message: Improve file found in multiple build targets warning
diff: -use std::collections::{BTreeMap, BTreeSet, HashMap, HashSet};
  +use std::collections::{BTreeMap, BTreeSet, HashMap};
  use std::fmt;
  use std::marker::PhantomData;
  use std::path::{Path, PathBuf};
  @@ -8,6 +8,7 @@ use std::str::{self, FromStr};
  use anyhow::{anyhow, bail, Context as _};
  use cargo_platform::Platform;
  use cargo_util::paths;
  +use itertools::Itertools;
  use lazycell::LazyCell;
  use log::{debug, trace};
  use semver::{self, VersionReq};
  @@ -1737,12 +1738,24 @@ impl TomlManifest {
  debug!("manifest has no build targets");
  }
  - if let Err(e) = unique_build_targets(&targets, package_root) {
  + if let Err(conflict_targets) = unique_build_targets(&targets, package_root) {
  + conflict_targets
  + .iter()
  + .for_each(|(target_path, conflicts)| {
  warnings.push(format!(
  - "file found to be present in multiple \
  - build targets: {}",
  - e
  + "file `{}` found to be present in multiple \
  + build targets:\n{}",
  + target_path.display().to_string(),
  + conflicts
  + .iter()
  + .map(|t| format!(
  + " * `{}` target `{}`",
  + t.kind().description(),
  + t.name(),
  + ))
  + .join("\n")
  ));
  + })
  }
  if let Some(links) = &package.links {
  @@ -2442,16 +2455,27 @@ 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::<_, Vec<_>>::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());
  + source_targets.entry(full).or_default().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(())
  }
  