init: improve error message and add tests

src/cargo/ops/cargo_new.rs

@@ -481,7 +481,7 @@ pub fn new(opts: &NewOptions, gctx: &GlobalContext) -> CargoResult<()> {
 
     mk(gctx, &mkopts).with_context(|| {
         format!(
-            "Failed to create package `{}` at `{}`",
+            "failed to create package `{}` at `{}`",
             name,
             path.display()
         )
@@ -513,7 +513,10 @@ pub fn init(opts: &NewOptions, gctx: &GlobalContext) -> CargoResult<NewProjectKi
         .status("Creating", format!("{} package", opts.kind))?;
 
     if path.join("Cargo.toml").exists() {
-        anyhow::bail!("`cargo init` cannot be run on existing Cargo packages")
+        anyhow::bail!(
+            "`cargo init` cannot be run on existing Cargo packages\n\
+             help: use `cargo new` to create a package in a new subdirectory"
+        )
     }
     check_path(path, &mut gctx.shell())?;
 
@@ -593,7 +596,7 @@ pub fn init(opts: &NewOptions, gctx: &GlobalContext) -> CargoResult<NewProjectKi
 
     mk(gctx, &mkopts).with_context(|| {
         format!(
-            "Failed to create package `{}` at `{}`",
+            "failed to create package `{}` at `{}`",
             name,
             path.display()
         )

tests/testsuite/cargo_init/error_on_existing_package.rs

@@ -0,0 +1,23 @@
+use crate::prelude::*;
+use cargo_test_support::paths;
+use cargo_test_support::str;
+use std::fs;
+
+#[cargo_test]
+fn init_error_on_existing_package() {
+    let project_root = paths::root().join("foo");
+    fs::create_dir_all(&project_root).unwrap();
+    fs::write(project_root.join("Cargo.toml"), "").unwrap();
+
+    snapbox::cmd::Command::cargo_ui()
+        .arg_line("init --color=never")
+        .current_dir(&project_root)
+        .assert()
+        .code(101)
+        .stderr_eq(str![[r#"
+    Creating binary (application) package
+error: `cargo init` cannot be run on existing Cargo packages
+help: use `cargo new` to create a package in a new subdirectory
+
+"#]]);
+}

tests/testsuite/cargo_init/mod.rs

@@ -13,6 +13,7 @@ mod confused_by_multiple_lib_files;
 mod creates_binary_when_both_binlib_present;
 mod creates_binary_when_instructed_and_has_lib_file;
 mod creates_library_when_instructed_and_has_bin_file;
+mod error_on_existing_package;
 mod explicit_bin_with_git;
 mod formats_source;
 mod fossil_autodetect;
@@ -35,6 +36,7 @@ mod no_filename;
 mod path_contains_separator;
 mod pijul_autodetect;
 mod reserved_name;
+mod reserved_name_core;
 mod simple_bin;
 mod simple_git;
 mod simple_git_ignore_exists;

tests/testsuite/cargo_init/reserved_name_core.rs

@@ -0,0 +1,30 @@
+use crate::prelude::*;
+use cargo_test_support::paths;
+use cargo_test_support::str;
+use std::fs;
+
+#[cargo_test]
+fn init_with_reserved_name_core() {
+    let project_root = paths::root().join("core");
+    fs::create_dir_all(&project_root).unwrap();
+
+    snapbox::cmd::Command::cargo_ui()
+        .arg_line("init --color=never")
+        .current_dir(&project_root)
+        .assert()
+        .stderr_eq(str![[r#"
+    Creating binary (application) package
+warning: package name `core` may be confused with the package with that name in Rust's standard library
+It is recommended to use a different name to avoid problems.
+note: the directory name is used as the package name
+help: to override the package name, pass `--name <pkgname>`
+help: to name the binary "core", use a valid package name, and set the binary name to be different from the package. This can be done by setting the binary filename to `src/bin/core.rs` or change the name in Cargo.toml with:
+
+    [[bin]]
+    name = "core"
+    path = "src/main.rs"
+
+note: see more `Cargo.toml` keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+"#]]);
+}