suggest ! instead of erroneous not on if/while block parse failure

src/libsyntax/parse/parser.rs

@@ -3211,6 +3211,21 @@ impl<'a> Parser<'a> {
         }
     }
 
+    /// If the condition of a `while` or `if` expression was an identifier
+    /// named `not`, that's a clue that the confused user likely meant `!`.
+    fn is_identifier_not(&self, cond: &Expr) -> bool {
+        if let ExprKind::Path(_, ref path) = cond.node {
+            if path.segments.len() == 1 { // just `not`, not `not::etc`
+                if let Some(segment) = path.segments.iter().next() {
+                    if segment.identifier.name.as_str() == "not" {
+                        return true;
+                    }
+                }
+            }
+        }
+        false
+    }
+
     /// Parse an 'if' or 'if let' expression ('if' token already eaten)
     pub fn parse_if_expr(&mut self, attrs: ThinVec<Attribute>) -> PResult<'a, P<Expr>> {
         if self.check_keyword(keywords::Let) {
@@ -3232,6 +3247,11 @@ impl<'a> Parser<'a> {
         }
         let not_block = self.token != token::OpenDelim(token::Brace);
         let thn = self.parse_block().map_err(|mut err| {
+            if self.is_identifier_not(&cond) {
+                err.span_suggestion(cond.span, // FIXME: replaced span doesn't incl. whitespace
+                                    "try replacing identifier `not` with the negation operator",
+                                    "!".to_owned());
+            }
             if not_block {
                 err.span_label(lo, "this `if` statement has a condition, but no block");
             }
@@ -3342,7 +3362,15 @@ impl<'a> Parser<'a> {
             return self.parse_while_let_expr(opt_label, span_lo, attrs);
         }
         let cond = self.parse_expr_res(Restrictions::NO_STRUCT_LITERAL, None)?;
-        let (iattrs, body) = self.parse_inner_attrs_and_block()?;
+        let (iattrs, body) = self.parse_inner_attrs_and_block()
+            .map_err(|mut err| {
+                if self.is_identifier_not(&cond) {
+                    let msg = "try replacing identifier `not` with the negation operator";
+                    // FIXME: replaced span doesn't include trailing whitespace
+                    err.span_suggestion(cond.span, msg, "!".to_owned());
+                }
+                err
+            })?;
         attrs.extend(iattrs);
         let span = span_lo.to(body.span);
         return Ok(self.mk_expr(span, ExprKind::While(cond, body, opt_label), attrs));

src/test/ui/did_you_mean/issue-46836-identifier-not-instead-of-negation.rs

@@ -0,0 +1,45 @@
+// Copyright 2018 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.
+
+fn gratitude() {
+    let for_you = false;
+    if not for_you {
+        //~^ ERROR expected `{`
+        println!("I couldn't");
+        //~^ ERROR expected one of
+    }
+}
+
+fn qualification() {
+    let the_worst = true;
+    while not the_worst {
+        //~^ ERROR expected one of
+        println!("still pretty bad");
+    }
+}
+
+fn defer() {
+    let department = false;
+    // `not` as one segment of a longer path doesn't trigger the smart help
+    if not::my department {
+        //~^ ERROR expected `{`
+        println!("pass");
+        //~^ ERROR expected one of
+    }
+}
+
+fn should_we() {
+    let not = true;
+    if not  // lack of braces is [sic]
+        println!("Then when?");
+    //~^ ERROR expected `{`
+}
+
+fn main() {}

src/test/ui/did_you_mean/issue-46836-identifier-not-instead-of-negation.stderr

@@ -0,0 +1,68 @@
+error: expected one of `,` or `}`, found `!`
+  --> $DIR/issue-46836-identifier-not-instead-of-negation.rs:15:16
+   |
+LL |         println!("I couldn't");
+   |                ^ expected one of `,` or `}` here
+
+error: expected `{`, found `for_you`
+  --> $DIR/issue-46836-identifier-not-instead-of-negation.rs:13:12
+   |
+LL |     if not for_you {
+   |     --     ^^^^^^^
+   |     |
+   |     this `if` statement has a condition, but no block
+help: try placing this code inside a block
+   |
+LL |     if not { for_you{println,}; }
+   |            ^^^^^^^^^^^^^^^^^^^^^^
+help: try replacing identifier `not` with the negation operator
+   |
+LL |     if ! for_you {
+   |        ^
+
+error: expected one of `!`, `.`, `::`, `?`, `{`, or an operator, found `the_worst`
+  --> $DIR/issue-46836-identifier-not-instead-of-negation.rs:22:15
+   |
+LL |     while not the_worst {
+   |           --- ^^^^^^^^^ expected one of `!`, `.`, `::`, `?`, `{`, or an operator here
+   |           |
+   |           help: try replacing identifier `not` with the negation operator: `!`
+
+error: expected one of `,` or `}`, found `!`
+  --> $DIR/issue-46836-identifier-not-instead-of-negation.rs:33:16
+   |
+LL |         println!("pass");
+   |                ^ expected one of `,` or `}` here
+
+error: expected `{`, found `department`
+  --> $DIR/issue-46836-identifier-not-instead-of-negation.rs:31:16
+   |
+LL |       if not::my department {
+   |       --         -^^^^^^^^^
+   |  _____|__________|
+   | |     |
+   | |     this `if` statement has a condition, but no block
+LL | |         //~^ ERROR expected `{`
+LL | |         println!("pass");
+LL | |         //~^ ERROR expected one of
+LL | |     }
+   | |_____- help: try placing this code inside a block: `{ department{println,}; }`
+
+error: expected `{`, found `println`
+  --> $DIR/issue-46836-identifier-not-instead-of-negation.rs:41:9
+   |
+LL |     if not  // lack of braces is [sic]
+   |     -- this `if` statement has a condition, but no block
+LL |         println!("Then when?");
+   |         ^^^^^^^
+help: try placing this code inside a block
+   |
+LL |         { println!("Then when?") }
+   |
+help: try replacing identifier `not` with the negation operator
+   |
+LL |     if !  // lack of braces is [sic]
+   |        ^
+
+error: aborting due to 6 previous errors
+