add `Span` information into `Qself`

src/librustc_resolve/lib.rs

@@ -3139,6 +3139,17 @@ fn resolve_qpath(&mut self,
                      span: Span,
                      global_by_default: bool)
                      -> Option<PathResolution> {
+        debug!(
+            "resolve_qpath(id={:?}, qself={:?}, path={:?}, \
+             ns={:?}, span={:?}, global_by_default={:?})",
+            id,
+            qself,
+            path,
+            ns,
+            span,
+            global_by_default,
+        );
+
         if let Some(qself) = qself {
             if qself.position == 0 {
                 // FIXME: Create some fake resolution that can't possibly be a type.
@@ -3231,6 +3242,15 @@ fn resolve_path(
         let mut allow_super = true;
         let mut second_binding = None;
 
+        debug!(
+            "resolve_path(path={:?}, opt_ns={:?}, record_used={:?}, path_span={:?}, crate_lint={:?})",
+            path,
+            opt_ns,
+            record_used,
+            path_span,
+            crate_lint,
+        );
+
         for (i, &ident) in path.iter().enumerate() {
             debug!("resolve_path ident {} {:?}", i, ident);
             let is_last = i == path.len() - 1;
@@ -4454,6 +4474,7 @@ pub enum MakeGlobMap {
     No,
 }
 
+#[derive(Debug)]
 enum CrateLint {
     /// Do not issue the lint
     No,

src/libsyntax/ast.rs

@@ -1211,6 +1211,11 @@ pub enum ExprKind {
 #[derive(Clone, PartialEq, Eq, RustcEncodable, RustcDecodable, Hash, Debug)]
 pub struct QSelf {
     pub ty: P<Ty>,
+
+    /// The span of `a::b::Trait` in a path like `<Vec<T> as
+    /// a::b::Trait>::AssociatedItem`; in the case where `position ==
+    /// 0`, this is an empty span.
+    pub path_span: Span,
     pub position: usize
 }
 

src/libsyntax/ext/build.rs

@@ -373,6 +373,7 @@ fn qpath_all(&self,
 
         (ast::QSelf {
             ty: self_type,
+            path_span: path.span,
             position: path.segments.len() - 1
         }, path)
     }

src/libsyntax/fold.rs

@@ -390,9 +390,10 @@ pub fn noop_fold_ty<T: Folder>(t: P<Ty>, fld: &mut T) -> P<Ty> {
             TyKind::Tup(tys) => TyKind::Tup(tys.move_map(|ty| fld.fold_ty(ty))),
             TyKind::Paren(ty) => TyKind::Paren(fld.fold_ty(ty)),
             TyKind::Path(qself, path) => {
-                let qself = qself.map(|QSelf { ty, position }| {
+                let qself = qself.map(|QSelf { ty, path_span, position }| {
                     QSelf {
                         ty: fld.fold_ty(ty),
+                        path_span: fld.new_span(path_span),
                         position,
                     }
                 });
@@ -1131,7 +1132,11 @@ pub fn noop_fold_pat<T: Folder>(p: P<Pat>, folder: &mut T) -> P<Pat> {
             }
             PatKind::Path(opt_qself, pth) => {
                 let opt_qself = opt_qself.map(|qself| {
-                    QSelf { ty: folder.fold_ty(qself.ty), position: qself.position }
+                    QSelf {
+                        ty: folder.fold_ty(qself.ty),
+                        path_span: folder.new_span(qself.path_span),
+                        position: qself.position,
+                    }
                 });
                 PatKind::Path(opt_qself, folder.fold_path(pth))
             }
@@ -1292,9 +1297,10 @@ pub fn noop_fold_expr<T: Folder>(Expr {id, node, span, attrs}: Expr, folder: &mu
                                 lim)
             }
             ExprKind::Path(qself, path) => {
-                let qself = qself.map(|QSelf { ty, position }| {
+                let qself = qself.map(|QSelf { ty, path_span, position }| {
                     QSelf {
                         ty: folder.fold_ty(ty),
+                        path_span: folder.new_span(path_span),
                         position,
                     }
                 });

src/libsyntax/parse/parser.rs

@@ -1715,8 +1715,11 @@ fn maybe_recover_from_bad_qpath<T: RecoverQPath>(&mut self, base: T, allow_recov
         self.parse_path_segments(&mut segments, T::PATH_STYLE, true)?;
 
         let span = ty.span.to(self.prev_span);
-        let recovered =
-            base.to_recovered(Some(QSelf { ty, position: 0 }), ast::Path { segments, span });
+        let path_span = span.to(span); // use an empty path since `position` == 0
+        let recovered = base.to_recovered(
+            Some(QSelf { ty, path_span, position: 0 }),
+            ast::Path { segments, span },
+        );
 
         self.diagnostic()
             .struct_span_err(span, "missing angle brackets in associated item path")
@@ -1905,21 +1908,32 @@ pub fn parse_path_segment_ident(&mut self) -> PResult<'a, ast::Ident> {
     /// `qualified_path = <type [as trait_ref]>::path`
     ///
     /// # Examples
+    /// `<T>::default`
     /// `<T as U>::a`
     /// `<T as U>::F::a<S>` (without disambiguator)
     /// `<T as U>::F::a::<S>` (with disambiguator)
     fn parse_qpath(&mut self, style: PathStyle) -> PResult<'a, (QSelf, ast::Path)> {
         let lo = self.prev_span;
         let ty = self.parse_ty()?;
-        let mut path = if self.eat_keyword(keywords::As) {
-            self.parse_path(PathStyle::Type)?
+
+        // `path` will contain the prefix of the path up to the `>`,
+        // if any (e.g., `U` in the `<T as U>::*` examples
+        // above). `path_span` has the span of that path, or an empty
+        // span in the case of something like `<T>::Bar`.
+        let (mut path, path_span);
+        if self.eat_keyword(keywords::As) {
+            let path_lo = self.span;
+            path = self.parse_path(PathStyle::Type)?;
+            path_span = path_lo.to(self.prev_span);
         } else {
-            ast::Path { segments: Vec::new(), span: syntax_pos::DUMMY_SP }
-        };
+            path = ast::Path { segments: Vec::new(), span: syntax_pos::DUMMY_SP };
+            path_span = self.span.to(self.span);
+        }
+
         self.expect(&token::Gt)?;
         self.expect(&token::ModSep)?;
 
-        let qself = QSelf { ty, position: path.segments.len() };
+        let qself = QSelf { ty, path_span, position: path.segments.len() };
         self.parse_path_segments(&mut path.segments, style, true)?;
 
         Ok((qself, ast::Path { segments: path.segments, span: lo.to(self.prev_span) }))