cargo run: on nightly, advertise the default-run feature

src/cargo/core/features.rs

@@ -392,7 +392,7 @@ thread_local!(
 /// - this is an `#[test]` that called `enable_nightly_features`
 /// - this is a integration test that uses `ProcessBuilder`
 ///       that called `masquerade_as_nightly_cargo`
-fn nightly_features_allowed() -> bool {
+pub fn nightly_features_allowed() -> bool {
     if ENABLE_NIGHTLY_FEATURES.with(|c| c.get()) {
         return true
     }

src/cargo/core/mod.rs

@@ -1,6 +1,10 @@
 pub use self::dependency::Dependency;
 pub use self::features::{CliUnstable, Edition, Feature, Features};
-pub use self::features::{maybe_allow_nightly_features, enable_nightly_features};
+pub use self::features::{
+    maybe_allow_nightly_features,
+    enable_nightly_features,
+    nightly_features_allowed
+};
 pub use self::manifest::{EitherManifest, VirtualManifest};
 pub use self::manifest::{LibKind, Manifest, Target, TargetKind};
 pub use self::package::{Package, PackageSet};

src/cargo/ops/cargo_run.rs

@@ -2,7 +2,7 @@ use std::path::Path;
 
 use ops;
 use util::{self, CargoResult, ProcessError};
-use core::{TargetKind, Workspace};
+use core::{TargetKind, Workspace, nightly_features_allowed};
 
 pub fn run(
     ws: &Workspace,
@@ -53,12 +53,22 @@ pub fn run(
     if bins.len() > 1 {
         if !options.filter.is_specific() {
             let names: Vec<&str> = bins.into_iter().map(|bin| bin.0).collect();
-            bail!(
+            if nightly_features_allowed() {
-                "`cargo run` requires that a project only have one \
+                bail!(
-                 executable; use the `--bin` option to specify which one \
+                    "`cargo run` could not determine which binary to run. \
-                 to run\navailable binaries: {}",
+                    Use the `--bin` option to specify a binary, \
-                 names.join(", ")
+                    or (on nightly) the `default-run` manifest key.\n\
-            )
+                    available binaries: {}",
+                    names.join(", ")
+                )
+            } else {
+                bail!(
+                    "`cargo run` requires that a project only have one \
+                    executable; use the `--bin` option to specify which one \
+                    to run\navailable binaries: {}",
+                    names.join(", ")
+                )
+            }
         } else {
             bail!(
                 "`cargo run` can run at most one executable, but \

tests/testsuite/run.rs

@@ -282,6 +282,17 @@ fn too_many_bins() {
              to specify which one to run\navailable binaries: [..]\n",
         ),
     );
+
+    assert_that(
+        p.cargo("run").masquerade_as_nightly_cargo(),
+        // Using [..] here because the order is not stable
+        execs().with_status(101).with_stderr(
+            "[ERROR] `cargo run` could not determine which binary to run. \
+             Use the `--bin` option to specify a binary, or (on \
+             nightly) the `default-run` manifest key.\
+             \navailable binaries: [..]\n",
+        ),
+    );
 }
 
 #[test]