Fix --manifest-path to point to Cargo.toml instead of a directory (#20)

Currently `cargo-subcommand` expects this argument to be a path to the
directory containing project(s), whereas it should actually point to a
file specifically named `Cargo.toml`.

Note that this path doesn't appear to be very strict in `cargo`: even if
pointing to a manifest file within a workspace, packages in the
surrounding workspace are still built, and are reachable through `-p`.

However, there's a strict requirement on the specified manifest to be
part of the workspace, which is not enforced in `cargo-subcommand` yet.

Author: MarijnS95

src/error.rs

@@ -9,6 +9,7 @@ pub enum Error {
     InvalidArgs,
     ManifestNotFound,
     RustcNotFound,
+    ManifestPathNotFound,
     GlobPatternError(&'static str),
     Glob(GlobError),
     Io(PathBuf, IoError),
@@ -20,6 +21,7 @@ impl Display for Error {
         f.write_str(match self {
             Self::InvalidArgs => "Invalid args.",
             Self::ManifestNotFound => "Didn't find Cargo.toml.",
+            Self::ManifestPathNotFound => "The manifest-path must be a path to a Cargo.toml file",
             Self::RustcNotFound => "Didn't find rustc.",
             Self::GlobPatternError(error) => error,
             Self::Glob(error) => return error.fmt(f),

src/subcommand.rs

@@ -3,6 +3,7 @@ use crate::artifact::{Artifact, CrateType};
 use crate::error::Error;
 use crate::profile::Profile;
 use crate::{utils, LocalizedConfig};
+use std::ffi::OsStr;
 use std::io::BufRead;
 use std::path::{Path, PathBuf};
 use std::process::Command;
@@ -34,11 +35,21 @@ impl Subcommand {
             args.exclude.is_empty(),
             "`--exclude` is not supported yet by `cargo-subcommand`"
         );
+
+        let manifest_path = args
+            .manifest_path
+            .clone()
+            .map(|path| {
+                if path.file_name() != Some(OsStr::new("Cargo.toml")) || !path.is_file() {
+                    Err(Error::ManifestPathNotFound)
+                } else {
+                    Ok(path)
+                }
+            })
+            .transpose()?;
+
         let (manifest_path, package) = utils::find_package(
-            &args
-                .manifest_path
-                .clone()
-                .unwrap_or_else(|| std::env::current_dir().unwrap()),
+            &manifest_path.unwrap_or_else(|| std::env::current_dir().unwrap()),
             args.package.get(0).map(|s| s.as_str()),
         )?;
         let root_dir = manifest_path.parent().unwrap();