fix(package): warn if symlinks checked out as plain text files (#14994)

### What does this PR try to resolve?

`cargo package` will warn users when git `core.symlinks` is `false`
and some symlinks were checked out as plain files during packaging.

Git config [`core.symlinks`] defaults to true when unset.
In git-for-windows (and git as well),
the config should be set to false explicitly when the repo was created,
if symlink support wasn't detected [^1].

We assume the config was always set at creation time and never changed.
So, if it is true, we don't bother users with any warning.

[^1]:
https://github.com/git-for-windows/git/blob/f1241afcc7956918d5da33ef74abd9cbba369247/setup.c#L2394-L2403

[`core.symlinks`]:
https://git-scm.com/docs/git-config#Documentation/git-config.txt-coresymlinks

### How should we test and review this PR?

CI passes.

This shares two commits 42dc4ef and
c8c8223 with #14981.

I didn't commit to fix all symlink issues all at once.
This PR demonstrates how we could leverage metadata in `PathEntry`.
Maybe later we can really follow plain-text symlinks and resolve the
issue.

### Additional information

cc #5664

Author: epage

src/cargo/ops/cargo_package/mod.rs

@@ -16,6 +16,7 @@ use crate::core::Workspace;
 use crate::core::{Package, PackageId, PackageSet, Resolve, SourceId};
 use crate::ops::lockfile::LOCKFILE_NAME;
 use crate::ops::registry::{infer_registry, RegistryOrIndex};
+use crate::sources::path::PathEntry;
 use crate::sources::registry::index::{IndexPackage, RegistryDependency};
 use crate::sources::{PathSource, CRATES_IO_REGISTRY};
 use crate::util::cache_lock::CacheLockMode;
@@ -396,7 +397,7 @@ fn prepare_archive(
 fn build_ar_list(
     ws: &Workspace<'_>,
     pkg: &Package,
-    src_files: Vec<PathBuf>,
+    src_files: Vec<PathEntry>,
     vcs_info: Option<vcs::VcsInfo>,
 ) -> CargoResult<Vec<ArchiveFile>> {
     let mut result = HashMap::new();
@@ -420,7 +421,7 @@ fn build_ar_list(
                     .push(ArchiveFile {
                         rel_path: rel_path.to_owned(),
                         rel_str: rel_str.to_owned(),
-                        contents: FileContents::OnDisk(src_file.clone()),
+                        contents: FileContents::OnDisk(src_file.to_path_buf()),
                     });
             }
         }

src/cargo/ops/cargo_package/vcs.rs

@@ -9,6 +9,7 @@ use serde::Serialize;
 use tracing::debug;
 
 use crate::core::Package;
+use crate::sources::PathEntry;
 use crate::CargoResult;
 use crate::GlobalContext;
 
@@ -41,7 +42,7 @@ pub struct GitVcsInfo {
 #[tracing::instrument(skip_all)]
 pub fn check_repo_state(
     p: &Package,
-    src_files: &[PathBuf],
+    src_files: &[PathEntry],
     gctx: &GlobalContext,
     opts: &PackageOpts<'_>,
 ) -> CargoResult<Option<VcsInfo>> {
@@ -91,6 +92,8 @@ pub fn check_repo_state(
         return Ok(None);
     }
 
+    warn_symlink_checked_out_as_plain_text_file(gctx, src_files, &repo)?;
+
     debug!(
         "found (git) Cargo.toml at `{}` in workdir `{}`",
         path.display(),
@@ -110,11 +113,57 @@ pub fn check_repo_state(
     return Ok(Some(VcsInfo { git, path_in_vcs }));
 }
 
+/// Warns if any symlinks were checked out as plain text files.
+///
+/// Git config [`core.symlinks`] defaults to true when unset.
+/// In git-for-windows (and git as well),
+/// the config should be set to false explicitly when the repo was created,
+/// if symlink support wasn't detected [^1].
+///
+/// We assume the config was always set at creation time and never changed.
+/// So, if it is true, we don't bother users with any warning.
+///
+/// [^1]: <https://github.com/git-for-windows/git/blob/f1241afcc7956918d5da33ef74abd9cbba369247/setup.c#L2394-L2403>
+///
+/// [`core.symlinks`]: https://git-scm.com/docs/git-config#Documentation/git-config.txt-coresymlinks
+fn warn_symlink_checked_out_as_plain_text_file(
+    gctx: &GlobalContext,
+    src_files: &[PathEntry],
+    repo: &git2::Repository,
+) -> CargoResult<()> {
+    if repo
+        .config()
+        .and_then(|c| c.get_bool("core.symlinks"))
+        .unwrap_or(true)
+    {
+        return Ok(());
+    }
+
+    if src_files.iter().any(|f| f.maybe_plain_text_symlink()) {
+        let mut shell = gctx.shell();
+        shell.warn(format_args!(
+            "found symbolic links that may be checked out as regular files for git repo at `{}`\n\
+            This might cause the `.crate` file to include incorrect or incomplete files",
+            repo.workdir().unwrap().display(),
+        ))?;
+        let extra_note = if cfg!(windows) {
+            "\nAnd on Windows, enable the Developer Mode to support symlinks"
+        } else {
+            ""
+        };
+        shell.note(format_args!(
+            "to avoid this, set the Git config `core.symlinks` to `true`{extra_note}",
+        ))?;
+    }
+
+    Ok(())
+}
+
 /// The real git status check starts from here.
 fn git(
     pkg: &Package,
     gctx: &GlobalContext,
-    src_files: &[PathBuf],
+    src_files: &[PathEntry],
     repo: &git2::Repository,
     opts: &PackageOpts<'_>,
 ) -> CargoResult<Option<GitVcsInfo>> {
@@ -136,6 +185,7 @@ fn git(
     let mut dirty_src_files: Vec<_> = src_files
         .iter()
         .filter(|src_file| dirty_files.iter().any(|path| src_file.starts_with(path)))
+        .map(|p| p.as_ref())
         .chain(dirty_metadata_paths(pkg, repo)?.iter())
         .map(|path| {
             pathdiff::diff_paths(path, cwd)

src/cargo/ops/vendor.rs

@@ -2,6 +2,7 @@ use crate::core::shell::Verbosity;
 use crate::core::{GitReference, Package, Workspace};
 use crate::ops;
 use crate::sources::path::PathSource;
+use crate::sources::PathEntry;
 use crate::sources::CRATES_IO_REGISTRY;
 use crate::util::cache_lock::CacheLockMode;
 use crate::util::{try_canonicalize, CargoResult, GlobalContext};
@@ -315,13 +316,14 @@ fn sync(
 fn cp_sources(
     pkg: &Package,
     src: &Path,
-    paths: &[PathBuf],
+    paths: &[PathEntry],
     dst: &Path,
     cksums: &mut BTreeMap<String, String>,
     tmp_buf: &mut [u8],
     gctx: &GlobalContext,
 ) -> CargoResult<()> {
     for p in paths {
+        let p = p.as_ref();
         let relative = p.strip_prefix(&src).unwrap();
 
         match relative.to_str() {

src/cargo/sources/mod.rs

@@ -29,6 +29,7 @@
 pub use self::config::SourceConfigMap;
 pub use self::directory::DirectorySource;
 pub use self::git::GitSource;
+pub use self::path::PathEntry;
 pub use self::path::PathSource;
 pub use self::path::RecursivePathSource;
 pub use self::registry::{