fix(add): Respect --locked

This is prep for #10901 to avoid the most common failure case for the
lock file.  We are assuming if the users action caused a change in the
manifes, then it will cause a change in the lock file.  This isn't
entirely true but close enough and I think these semantics can make
sense.

Author: epage

src/cargo/ops/cargo_add/manifest.rs

@@ -238,8 +238,7 @@ impl str::FromStr for Manifest {
 
 impl std::fmt::Display for Manifest {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
-        let s = self.data.to_string();
-        s.fmt(f)
+        self.data.fmt(f)
     }
 }
 
@@ -433,6 +432,12 @@ impl LocalManifest {
     }
 }
 
+impl std::fmt::Display for LocalManifest {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        self.manifest.fmt(f)
+    }
+}
+
 #[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord)]
 enum DependencyStatus {
     None,

src/cargo/ops/cargo_add/mod.rs

@@ -62,6 +62,7 @@ pub fn add(workspace: &Workspace<'_>, options: &AddOptions<'_>) -> CargoResult<(
 
     let manifest_path = options.spec.manifest_path().to_path_buf();
     let mut manifest = LocalManifest::try_new(&manifest_path)?;
+    let original_raw_manifest = manifest.to_string();
     let legacy = manifest.get_legacy_sections();
     if !legacy.is_empty() {
         anyhow::bail!(
@@ -142,6 +143,16 @@ pub fn add(workspace: &Workspace<'_>, options: &AddOptions<'_>) -> CargoResult<(
         }
     }
 
+    if options.config.locked() {
+        let new_raw_manifest = manifest.to_string();
+        if original_raw_manifest != new_raw_manifest {
+            anyhow::bail!(
+                "the manifest file {} needs to be updated but --locked was passed to prevent this",
+                manifest.path.display()
+            );
+        }
+    }
+
     if options.dry_run {
         options.config.shell().warn("aborting add due to dry run")?;
     } else {

tests/testsuite/cargo_add/locked_changed/in

@@ -0,0 +1 @@
+../add-basic.in

tests/testsuite/cargo_add/locked_changed/mod.rs

@@ -0,0 +1,25 @@
+use cargo_test_support::compare::assert_ui;
+use cargo_test_support::prelude::*;
+use cargo_test_support::Project;
+
+use crate::cargo_add::init_registry;
+use cargo_test_support::curr_dir;
+
+#[cargo_test]
+fn locked_changed() {
+    init_registry();
+    let project = Project::from_template(curr_dir!().join("in"));
+    let project_root = project.root();
+    let cwd = &project_root;
+
+    snapbox::cmd::Command::cargo_ui()
+        .arg("add")
+        .arg_line("my-package --locked")
+        .current_dir(cwd)
+        .assert()
+        .failure()
+        .stdout_matches_path(curr_dir!().join("stdout.log"))
+        .stderr_matches_path(curr_dir!().join("stderr.log"));
+
+    assert_ui().subset_matches(curr_dir!().join("out"), &project_root);
+}

tests/testsuite/cargo_add/locked_changed/out/Cargo.toml

@@ -0,0 +1,5 @@
+[workspace]
+
+[package]
+name = "cargo-list-test-fixture"
+version = "0.0.0"

tests/testsuite/cargo_add/locked_changed/stderr.log

@@ -0,0 +1,3 @@
+    Updating `dummy-registry` index
+      Adding my-package v99999.0.0 to dependencies.
+error: the manifest file [ROOT]/case/Cargo.toml needs to be updated but --locked was passed to prevent this

tests/testsuite/cargo_add/locked_changed/stdout.log

@@ -0,0 +1,8 @@
+[workspace]
+
+[package]
+name = "cargo-list-test-fixture"
+version = "0.0.0"
+
+[dependencies]
+my-package = "99999.0.0"

tests/testsuite/cargo_add/locked_unchanged/in/Cargo.toml

@@ -0,0 +1,25 @@
+use cargo_test_support::compare::assert_ui;
+use cargo_test_support::prelude::*;
+use cargo_test_support::Project;
+
+use crate::cargo_add::init_registry;
+use cargo_test_support::curr_dir;
+
+#[cargo_test]
+fn locked_unchanged() {
+    init_registry();
+    let project = Project::from_template(curr_dir!().join("in"));
+    let project_root = project.root();
+    let cwd = &project_root;
+
+    snapbox::cmd::Command::cargo_ui()
+        .arg("add")
+        .arg_line("my-package --locked")
+        .current_dir(cwd)
+        .assert()
+        .success()
+        .stdout_matches_path(curr_dir!().join("stdout.log"))
+        .stderr_matches_path(curr_dir!().join("stderr.log"));
+
+    assert_ui().subset_matches(curr_dir!().join("out"), &project_root);
+}