Add configuration-file support to the 'task' command

cli/src/settings.rs

@@ -0,0 +1,35 @@
+use config::{Config, Environment, File, FileSourceFile};
+use failure::Fallible;
+use std::env;
+use std::path::PathBuf;
+
+pub(crate) fn read_settings() -> Fallible<Config> {
+    let mut settings = Config::default();
+
+    // set up defaults
+    if let Some(mut dir) = dirs::data_local_dir() {
+        dir.push("taskchampion");
+        settings.set_default(
+            "data_dir",
+            // the config crate does not support non-string paths
+            dir.to_str().expect("data_local_dir is not utf-8"),
+        )?;
+    }
+
+    // load either from the path in TASKCHAMPION_CONFIG, or from CONFIG_DIR/taskchampion
+    if let Some(config_file) = env::var_os("TASKCHAMPION_CONFIG") {
+        let config_file: PathBuf = config_file.into();
+        let config_file: File<FileSourceFile> = config_file.into();
+        settings.merge(config_file.required(true))?;
+        env::remove_var("TASKCHAMPION_CONFIG");
+    } else if let Some(mut dir) = dirs::config_dir() {
+        dir.push("taskchampion");
+        let config_file: File<FileSourceFile> = dir.into();
+        settings.merge(config_file.required(false))?;
+    }
+
+    // merge environment variables
+    settings.merge(Environment::with_prefix("TASKCHAMPION"))?;
+
+    Ok(settings)
+}