Improve suggestion for missing fmt str in println

Avoid using `concat!(fmt, "\n")` to improve the diagnostics being
emitted when the first `println!()` argument isn't a formatting string
literal.

1 files changed, 15 insertions, 10 deletions

diff --git a/src/libsyntax/ext/base.rs b/src/libsyntax/ext/base.rs
index 5ec44fb5898..b55c4f99206 100644
--- a/src/libsyntax/ext/base.rs
+++ b/src/libsyntax/ext/base.rs
@@ -959,29 +959,34 @@ impl<'a> ExtCtxt<'a> {
 /// Extract a string literal from the macro expanded version of `expr`,
 /// emitting `err_msg` if `expr` is not a string literal. This does not stop
 /// compilation on error, merely emits a non-fatal error and returns None.
-pub fn expr_to_spanned_string(cx: &mut ExtCtxt, expr: P<ast::Expr>, err_msg: &str)
-                              -> Option<Spanned<(Symbol, ast::StrStyle)>> {
+pub fn expr_to_spanned_string<'a>(
+    cx: &'a mut ExtCtxt,
+    expr: P<ast::Expr>,
+    err_msg: &str,
+) -> Result<Spanned<(Symbol, ast::StrStyle)>, DiagnosticBuilder<'a>> {
     // Update `expr.span`'s ctxt now in case expr is an `include!` macro invocation.
     let expr = expr.map(|mut expr| {
         expr.span = expr.span.apply_mark(cx.current_expansion.mark);
         expr
     });
 
-    // we want to be able to handle e.g. concat("foo", "bar")
+    // we want to be able to handle e.g. `concat!("foo", "bar")`
     let expr = cx.expander().fold_expr(expr);
-    match expr.node {
+    Err(match expr.node {
         ast::ExprKind::Lit(ref l) => match l.node {
-            ast::LitKind::Str(s, style) => return Some(respan(expr.span, (s, style))),
-            _ => cx.span_err(l.span, err_msg)
+            ast::LitKind::Str(s, style) => return Ok(respan(expr.span, (s, style))),
+            _ => cx.struct_span_err(l.span, err_msg)
         },
-        _ => cx.span_err(expr.span, err_msg)
-    }
-    None
+        _ => cx.struct_span_err(expr.span, err_msg)
+    })
 }
 
 pub fn expr_to_string(cx: &mut ExtCtxt, expr: P<ast::Expr>, err_msg: &str)
                       -> Option<(Symbol, ast::StrStyle)> {
-    expr_to_spanned_string(cx, expr, err_msg).map(|s| s.node)
+    expr_to_spanned_string(cx, expr, err_msg)
+        .map_err(|mut err| err.emit())
+        .ok()
+        .map(|s| s.node)
 }
 
 /// Non-fatally assert that `tts` is empty. Note that this function