feat: Use salsa for incremental compilation

https://github.com/salsa-rs/salsa

Cargo.toml

@@ -40,6 +40,7 @@ itertools = "0.8"
 futures = "0.1.11"
 codespan = "0.3"
 codespan-reporting = "0.3"
+salsa = { version = "0.13", path = "../salsa" }
 
 serde = { version = "1.0.0", optional = true }
 serde_state = { version = "0.4", optional = true }

base/src/error.rs

@@ -16,7 +16,7 @@ use codespan_reporting::{Diagnostic, Label};
 use crate::pos::{BytePos, Span, Spanned};
 
 /// An error type which can represent multiple errors.
-#[derive(Clone, Debug, PartialEq)]
+#[derive(Clone, Debug, Eq, PartialEq, Hash)]
 pub struct Errors<T> {
     errors: Vec<T>,
 }
@@ -159,6 +159,8 @@ pub struct InFile<E> {
     error: Errors<Spanned<E, BytePos>>,
 }
 
+impl<E> Eq for InFile<E> where E: Eq {}
+
 impl<E> PartialEq for InFile<E>
 where
     E: PartialEq,
@@ -168,6 +170,19 @@ where
     }
 }
 
+impl<E> std::hash::Hash for InFile<E>
+where
+    E: std::hash::Hash,
+{
+    #[inline(always)]
+    fn hash<H>(&self, state: &mut H)
+    where
+        H: std::hash::Hasher,
+    {
+        self.error.hash(state)
+    }
+}
+
 impl<E: fmt::Display> InFile<E> {
     /// Creates a new `InFile` error which states that the error occurred in `file` using the file
     /// contents in `source` to provide a context to the span.

base/src/pos.rs

@@ -46,6 +46,21 @@ pub struct Spanned<T, Pos> {
     pub value: T,
 }
 
+impl<T, Pos> std::hash::Hash for Spanned<T, Pos>
+where
+    T: std::hash::Hash,
+    Pos: std::hash::Hash + Copy,
+{
+    fn hash<H>(&self, state: &mut H)
+    where
+        H: std::hash::Hasher,
+    {
+        self.span.start().hash(state);
+        self.span.end().hash(state);
+        self.value.hash(state);
+    }
+}
+
 impl<T, Pos> Spanned<T, Pos> {
     pub fn map<U, F>(self, mut f: F) -> Spanned<U, Pos>
     where