Fix format macro expansions spans to be macro-generated

New Exprs generated as part of the format macro expansion should get the macro
expansion span which has an expansion context, not the span of the format string
which does not.

src/libsyntax_ext/format.rs

@@ -695,7 +695,7 @@ impl<'a, 'b> Context<'a, 'b> {
         // Now create a vector containing all the arguments
         let args = locals.into_iter().chain(counts.into_iter());
 
-        let args_array = self.ecx.expr_vec(self.fmtsp, args.collect());
+        let args_array = self.ecx.expr_vec(self.macsp, args.collect());
 
         // Constructs an AST equivalent to:
         //
@@ -724,12 +724,12 @@ impl<'a, 'b> Context<'a, 'b> {
         //
         // But the nested match expression is proved to perform not as well
         // as series of let's; the first approach does.
-        let pat = self.ecx.pat_tuple(self.fmtsp, pats);
-        let arm = self.ecx.arm(self.fmtsp, pat, args_array);
-        let head = self.ecx.expr(self.fmtsp, ast::ExprKind::Tup(heads));
-        let result = self.ecx.expr_match(self.fmtsp, head, vec![arm]);
+        let pat = self.ecx.pat_tuple(self.macsp, pats);
+        let arm = self.ecx.arm(self.macsp, pat, args_array);
+        let head = self.ecx.expr(self.macsp, ast::ExprKind::Tup(heads));
+        let result = self.ecx.expr_match(self.macsp, head, vec![arm]);
 
-        let args_slice = self.ecx.expr_addr_of(self.fmtsp, result);
+        let args_slice = self.ecx.expr_addr_of(self.macsp, result);
 
         // Now create the fmt::Arguments struct with all our locals we created.
         let (fn_name, fn_args) = if self.all_pieces_simple {

src/test/ui/issues/issue-27592.rs

@@ -15,5 +15,5 @@ impl ::std::fmt::Write for Stream {
 fn main() {
     write(|| format_args!("{}", String::from("Hello world")));
     //~^ ERROR cannot return value referencing temporary value
-    //~| ERROR cannot return value referencing temporary value
+    //~| ERROR cannot return reference to temporary value
 }

src/test/ui/issues/issue-27592.stderr

@@ -7,14 +7,11 @@ LL |     write(|| format_args!("{}", String::from("Hello world")));
    |              |                  temporary value created here
    |              returns a value referencing data owned by the current function
 
-error[E0515]: cannot return value referencing temporary value
+error[E0515]: cannot return reference to temporary value
   --> $DIR/issue-27592.rs:16:14
    |
 LL |     write(|| format_args!("{}", String::from("Hello world")));
-   |              ^^^^^^^^^^^^^----^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   |              |            |
-   |              |            temporary value created here
-   |              returns a value referencing data owned by the current function
+   |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ returns a reference to data owned by the current function
 
 error: aborting due to 2 previous errors