config reading: use same_file for suppressing "both files" warning

This is 100% reliable on Unix, and better on Windows.

(In this commit I avoid reindenting things to make review easier; the
formatting will be fixed in the next commit.)

Fixes rust-lang#13667

Cargo.toml

@@ -184,6 +184,7 @@ rand.workspace = true
 regex.workspace = true
 rusqlite.workspace = true
 rustfix.workspace = true
+same-file.workspace = true
 semver.workspace = true
 serde = { workspace = true, features = ["derive"] }
 serde-untagged.workspace = true

src/cargo/util/context/mod.rs

@@ -1542,36 +1542,32 @@ impl GlobalContext {
         //
         // Instead, if we got an error, we should bail, but perhaps
         // that might be too much risk of being a breaking change.
-        if possible.exists() {
+        if let Ok(possible_handle) = same_file::Handle::from_path(&possible) {
             if warn {
+                if let Ok(possible_with_extension_handle) =
+                    same_file::Handle::from_path(&possible_with_extension)
+                {
                 // We don't want to print a warning if the version
                 // without the extension is just a symlink to the version
                 // WITH an extension, which people may want to do to
                 // support multiple Cargo versions at once and not
                 // get a warning.
-                let skip_warning = if let Ok(target_path) = fs::read_link(&possible) {
-                    target_path == possible_with_extension
-                } else {
-                    false
-                };
-
-                if !skip_warning {
-                    if possible_with_extension.exists() {
+                    if possible_handle != possible_with_extension_handle {
                         self.shell().warn(format!(
                             "both `{}` and `{}` exist. Using `{}`",
                             possible.display(),
                             possible_with_extension.display(),
                             possible.display()
                         ))?;
-                    } else {
+                    }
+                } else {
                         self.shell().warn(format!(
                             "`{}` is deprecated in favor of `{filename_without_extension}.toml`",
                             possible.display(),
                         ))?;
                         self.shell().note(
                             format!("if you need to support cargo 1.38 or earlier, you can symlink `{filename_without_extension}` to `{filename_without_extension}.toml`"),
                         )?;
-                    }
                 }
             }
 

tests/testsuite/config.rs

@@ -348,12 +348,8 @@ f1 = 1
     assert_eq!(gctx.get::<Option<i32>>("foo.f1").unwrap(), Some(1));
 
     // It should NOT have warned for the symlink.
-    // But, currently it does!
     let output = read_output(gctx);
-    let expected = "\
-[WARNING] both `[..]/.cargo/config` and `[..]/.cargo/config.toml` exist. Using `[..]/.cargo/config`
-";
-    assert_match(expected, &output);
+    assert_match("", &output);
 }
 
 #[cargo_test]
@@ -378,12 +374,8 @@ f1 = 1
     assert_eq!(gctx.get::<Option<i32>>("foo.f1").unwrap(), Some(1));
 
     // It should NOT have warned for this situation.
-    // But, currently it does!
     let output = read_output(gctx);
-    let expected = "\
-[WARNING] both `[..]/.cargo/config` and `[..]/.cargo/config.toml` exist. Using `[..]/.cargo/config`
-";
-    assert_match(expected, &output);
+    assert_match("", &output);
 }
 
 #[cargo_test]