Auto merge of #117814 - RalfJung:rustc-logger-without-set-var, r=TaKO8Ki

rustc_log: provide a way to init logging based on the values, not names, of the env vars

Miri wants to affect how rustc does logging. So far this required setting environment variables before calling `rustc_driver::init_rustc_env_logger`. However, `set_var` is a function one should really [avoid calling](https://github.com/rust-lang/rust/issues/90308), so this adds the necessary APIs to rustc such that Miri can just pass it the *values* of all the log-relevant environment variables, rather than having to change the global environment.

2 files changed, 39 insertions, 16 deletions

diff --git a/compiler/rustc_driver_impl/src/lib.rs b/compiler/rustc_driver_impl/src/lib.rs
index 7905a0e3924..17a53aafe9a 100644
--- a/compiler/rustc_driver_impl/src/lib.rs
+++ b/compiler/rustc_driver_impl/src/lib.rs
@@ -1520,14 +1520,14 @@ fn report_ice(
 /// This allows tools to enable rust logging without having to magically match rustc's
 /// tracing crate version.
 pub fn init_rustc_env_logger(handler: &EarlyErrorHandler) {
-    init_env_logger(handler, "RUSTC_LOG");
+    init_logger(handler, rustc_log::LoggerConfig::from_env("RUSTC_LOG"));
 }
 
 /// This allows tools to enable rust logging without having to magically match rustc's
-/// tracing crate version. In contrast to `init_rustc_env_logger` it allows you to choose an env var
-/// other than `RUSTC_LOG`.
-pub fn init_env_logger(handler: &EarlyErrorHandler, env: &str) {
-    if let Err(error) = rustc_log::init_env_logger(env) {
+/// tracing crate version. In contrast to `init_rustc_env_logger` it allows you to choose
+/// the values directly rather than having to set an environment variable.
+pub fn init_logger(handler: &EarlyErrorHandler, cfg: rustc_log::LoggerConfig) {
+    if let Err(error) = rustc_log::init_logger(cfg) {
         handler.early_error(error.to_string());
     }
 }
diff --git a/compiler/rustc_log/src/lib.rs b/compiler/rustc_log/src/lib.rs
index 0c9ec556549..e3464cb8a2d 100644
--- a/compiler/rustc_log/src/lib.rs
+++ b/compiler/rustc_log/src/lib.rs
@@ -14,7 +14,7 @@
 //!
 //! ```
 //! fn main() {
-//!     rustc_log::init_env_logger("LOG").unwrap();
+//!     rustc_log::init_logger(rustc_log::LoggerConfig::from_env("LOG")).unwrap();
 //!
 //!     let edition = rustc_span::edition::Edition::Edition2021;
 //!     rustc_span::create_session_globals_then(edition, || {
@@ -52,13 +52,36 @@ use tracing_subscriber::fmt::{
 };
 use tracing_subscriber::layer::SubscriberExt;
 
-pub fn init_env_logger(env: &str) -> Result<(), Error> {
-    let filter = match env::var(env) {
+/// The values of all the environment variables that matter for configuring a logger.
+/// Errors are explicitly preserved so that we can share error handling.
+pub struct LoggerConfig {
+    pub filter: Result<String, VarError>,
+    pub color_logs: Result<String, VarError>,
+    pub verbose_entry_exit: Result<String, VarError>,
+    pub verbose_thread_ids: Result<String, VarError>,
+    pub backtrace: Result<String, VarError>,
+}
+
+impl LoggerConfig {
+    pub fn from_env(env: &str) -> Self {
+        LoggerConfig {
+            filter: env::var(env),
+            color_logs: env::var(format!("{env}_COLOR")),
+            verbose_entry_exit: env::var(format!("{env}_ENTRY_EXIT")),
+            verbose_thread_ids: env::var(format!("{env}_THREAD_IDS")),
+            backtrace: env::var(format!("{env}_BACKTRACE")),
+        }
+    }
+}
+
+/// Initialize the logger with the given values for the filter, coloring, and other options env variables.
+pub fn init_logger(cfg: LoggerConfig) -> Result<(), Error> {
+    let filter = match cfg.filter {
         Ok(env) => EnvFilter::new(env),
         _ => EnvFilter::default().add_directive(Directive::from(LevelFilter::WARN)),
     };
 
-    let color_logs = match env::var(String::from(env) + "_COLOR") {
+    let color_logs = match cfg.color_logs {
         Ok(value) => match value.as_ref() {
             "always" => true,
             "never" => false,
@@ -69,14 +92,14 @@ pub fn init_env_logger(env: &str) -> Result<(), Error> {
         Err(VarError::NotUnicode(_value)) => return Err(Error::NonUnicodeColorValue),
     };
 
-    let verbose_entry_exit = match env::var_os(String::from(env) + "_ENTRY_EXIT") {
-        None => false,
-        Some(v) => &v != "0",
+    let verbose_entry_exit = match cfg.verbose_entry_exit {
+        Ok(v) => &v != "0",
+        Err(_) => false,
     };
 
-    let verbose_thread_ids = match env::var_os(String::from(env) + "_THREAD_IDS") {
-        None => false,
-        Some(v) => &v == "1",
+    let verbose_thread_ids = match cfg.verbose_thread_ids {
+        Ok(v) => &v == "1",
+        Err(_) => false,
     };
 
     let layer = tracing_tree::HierarchicalLayer::default()
@@ -91,7 +114,7 @@ pub fn init_env_logger(env: &str) -> Result<(), Error> {
         .with_thread_names(verbose_thread_ids);
 
     let subscriber = tracing_subscriber::Registry::default().with(filter).with(layer);
-    match env::var(format!("{env}_BACKTRACE")) {
+    match cfg.backtrace {
         Ok(str) => {
             let fmt_layer = tracing_subscriber::fmt::layer()
                 .with_writer(io::stderr)