[DUX-1511] Add the ability to track warnings through re-compilation

Closes #148

Author: jkeuhlen

.gitignore

@@ -1,3 +1,4 @@
 /target
 /result*
 lcov.info
+.direnv/

docs/cli.md

@@ -37,6 +37,10 @@ Don't reload for `README.md` files:
 
     ghciwatch --reload-glob '!src/**/README.md'
 
+Track warnings across recompilations to prevent them from disappearing:
+
+    ghciwatch --track-warnings
+
 ## Arguments
 <dl>
 
@@ -88,6 +92,19 @@ Don't interrupt reloads when files change.
 
 Depending on your workflow, `ghciwatch` may feel more responsive with this set.
 
+</dd>
+<dt><a id="--track-warnings" href="#--track-warnings"><code>--track-warnings</code></a></dt><dd>
+
+Track warnings across recompilations.
+
+When enabled, warnings will be preserved in memory even when files are recompiled due to dependency changes, helping prevent "ephemeral warnings" from being missed.
+
+This feature addresses the common issue where GHC warnings disappear when files are recompiled due to dependency changes (without the file itself changing). With `--track-warnings`, warnings persist until the file is directly modified or removed.
+
+Tracked warnings are displayed with the same formatting and colors as fresh warnings, and are included in the error file output when using `--error-file`.
+
+Can also be enabled by setting the `GHCIWATCH_TRACK_WARNINGS` environment variable to any value.
+
 </dd>
 <dt><a id="--completions" href="#--completions"><code>--completions &lt;COMPLETIONS&gt;</code></a></dt><dd>
 

src/cli.rs

@@ -96,6 +96,13 @@ pub struct Opts {
     #[arg(long, hide = true)]
     pub tui: bool,
 
+    /// Track warnings across recompilations.
+    ///
+    /// When enabled, warnings will be preserved in memory even when files are recompiled
+    /// due to dependency changes, helping prevent "ephemeral warnings" from being missed.
+    #[arg(long, env = "GHCIWATCH_TRACK_WARNINGS")]
+    pub track_warnings: bool,
+
     /// Generate Markdown CLI documentation.
     #[cfg(feature = "clap-markdown")]
     #[arg(long, hide = true)]

src/ghci/compilation_log.rs

@@ -1,5 +1,6 @@
 use crate::ghci::parse::CompilationResult;
 use crate::ghci::parse::CompilationSummary;
+use crate::ghci::parse::CompilingModule;
 use crate::ghci::parse::GhcDiagnostic;
 use crate::ghci::parse::GhcMessage;
 use crate::ghci::parse::Severity;
@@ -9,6 +10,7 @@ use crate::ghci::parse::Severity;
 pub struct CompilationLog {
     pub summary: Option<CompilationSummary>,
     pub diagnostics: Vec<GhcDiagnostic>,
+    pub compiled_modules: Vec<CompilingModule>,
 }
 
 impl CompilationLog {
@@ -24,6 +26,7 @@ impl Extend<GhcMessage> for CompilationLog {
             match message {
                 GhcMessage::Compiling(module) => {
                     tracing::debug!(module = %module.name, path = %module.path, "Compiling");
+                    self.compiled_modules.push(module);
                 }
                 GhcMessage::Diagnostic(diagnostic) => {
                     if let GhcDiagnostic {

src/ghci/error_log.rs

@@ -1,3 +1,4 @@
+use camino::Utf8Path;
 use camino::Utf8PathBuf;
 use miette::IntoDiagnostic;
 use tokio::fs::File;
@@ -23,6 +24,11 @@ impl ErrorLog {
         Self { path }
     }
 
+    /// Get the path for this error log writer, if any.
+    pub fn path(&self) -> Option<&Utf8Path> {
+        self.path.as_deref()
+    }
+
     /// Write the error log, if any, with the given compilation summary and diagnostic messages.
     #[instrument(skip(self, log), name = "error_log_write", level = "debug")]
     pub async fn write(&mut self, log: &CompilationLog) -> miette::Result<()> {