Stop sorting Spans' SyntaxContext, as that is incompatible with incremental

Author: oli-obk

Diff for: Cargo.lock

@@ -4631,6 +4631,7 @@ dependencies = [
 name = "rustc_span"
 version = "0.0.0"
 dependencies = [
+ "derivative",
  "indexmap",
  "itoa",
  "md-5",

Diff for: compiler/rustc_span/Cargo.toml

@@ -5,6 +5,7 @@ edition = "2021"
 
 [dependencies]
 # tidy-alphabetical-start
+derivative = "2.2.0"
 indexmap = { version = "2.0.0" }
 itoa = "1.0"
 md5 = { package = "md-5", version = "0.10.0" }

Diff for: compiler/rustc_span/src/hygiene.rs

@@ -42,9 +42,15 @@ use std::fmt;
 use std::hash::Hash;
 
 /// A `SyntaxContext` represents a chain of pairs `(ExpnId, Transparency)` named "marks".
-#[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
+#[derive(Clone, Copy, PartialEq, Eq, Hash)]
 pub struct SyntaxContext(u32);
 
+// To ensure correctness of incremental compilation,
+// `SyntaxContext` must not implement `Ord` or `PartialOrd`.
+// See https://github.com/rust-lang/rust/issues/90317.
+impl !Ord for SyntaxContext {}
+impl !PartialOrd for SyntaxContext {}
+
 #[derive(Debug, Encodable, Decodable, Clone)]
 pub struct SyntaxContextData {
     outer_expn: ExpnId,

Diff for: compiler/rustc_span/src/lib.rs

@@ -469,46 +469,23 @@ impl FileName {
 /// `SpanData` is public because `Span` uses a thread-local interner and can't be
 /// sent to other threads, but some pieces of performance infra run in a separate thread.
 /// Using `Span` is generally preferred.
-#[derive(Clone, Copy, Hash, PartialEq, Eq)]
+#[derive(Clone, Copy, Hash, PartialEq, Eq, derivative::Derivative)]
+#[derivative(PartialOrd, Ord)]
 pub struct SpanData {
     pub lo: BytePos,
     pub hi: BytePos,
     /// Information about where the macro came from, if this piece of
     /// code was created by a macro expansion.
+    #[derivative(PartialOrd = "ignore", Ord = "ignore")]
+    // `SyntaxContext` does not implement `Ord`.
+    // The other fields are enough to determine in-file order.
     pub ctxt: SyntaxContext,
+    #[derivative(PartialOrd = "ignore", Ord = "ignore")]
+    // `LocalDefId` does not implement `Ord`.
+    // The other fields are enough to determine in-file order.
     pub parent: Option<LocalDefId>,
 }
 
-// Order spans by position in the file.
-impl Ord for SpanData {
-    fn cmp(&self, other: &Self) -> Ordering {
-        let SpanData {
-            lo: s_lo,
-            hi: s_hi,
-            ctxt: s_ctxt,
-            // `LocalDefId` does not implement `Ord`.
-            // The other fields are enough to determine in-file order.
-            parent: _,
-        } = self;
-        let SpanData {
-            lo: o_lo,
-            hi: o_hi,
-            ctxt: o_ctxt,
-            // `LocalDefId` does not implement `Ord`.
-            // The other fields are enough to determine in-file order.
-            parent: _,
-        } = other;
-
-        (s_lo, s_hi, s_ctxt).cmp(&(o_lo, o_hi, o_ctxt))
-    }
-}
-
-impl PartialOrd for SpanData {
-    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
-        Some(self.cmp(other))
-    }
-}
-
 impl SpanData {
     #[inline]
     pub fn span(&self) -> Span {