(101)
+        .with_stderr_data(str![[r#"[ERROR] failed to parse manifest at `[ROOT]/foo/Cargo.toml`
+
+Caused by:
+  path `[ROOT]/foo/examples/fuzz` for example `fuzz` is a directory, but a source file was expected.
+  Consider setting the path to the intended entrypoint file instead: `[ROOT]/foo/examples/fuzz/.../main.rs`
+
+"#]]).run();
+}