Improve file found in multiple build targets warning

Signed-off-by: hi-rustin <rustin.liu@gmail.com>

src/cargo/util/toml/mod.rs

@@ -1,4 +1,5 @@
-use std::collections::{BTreeMap, BTreeSet, HashMap, HashSet};
+use std::collections::hash_map::Entry;
+use std::collections::{BTreeMap, BTreeSet, HashMap};
 use std::fmt;
 use std::marker::PhantomData;
 use std::path::{Path, PathBuf};
@@ -1737,12 +1738,22 @@ impl TomlManifest {
             debug!("manifest has no build targets");
         }
 
-        if let Err(e) = unique_build_targets(&targets, package_root) {
-            warnings.push(format!(
-                "file found to be present in multiple \
-                 build targets: {}",
-                e
-            ));
+        if let Err(conflict_targets) = unique_build_targets(&targets, package_root) {
+            conflict_targets.iter().for_each(|c| {
+                warnings.push(format!(
+                    "file `{}` found to be present in multiple \
+                 build targets:\n{}",
+                    c.0.display().to_string(),
+                    c.1.iter()
+                        .map(|t| format!(
+                            "  * This file exist in `{}` as a `{}` target.",
+                            t.name(),
+                            t.kind().description()
+                        ))
+                        .collect::<Vec<String>>()
+                        .join("\n")
+                ));
+            })
         }
 
         if let Some(links) = &package.links {
@@ -2442,16 +2453,34 @@ fn default_readme_from_package_root(package_root: &Path) -> Option<String> {
 
 /// Checks a list of build targets, and ensures the target names are unique within a vector.
 /// If not, the name of the offending build target is returned.
-fn unique_build_targets(targets: &[Target], package_root: &Path) -> Result<(), String> {
-    let mut seen = HashSet::new();
+fn unique_build_targets(
+    targets: &[Target],
+    package_root: &Path,
+) -> Result<(), HashMap<PathBuf, Vec<Target>>> {
+    let mut source_targets = HashMap::new();
     for target in targets {
         if let TargetSourcePath::Path(path) = target.src_path() {
             let full = package_root.join(path);
-            if !seen.insert(full.clone()) {
-                return Err(full.display().to_string());
+            match source_targets.entry(full) {
+                Entry::Vacant(e) => {
+                    e.insert(vec![target.clone()]);
+                }
+                Entry::Occupied(mut e) => {
+                    e.get_mut().push(target.clone());
+                }
             }
         }
     }
+
+    let conflict_targets = source_targets
+        .into_iter()
+        .filter(|(_, targets)| targets.len() > 1)
+        .collect::<HashMap<_, _>>();
+
+    if !conflict_targets.is_empty() {
+        return Err(conflict_targets);
+    }
+
     Ok(())
 }