proc_macro: Stay on the "use the cache" path more

Discovered in #50061 we're falling off the "happy path" of using a stringified
token stream more often than we should. This was due to the fact that a
user-written token like `0xf` is equality-different from the stringified token
of `15` (despite being semantically equivalent).

This patch updates the call to `eq_unspanned` with an even more awful solution,
`probably_equal_for_proc_macro`, which ignores the value of each token and
basically only compares the structure of the token stream, assuming that the AST
doesn't change just one token at a time.

While this is a step towards fixing #50061 there is still one regression
from #49154 which needs to be fixed.

src/libsyntax/tokenstream.rs

@@ -124,6 +124,24 @@ impl TokenTree {
         }
     }
 
+    // See comments in `interpolated_to_tokenstream` for why we care about
+    // *probably* equal here rather than actual equality
+    //
+    // This is otherwise the same as `eq_unspanned`, only recursing with a
+    // different method.
+    pub fn probably_equal_for_proc_macro(&self, other: &TokenTree) -> bool {
+        match (self, other) {
+            (&TokenTree::Token(_, ref tk), &TokenTree::Token(_, ref tk2)) => {
+                tk.probably_equal_for_proc_macro(tk2)
+            }
+            (&TokenTree::Delimited(_, ref dl), &TokenTree::Delimited(_, ref dl2)) => {
+                dl.delim == dl2.delim &&
+                dl.stream().probably_equal_for_proc_macro(&dl2.stream())
+            }
+            (_, _) => false,
+        }
+    }
+
     /// Retrieve the TokenTree's span.
     pub fn span(&self) -> Span {
         match *self {
@@ -250,6 +268,22 @@ impl TokenStream {
         t1.next().is_none() && t2.next().is_none()
     }
 
+    // See comments in `interpolated_to_tokenstream` for why we care about
+    // *probably* equal here rather than actual equality
+    //
+    // This is otherwise the same as `eq_unspanned`, only recursing with a
+    // different method.
+    pub fn probably_equal_for_proc_macro(&self, other: &TokenStream) -> bool {
+        let mut t1 = self.trees();
+        let mut t2 = other.trees();
+        for (t1, t2) in t1.by_ref().zip(t2.by_ref()) {
+            if !t1.probably_equal_for_proc_macro(&t2) {
+                return false;
+            }
+        }
+        t1.next().is_none() && t2.next().is_none()
+    }
+
     /// Precondition: `self` consists of a single token tree.
     /// Returns true if the token tree is a joint operation w.r.t. `proc_macro::TokenNode`.
     pub fn as_tree(self) -> (TokenTree, bool /* joint? */) {