feat: InMemoryEmitter (#451)

Adds an `InMemoryEmitter` that pushes diagnostics to a shared buffer.
Mapping spans to sources is done outside of the emitter itself.

---------

Co-authored-by: DaniPopes <[email protected]>

Author: onbjerg

crates/interface/src/diagnostics/emitter/mem.rs

@@ -0,0 +1,29 @@
+use crate::diagnostics::{Diag, Emitter};
+use solar_data_structures::sync::RwLock;
+use std::sync::Arc;
+
+/// An in-memory diagnostics emitter.
+///
+/// Diagnostics are pushed to a shared buffer as-is.
+///
+/// # Warning
+///
+/// Do **NOT** hold a read lock on the buffer across compiler passes as this will prevent the
+/// compiler from pushing diagnostics.
+pub struct InMemoryEmitter {
+    buffer: Arc<RwLock<Vec<Diag>>>,
+}
+
+impl InMemoryEmitter {
+    /// Creates a new emitter, returning the emitter itself and the buffer.
+    pub fn new() -> (Self, Arc<RwLock<Vec<Diag>>>) {
+        let buffer = Default::default();
+        (Self { buffer: Arc::clone(&buffer) }, buffer)
+    }
+}
+
+impl Emitter for InMemoryEmitter {
+    fn emit_diagnostic(&mut self, diagnostic: &crate::diagnostics::Diag) {
+        self.buffer.write().push(diagnostic.clone());
+    }
+}

crates/interface/src/diagnostics/emitter/mod.rs

@@ -10,6 +10,9 @@ mod json;
 #[cfg(feature = "json")]
 pub use json::JsonEmitter;
 
+mod mem;
+pub use mem::InMemoryEmitter;
+
 mod rustc;
 
 /// Dynamic diagnostic emitter. See [`Emitter`].

crates/interface/src/diagnostics/mod.rs

@@ -20,7 +20,8 @@ mod emitter;
 #[cfg(feature = "json")]
 pub use emitter::JsonEmitter;
 pub use emitter::{
-    DynEmitter, Emitter, HumanBufferEmitter, HumanEmitter, LocalEmitter, SilentEmitter,
+    DynEmitter, Emitter, HumanBufferEmitter, HumanEmitter, InMemoryEmitter, LocalEmitter,
+    SilentEmitter,
 };
 
 mod message;