Auto merge of #49258 - zackmdavis:not_going_to_recover, r=petrochenkov

suggest `!` for erroneous identifier `not`

![not_recovery](https://user-images.githubusercontent.com/1076988/37753255-3b669c42-2d59-11e8-9071-efad8eaf3086.png)

This supersedes #48858.

r? @petrochenkov

2 files changed, 57 insertions, 2 deletions

diff --git a/src/libsyntax/parse/parser.rs b/src/libsyntax/parse/parser.rs
index e6da5bcaa3a..61de50e8e6a 100644
--- a/src/libsyntax/parse/parser.rs
+++ b/src/libsyntax/parse/parser.rs
@@ -2830,7 +2830,48 @@ impl<'a> Parser<'a> {
                 let (span, e) = self.interpolated_or_expr_span(e)?;
                 (lo.to(span), ExprKind::Box(e))
             }
-            _ => return self.parse_dot_or_call_expr(Some(attrs))
+            token::Ident(..) if self.token.is_ident_named("not") => {
+                // `not` is just an ordinary identifier in Rust-the-language,
+                // but as `rustc`-the-compiler, we can issue clever diagnostics
+                // for confused users who really want to say `!`
+                let token_cannot_continue_expr = |t: &token::Token| match *t {
+                    // These tokens can start an expression after `!`, but
+                    // can't continue an expression after an ident
+                    token::Ident(ident, is_raw) => token::ident_can_begin_expr(ident, is_raw),
+                    token::Literal(..) | token::Pound => true,
+                    token::Interpolated(ref nt) => match nt.0 {
+                        token::NtIdent(..) | token::NtExpr(..) |
+                        token::NtBlock(..) | token::NtPath(..) => true,
+                        _ => false,
+                    },
+                    _ => false
+                };
+                let cannot_continue_expr = self.look_ahead(1, token_cannot_continue_expr);
+                if cannot_continue_expr {
+                    self.bump();
+                    // Emit the error ...
+                    let mut err = self.diagnostic()
+                        .struct_span_err(self.span,
+                                         &format!("unexpected {} after identifier",
+                                                  self.this_token_descr()));
+                    // span the `not` plus trailing whitespace to avoid
+                    // trailing whitespace after the `!` in our suggestion
+                    let to_replace = self.sess.codemap()
+                        .span_until_non_whitespace(lo.to(self.span));
+                    err.span_suggestion_short(to_replace,
+                                              "use `!` to perform logical negation",
+                                              "!".to_owned());
+                    err.emit();
+                    // —and recover! (just as if we were in the block
+                    // for the `token::Not` arm)
+                    let e = self.parse_prefix_expr(None);
+                    let (span, e) = self.interpolated_or_expr_span(e)?;
+                    (lo.to(span), self.mk_unary(UnOp::Not, e))
+                } else {
+                    return self.parse_dot_or_call_expr(Some(attrs));
+                }
+            }
+            _ => { return self.parse_dot_or_call_expr(Some(attrs)); }
         };
         return Ok(self.mk_expr(lo.to(hi), ex, attrs));
     }
@@ -4486,6 +4527,11 @@ impl<'a> Parser<'a> {
             // Which is valid in other languages, but not Rust.
             match self.parse_stmt_without_recovery(false) {
                 Ok(Some(stmt)) => {
+                    if self.look_ahead(1, |t| t == &token::OpenDelim(token::Brace)) {
+                        // if the next token is an open brace (e.g., `if a b {`), the place-
+                        // inside-a-block suggestion would be more likely wrong than right
+                        return Err(e);
+                    }
                     let mut stmt_span = stmt.span;
                     // expand the span to include the semicolon, if it exists
                     if self.eat(&token::Semi) {
diff --git a/src/libsyntax/parse/token.rs b/src/libsyntax/parse/token.rs
index 6544619af9c..df0ea05005c 100644
--- a/src/libsyntax/parse/token.rs
+++ b/src/libsyntax/parse/token.rs
@@ -91,7 +91,7 @@ impl Lit {
     }
 }
 
-fn ident_can_begin_expr(ident: ast::Ident, is_raw: bool) -> bool {
+pub(crate) fn ident_can_begin_expr(ident: ast::Ident, is_raw: bool) -> bool {
     let ident_token: Token = Ident(ident, is_raw);
 
     !ident_token.is_reserved_ident() ||
@@ -348,6 +348,15 @@ impl Token {
         self.lifetime().is_some()
     }
 
+    /// Returns `true` if the token is a identifier whose name is the given
+    /// string slice.
+    pub fn is_ident_named(&self, name: &str) -> bool {
+        match self.ident() {
+            Some((ident, _)) => ident.name.as_str() == name,
+            None => false
+        }
+    }
+
     /// Returns `true` if the token is a documentation comment.
     pub fn is_doc_comment(&self) -> bool {
         match *self {