Span::resolved_at and Span::located_at to combine behavior of two spans

Author: dtolnay

src/lib.rs

@@ -162,6 +162,18 @@ impl Span {
         Span(imp::Span::def_site())
     }
 
+    /// Creates a new span with the same line/column information as `self` but
+    /// that resolves symbols as though it were at `other`.
+    pub fn resolved_at(&self, other: Span) -> Span {
+        Span(self.0.resolved_at(other.0))
+    }
+
+    /// Creates a new span with the same name resolution behavior as `self` but
+    /// with the line/column information of `other`.
+    pub fn located_at(&self, other: Span) -> Span {
+        Span(self.0.located_at(other.0))
+    }
+
     /// This method is only available when the `"nightly"` feature is enabled.
     #[cfg(feature = "nightly")]
     pub fn unstable(self) -> proc_macro::Span {

src/stable.rs

@@ -334,6 +334,17 @@ impl Span {
         Span::call_site()
     }
 
+    pub fn resolved_at(&self, _other: Span) -> Span {
+        // Stable spans consist only of line/column information, so
+        // `resolved_at` and `located_at` only select which span the
+        // caller wants line/column information from.
+        *self
+    }
+
+    pub fn located_at(&self, other: Span) -> Span {
+        other
+    }
+
     #[cfg(procmacro2_semver_exempt)]
     pub fn source_file(&self) -> SourceFile {
         CODEMAP.with(|cm| {

src/unstable.rs

@@ -225,6 +225,14 @@ impl Span {
         Span(proc_macro::Span::def_site())
     }
 
+    pub fn resolved_at(&self, other: Span) -> Span {
+        Span(self.0.resolved_at(other.0))
+    }
+
+    pub fn located_at(&self, other: Span) -> Span {
+        Span(self.0.located_at(other.0))
+    }
+
     pub fn unstable(self) -> proc_macro::Span {
         self.0
     }