Use the macro structure spans instead of the invocation

src/libsyntax/ext/expand.rs

@@ -1046,6 +1046,7 @@ impl<'feat> ExpansionConfig<'feat> {
 }
 
 // A Marker adds the given mark to the syntax context.
+#[derive(Debug)]
 pub struct Marker(pub Mark);
 
 impl Folder for Marker {

src/libsyntax/ext/tt/macro_rules.rs

@@ -119,8 +119,21 @@ fn generic_extension<'cx>(cx: &'cx mut ExtCtxt,
                     quoted::TokenTree::Delimited(_, ref delimed) => delimed.tts.clone(),
                     _ => cx.span_bug(sp, "malformed macro rhs"),
                 };
+
                 // rhs has holes ( `$id` and `$(...)` that need filled)
-                let tts = transcribe(cx, Some(named_matches), rhs);
+                let mut tts = transcribe(cx, Some(named_matches), rhs.clone());
+
+                // Replace all the tokens for the corresponding positions in the macro, to maintain
+                // proper positions in error reporting, while maintaining the macro_backtrace.
+                if rhs.len() == tts.len() {
+                    tts = tts.map_pos(|i, tt| {
+                        let mut tt = tt.clone();
+                        let mut sp = rhs[i].span();
+                        sp.ctxt = tt.span().ctxt;
+                        tt.set_span(sp);
+                        tt
+                    });
+                }
 
                 if cx.trace_macros() {
                     trace_macros_note(cx, sp, format!("to `{}`", tts));

src/libsyntax/ext/tt/quoted.rs

@@ -128,7 +128,7 @@ impl TokenTree {
         }
     }
 
-    /// Retrieve the TokenTree's span.
+    /// Retrieve the `TokenTree`'s span.
     pub fn span(&self) -> Span {
         match *self {
             TokenTree::Token(sp, _) |

src/libsyntax/parse/parser.rs

@@ -150,7 +150,7 @@ fn maybe_append(mut lhs: Vec<Attribute>, rhs: Option<Vec<Attribute>>)
     lhs
 }
 
-#[derive(Clone, Copy, PartialEq)]
+#[derive(Debug, Clone, Copy, PartialEq)]
 enum PrevTokenKind {
     DocComment,
     Comma,
@@ -6090,8 +6090,7 @@ impl<'a> Parser<'a> {
             let (delim, tts) = self.expect_delimited_token_tree()?;
             if delim != token::Brace {
                 if !self.eat(&token::Semi) {
-                    let prev_span = self.prev_span;
-                    self.span_err(prev_span,
+                    self.span_err(self.prev_span,
                                   "macros that expand to items must either \
                                    be surrounded with braces or followed by \
                                    a semicolon");
@@ -6108,8 +6107,7 @@ impl<'a> Parser<'a> {
         match visibility {
             Visibility::Inherited => {}
             _ => {
-                let prev_span = self.prev_span;
-                return Err(self.span_fatal(prev_span, "unmatched visibility `pub`"));
+                return Err(self.span_fatal(self.prev_span, "unmatched visibility `pub`"));
             }
         }
 

src/libsyntax/tokenstream.rs

@@ -131,6 +131,15 @@ impl TokenTree {
         }
     }
 
+    /// Modify the `TokenTree`'s span inplace.
+    pub fn set_span(&mut self, span: Span) {
+        match *self {
+            TokenTree::Token(ref mut sp, _) | TokenTree::Delimited(ref mut sp, _) => {
+                *sp = span;
+            }
+        }
+    }
+
     /// Indicates if the stream is a token that is equal to the provided token.
     pub fn eq_token(&self, t: Token) -> bool {
         match *self {
@@ -190,6 +199,14 @@ impl PartialEq<TokenStream> for TokenStream {
 }
 
 impl TokenStream {
+    pub fn len(&self) -> usize {
+        if let TokenStreamKind::Stream(ref slice) = self.kind {
+            slice.len()
+        } else {
+            0
+        }
+    }
+
     pub fn empty() -> TokenStream {
         TokenStream { kind: TokenStreamKind::Empty }
     }
@@ -241,6 +258,21 @@ impl TokenStream {
         }
     }
 
+    pub fn map_pos<F: FnMut(usize, TokenTree) -> TokenTree>(self, mut f: F) -> TokenStream {
+        let mut trees = self.into_trees();
+        let mut result = Vec::new();
+        let mut i = 0;
+        while let Some(stream) = trees.next_as_stream() {
+            result.push(match stream.kind {
+                TokenStreamKind::Tree(tree) => f(i, tree).into(),
+                TokenStreamKind::JointTree(tree) => f(i, tree).joint(),
+                _ => unreachable!()
+            });
+            i += 1;
+        }
+        TokenStream::concat(result)
+    }
+
     pub fn map<F: FnMut(TokenTree) -> TokenTree>(self, mut f: F) -> TokenStream {
         let mut trees = self.into_trees();
         let mut result = Vec::new();

src/libsyntax_pos/lib.rs

@@ -100,6 +100,7 @@ impl Span {
         if self.source_equal(&DUMMY_SP) { other } else { self }
     }
 
+    /// Return true if `self` fully encloses `other`.
     pub fn contains(self, other: Span) -> bool {
         self.lo <= other.lo && other.hi <= self.hi
     }
@@ -184,15 +185,32 @@ impl Span {
         result
     }
 
-    pub fn to(self, end: Span) -> Span {
-        // FIXME(jseyfried): self.ctxt should always equal end.ctxt here (c.f. issue #23480)
-        if self.ctxt == SyntaxContext::empty() {
-            Span { lo: self.lo, ..end }
-        } else {
-            Span { hi: end.hi, ..self }
-        }
+    pub fn empty_ctxt(&self) -> bool {
+        self.ctxt == SyntaxContext::empty()
     }
 
+    /// Return a `Span` that would enclose both `self` and `end`.
+    pub fn to(self, end: Span) -> Span {
+        let lo = if self.lo < end.lo {
+            self.lo
+        } else {
+            end.lo
+        };
+        let hi = if self.hi > end.hi {
+            self.hi
+        } else {
+            end.hi
+        };
+        // FIXME(jseyfried): self.ctxt should always equal end.ctxt here (c.f. issue #23480)
+        let ctxt = if self.ctxt == SyntaxContext::empty() {
+            end.ctxt
+        } else {
+            self.ctxt
+        };
+        Span {lo, hi, ctxt}
+    }
+
+    /// Return a `Span` between the end of `self` to the beginning of `end`.
     pub fn between(self, end: Span) -> Span {
         Span {
             lo: self.hi,
@@ -205,6 +223,7 @@ impl Span {
         }
     }
 
+    /// Return a `Span` between the beginning of `self` to the beginning of `end`.
     pub fn until(self, end: Span) -> Span {
         Span {
             lo: self.lo,
@@ -852,6 +871,7 @@ pub struct FileLines {
 thread_local!(pub static SPAN_DEBUG: Cell<fn(Span, &mut fmt::Formatter) -> fmt::Result> =
                 Cell::new(default_span_debug));
 
+#[derive(Debug)]
 pub struct MacroBacktrace {
     /// span where macro was applied to generate this code
     pub call_site: Span,

src/test/ui/span/macro-span-replacement.rs

@@ -0,0 +1,19 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+macro_rules! m {
+    ($a:tt $b:tt) => {
+        $b $a;
+    }
+}
+
+fn main() {
+    m!(S struct);
+}

src/test/ui/span/macro-span-replacement.stderr

@@ -0,0 +1,11 @@
+warning: struct is never used: `S`
+  --> $DIR/macro-span-replacement.rs:13:9
+   |
+13 |         $b $a;
+   |         ^^^^^^
+...
+18 |     m!(S struct);
+   |     ------------- in this macro invocation
+   |
+   = note: #[warn(dead_code)] on by default
+