feat(cli): Accept 'cargo Cargo.toml'

This wasn't in the original Pre-RFC but in terms of consistently
accepting manifests everything, I think this makes sense.

src/bin/cargo/cli.rs

@@ -492,7 +492,7 @@ pub fn cli() -> Command {
     let usage = if is_rustup {
         "cargo [+toolchain] [OPTIONS] [COMMAND]\n       cargo [+toolchain] [OPTIONS] -Zscript <MANIFEST_RS> [ARGS]..."
     } else {
-        "cargo [OPTIONS] [COMMAND]\n       cargo [OPTIONS] -Zscript <MANIFEST_RS> [ARGS]..."
+        "cargo [OPTIONS] [COMMAND]\n       cargo [OPTIONS] -Zscript <MANIFEST> [ARGS]..."
     };
     Command::new("cargo")
         // Subcommands all count their args' display order independently (from 0),

src/bin/cargo/commands/run.rs

@@ -87,7 +87,9 @@ pub fn exec(config: &mut Config, args: &ArgMatches) -> CliResult {
 
 pub fn is_manifest_command(arg: &str) -> bool {
     let path = Path::new(arg);
-    1 < path.components().count() || path.extension() == Some(OsStr::new("rs"))
+    1 < path.components().count()
+        || path.extension() == Some(OsStr::new("rs"))
+        || path.file_name() == Some(OsStr::new("Cargo.toml"))
 }
 
 pub fn exec_manifest_command(config: &Config, cmd: &str, args: &[OsString]) -> CliResult {

src/doc/src/reference/unstable.md

@@ -1462,16 +1462,19 @@ persistent lockfile.
 
 #### Manifest-commands
 
-You may pass single-file packages directly to the `cargo` command, without subcommand.  This is mostly intended for being put in `#!` lines.
+You may pass a manifest directly to the `cargo` command, without a subcommand,
+like `foo/Cargo.toml` or a single-file package like `foo.rs`.  This is mostly
+intended for being put in `#!` lines.
 
 The precedence for how to interpret `cargo <subcommand>` is
 1. Built-in xor single-file packages
 2. Aliases
 3. External subcommands
 
-A parameter is identified as a single-file package if it has one of:
+A parameter is identified as a manifest-command if it has one of:
 - Path separators
 - A `.rs` extension
+- The file name is `Cargo.toml`
 
 ### `[lints]`
 

tests/testsuite/script.rs

@@ -73,13 +73,16 @@ fn basic_cargo_toml() {
 
     p.cargo("-Zscript Cargo.toml")
         .masquerade_as_nightly_cargo(&["script"])
-        .with_status(101)
-        .with_stdout("")
+        .with_stdout(
+            r#"bin: target/debug/foo[EXE]
+args: []
+"#,
+        )
         .with_stderr(
             "\
-error: no such command: `Cargo.toml`
-
-<tab>View all installed commands with `cargo --list`
+[COMPILING] foo v0.0.1 ([ROOT]/foo)
+[FINISHED] dev [unoptimized + debuginfo] target(s) in [..]s
+[RUNNING] `target/debug/foo[EXE]`
 ",
         )
         .run();