add --dry-run option to cargo update

src/bin/cargo/commands/publish.rs

@@ -19,7 +19,7 @@ pub fn cli() -> App {
         .arg_target_dir()
         .arg_manifest_path()
         .arg_jobs()
-        .arg(opt("dry-run", "Perform all checks without uploading"))
+        .arg_dry_run("Perform all checks without uploading")
         .arg(opt("registry", "Registry to publish to").value_name("REGISTRY"))
 }
 

src/bin/cargo/commands/update.rs

@@ -10,6 +10,7 @@ pub fn cli() -> App {
             "aggressive",
             "Force updating all dependencies of <name> as well",
         ))
+        .arg_dry_run("Don't actually write the lockfile")
         .arg(opt("precise", "Update a single dependency to exactly PRECISE").value_name("PRECISE"))
         .arg_manifest_path()
         .after_help(
@@ -44,6 +45,7 @@ pub fn exec(config: &mut Config, args: &ArgMatches<'_>) -> CliResult {
         aggressive: args.is_present("aggressive"),
         precise: args.value_of("precise"),
         to_update: values(args, "package"),
+        dry_run: args.is_present("dry-run"),
         config,
     };
     ops::update_lockfile(&ws, &update_opts)?;

src/cargo/ops/cargo_generate_lockfile.rs

@@ -15,6 +15,7 @@ pub struct UpdateOptions<'a> {
     pub to_update: Vec<String>,
     pub precise: Option<&'a str>,
     pub aggressive: bool,
+    pub dry_run: bool,
 }
 
 pub fn generate_lockfile(ws: &Workspace) -> CargoResult<()> {
@@ -118,8 +119,13 @@ pub fn update_lockfile(ws: &Workspace, opts: &UpdateOptions) -> CargoResult<()>
             }
         }
     }
-
-    ops::write_pkg_lockfile(ws, &resolve)?;
+    if opts.dry_run {
+        opts.config
+            .shell()
+            .warn("not updating lockfile due to dry run")?;
+    } else {
+        ops::write_pkg_lockfile(ws, &resolve)?;
+    }
     return Ok(());
 
     fn fill_with_deps<'a>(

src/cargo/util/command_prelude.rs

@@ -177,6 +177,10 @@ pub trait AppExt: Sized {
                     .hidden(true),
             )
     }
+
+    fn arg_dry_run(self, dry_run: &'static str) -> Self {
+        self._arg(opt("dry-run", dry_run))
+    }
 }
 
 impl AppExt for App {

tests/testsuite/update.rs

@@ -415,3 +415,58 @@ fn preserve_top_comment() {
 
     assert!(lockfile == lockfile2);
 }
+
+#[test]
+fn dry_run_update() {
+    Package::new("log", "0.1.0").publish();
+    Package::new("serde", "0.1.0").dep("log", "0.1").publish();
+
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                [package]
+                name = "bar"
+                version = "0.0.1"
+                authors = []
+
+                [dependencies]
+                serde = "0.1"
+                log = "0.1"
+                foo = { path = "foo" }
+            "#,
+        )
+        .file("src/lib.rs", "")
+        .file(
+            "foo/Cargo.toml",
+            r#"
+                [package]
+                name = "foo"
+                version = "0.0.1"
+                authors = []
+
+                [dependencies]
+                serde = "0.1"
+            "#,
+        )
+        .file("foo/src/lib.rs", "")
+        .build();
+
+    p.cargo("build").run();
+    let old_lockfile = p.read_file("Cargo.lock");
+
+    Package::new("log", "0.1.1").publish();
+    Package::new("serde", "0.1.1").dep("log", "0.1").publish();
+
+    p.cargo("update -p serde --dry-run")
+        .with_stderr(
+            "\
+[UPDATING] `[..]` index
+[UPDATING] serde v0.1.0 -> v0.1.1
+[WARNING] not updating lockfile due to dry run
+",
+        )
+        .run();
+    let new_lockfile = p.read_file("Cargo.lock");
+    assert_eq!(old_lockfile, new_lockfile)
+}