feat: Add environment variable substitution in config files (#1577)

closes #942 

This PR brings support for environment variable substitution in config
files. This simply uses the [subst](https://github.com/fizyr/subst)
crate to parse all environment variables before parsing the toml, this
allows to define toml values in environment variables for more
flexibility.

Author: rdelaage

Cargo.lock

@@ -139,6 +139,7 @@ opentelemetry = { version = "0.30.0", default-features = false, features = ["met
 opentelemetry-otlp = { version = "0.30.0", features = ["metrics"], optional = true }
 opentelemetry_sdk = { version = "0.30.0", default-features = false, features = ["metrics"], optional = true }
 rhai = { version = "1", features = ["sync", "serde", "no_optimize", "no_module", "no_custom_syntax", "only_i64"], optional = true }
+subst = "0.3.8"
 
 [dev-dependencies]
 abscissa_core = { version = "0.8.2", default-features = false, features = ["testing"] }

Cargo.toml

@@ -13,7 +13,7 @@ use std::{
     path::PathBuf,
 };
 
-use abscissa_core::{FrameworkError, config::Config, path::AbsPathBuf};
+use abscissa_core::{FrameworkError, FrameworkErrorKind, config::Config, path::AbsPathBuf};
 use anyhow::{Result, anyhow};
 use clap::{Parser, ValueHint};
 use conflate::Merge;
@@ -120,7 +120,17 @@ impl RusticConfig {
 
         if let Some(path) = paths.iter().find(|path| path.exists()) {
             merge_logs.push((Level::Info, format!("using config {}", path.display())));
-            let mut config = Self::load_toml_file(AbsPathBuf::canonicalize(path)?)?;
+            let config_content = subst::substitute(
+                &std::fs::read_to_string(AbsPathBuf::canonicalize(path)?)?,
+                &subst::Env,
+            )
+            .map_err(|e| {
+                abscissa_core::error::context::Context::new(
+                    FrameworkErrorKind::ParseError,
+                    Some(Box::new(e)),
+                )
+            })?;
+            let mut config = Self::load_toml(config_content)?;
             // if "use_profile" is defined in config file, merge the referenced profiles first
             for profile in &config.global.use_profiles.clone() {
                 config.merge_profile(profile, merge_logs, Level::Warn)?;