Rollup merge of #40815 - estebank:issue-40006, r=GuillaumeGomez

Identify missing item category in `impl`s

```rust
struct S;
impl S {
    pub hello_method(&self) {
        println!("Hello");
    }
}
fn main() { S.hello_method(); }
```

```rust
error: missing `fn` for method declaration
 --> file.rs:3:4
  |
3 |     pub hello_method(&self) {
  |        ^ missing `fn`
```

Fix #40006. r? @pnkfelix CC @jonathandturner @GuillaumeGomez

src/libsyntax_pos/lib.rs

@@ -189,6 +189,30 @@ impl Span {
             Span { hi: end.hi, ..self }
         }
     }
+
+    pub fn between(self, end: Span) -> Span {
+        Span {
+            lo: self.hi,
+            hi: end.lo,
+            ctxt: if end.ctxt == SyntaxContext::empty() {
+                end.ctxt
+            } else {
+                self.ctxt
+            }
+        }
+    }
+
+    pub fn until(self, end: Span) -> Span {
+        Span {
+            lo: self.lo,
+            hi: end.lo,
+            ctxt: if end.ctxt == SyntaxContext::empty() {
+                end.ctxt
+            } else {
+                self.ctxt
+            }
+        }
+    }
 }
 
 #[derive(Clone, Debug)]