(take 2) libsyntax: note that let a = (let b = something) is invalid

src/libsyntax/errors/emitter.rs

@@ -10,7 +10,7 @@
 
 use self::Destination::*;
 
-use codemap::{self, COMMAND_LINE_SP, COMMAND_LINE_EXPN, Pos, Span};
+use codemap::{self, COMMAND_LINE_SP, COMMAND_LINE_EXPN, DUMMY_SP, Pos, Span};
 use diagnostics;
 
 use errors::{Level, RenderSpan, DiagnosticBuilder};
@@ -109,8 +109,8 @@ impl Emitter for EmitterWriter {
             lvl: Level) {
         let error = match sp {
             Some(COMMAND_LINE_SP) => self.emit_(FileLine(COMMAND_LINE_SP), msg, code, lvl),
+            Some(DUMMY_SP) | None => print_diagnostic(&mut self.dst, "", lvl, msg, code),
             Some(sp) => self.emit_(FullSpan(sp), msg, code, lvl),
-            None => print_diagnostic(&mut self.dst, "", lvl, msg, code),
         };
 
         if let Err(e) = error {

src/libsyntax/parse/parser.rs

@@ -2218,6 +2218,12 @@ impl<'a> Parser<'a> {
                         ex = ExprBreak(None);
                     }
                     hi = self.last_span.hi;
+                } else if self.token.is_keyword(keywords::Let) {
+                    // Catch this syntax error here, instead of in `check_strict_keywords`, so
+                    // that we can explicitly mention that let is not to be used as an expression
+                    let mut db = self.fatal("expected expression, found statement (`let`)");
+                    db.note("variable declaration using `let` is a statement");
+                    return Err(db);
                 } else if self.check(&token::ModSep) ||
                         self.token.is_ident() &&
                         !self.check_keyword(keywords::True) &&

src/test/run-fail-fulldeps/qquote.rs

@@ -10,7 +10,7 @@
 
 // ignore-cross-compile
 
-// error-pattern:expected identifier, found keyword `let`
+// error-pattern:expected expression, found statement (`let`)
 
 #![feature(quote, rustc_private)]