From ad3f3c7fb67329778bf26771cd4fbfc68e175daf Mon Sep 17 00:00:00 2001
From: Alex Macleod <alex@macleod.io>
Date: Thu, 25 Jul 2024 17:53:29 +0000
Subject: [PATCH] Skip locking span interner for some syntax context checks

---
 compiler/rustc_span/src/lib.rs           |  6 ------
 compiler/rustc_span/src/span_encoding.rs | 14 +++++++++++---
 2 files changed, 11 insertions(+), 9 deletions(-)

diff --git a/compiler/rustc_span/src/lib.rs b/compiler/rustc_span/src/lib.rs
index 7c8ac3be4be..6cb8491067d 100644
--- a/compiler/rustc_span/src/lib.rs
+++ b/compiler/rustc_span/src/lib.rs
@@ -564,12 +564,6 @@ impl Span {
         !self.is_dummy() && sm.is_span_accessible(self)
     }
 
-    /// Returns `true` if this span comes from any kind of macro, desugaring or inlining.
-    #[inline]
-    pub fn from_expansion(self) -> bool {
-        !self.ctxt().is_root()
-    }
-
     /// Returns `true` if `span` originates in a derive-macro's expansion.
     pub fn in_derive_expansion(self) -> bool {
         matches!(self.ctxt().outer_expn_data().kind, ExpnKind::Macro(MacroKind::Derive, _))
diff --git a/compiler/rustc_span/src/span_encoding.rs b/compiler/rustc_span/src/span_encoding.rs
index 53d7b7511a6..f56da203fa8 100644
--- a/compiler/rustc_span/src/span_encoding.rs
+++ b/compiler/rustc_span/src/span_encoding.rs
@@ -305,6 +305,13 @@ impl Span {
         }
     }
 
+    /// Returns `true` if this span comes from any kind of macro, desugaring or inlining.
+    #[inline]
+    pub fn from_expansion(self) -> bool {
+        // If the span is fully inferred then ctxt > MAX_CTXT
+        self.inline_ctxt().map_or(true, |ctxt| !ctxt.is_root())
+    }
+
     /// Returns `true` if this is a dummy span with any hygienic context.
     #[inline]
     pub fn is_dummy(self) -> bool {
@@ -372,9 +379,10 @@ impl Span {
     pub fn eq_ctxt(self, other: Span) -> bool {
         match (self.inline_ctxt(), other.inline_ctxt()) {
             (Ok(ctxt1), Ok(ctxt2)) => ctxt1 == ctxt2,
-            (Ok(ctxt), Err(index)) | (Err(index), Ok(ctxt)) => {
-                with_span_interner(|interner| ctxt == interner.spans[index].ctxt)
-            }
+            // If `inline_ctxt` returns `Ok` the context is <= MAX_CTXT.
+            // If it returns `Err` the span is fully interned and the context is > MAX_CTXT.
+            // As these do not overlap an `Ok` and `Err` result cannot have an equal context.
+            (Ok(_), Err(_)) | (Err(_), Ok(_)) => false,
             (Err(index1), Err(index2)) => with_span_interner(|interner| {
                 interner.spans[index1].ctxt == interner.spans[index2].ctxt
             }),