Added warning/error for if-let-chain ambiguity.

With eRFC 2497, previously accepted ambigious syntax regarding use of
`&&` and `||` in if-let and while-let statements should now warn
or error depending on the edition.

This commit takes a naive approach to detecting ambigious use of `&&`
or `||` and will probably need fine tuned to handle all cases.

src/libsyntax/parse/parser.rs

@@ -3327,6 +3327,8 @@ impl<'a> Parser<'a> {
         let pats = self.parse_pats()?;
         self.expect(&token::Eq)?;
         let expr = self.parse_expr_res(Restrictions::NO_STRUCT_LITERAL, None)?;
+        self.while_if_let_ambiguity(&expr);
+
         let thn = self.parse_block()?;
         let (hi, els) = if self.eat_keyword(keywords::Else) {
             let expr = self.parse_else_expr()?;
@@ -3337,6 +3339,56 @@ impl<'a> Parser<'a> {
         Ok(self.mk_expr(lo.to(hi), ExprKind::IfLet(pats, expr, thn, els), attrs))
     }
 
+    /// With eRFC 2497, we need to check whether an expression is ambigious and warn or error
+    /// depending on the edition, this function handles that.
+    fn while_if_let_ambiguity(&self, expr: &P<Expr>) {
+        if let Some((span, op_kind)) = self.while_if_let_expr_ambiguity(&expr) {
+            let message = format!("ambigious use of `{}`", op_kind.to_string());
+            let mut err = if self.span.edition() >= Edition::Edition2018 {
+                self.diagnostic().struct_span_err(span, &message)
+            } else {
+                self.diagnostic().struct_span_warn(span, &message)
+            };
+
+            let note = if self.span.edition() >= Edition::Edition2018 {
+                "This will be a error until the `let_chains` feature is stabilized."
+            } else {
+                "This will be a error in Rust 2018 until the `let_chains` feature is stabilized."
+            };
+            err.note(note);
+
+            if let Ok(snippet) = self.sess.source_map().span_to_snippet(span) {
+                err.span_suggestion(
+                    span, "consider adding parenthesis", format!("({})", snippet),
+                );
+            }
+
+            err.emit();
+        }
+    }
+
+    /// With eRFC 2497 adding if-let chains, there is a requirement that the parsing of
+    /// `&&` and `||` in a if-let statement be unambigious. This function returns a span and
+    /// a `BinOpKind` (either `&&` or `||` depending on what was ambigious) if it is determined
+    /// that the current expression parsed is ambigious and will break in future.
+    fn while_if_let_expr_ambiguity(&self, expr: &P<Expr>) -> Option<(Span, BinOpKind)> {
+        debug!("while_if_let_expr_ambiguity: expr.node: {:?}", expr.node);
+        match &expr.node {
+            ExprKind::Binary(op, _, _) if op.node == BinOpKind::And || op.node == BinOpKind::Or => {
+                Some((expr.span, op.node))
+            },
+            ExprKind::Range(ref lhs, ref rhs, _) => {
+                let lhs_ambigious = lhs.as_ref()
+                    .and_then(|lhs| self.while_if_let_expr_ambiguity(lhs));
+                let rhs_ambigious = rhs.as_ref()
+                    .and_then(|rhs| self.while_if_let_expr_ambiguity(rhs));
+
+                lhs_ambigious.or(rhs_ambigious)
+            }
+            _ => None,
+        }
+    }
+
     // `move |args| expr`
     fn parse_lambda_expr(&mut self,
                              attrs: ThinVec<Attribute>)
@@ -3437,6 +3489,7 @@ impl<'a> Parser<'a> {
         let pats = self.parse_pats()?;
         self.expect(&token::Eq)?;
         let expr = self.parse_expr_res(Restrictions::NO_STRUCT_LITERAL, None)?;
+        self.while_if_let_ambiguity(&expr);
         let (iattrs, body) = self.parse_inner_attrs_and_block()?;
         attrs.extend(iattrs);
         let span = span_lo.to(body.span);

src/test/ui/rfc-2497-if-let-chains/syntax-ambiguity-2015.rs

@@ -0,0 +1,50 @@
+// Copyright 2016 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.
+
+// edition:2015
+// compile-pass
+
+// Enabling `ireffutable_let_patterns` isn't necessary for what this tests, but it makes coming up
+// with examples easier.
+#![feature(irrefutable_let_patterns)]
+
+#[allow(irrefutable_let_patterns)]
+fn main() {
+    use std::ops::Range;
+
+    if let Range { start: _, end: _ } = true..true && false { }
+    //~^ WARN error in 2018
+
+    if let Range { start: _, end: _ } = true..true || false { }
+    //~^ WARN error in 2018
+
+    while let Range { start: _, end: _ } = true..true && false { }
+    //~^ WARN error in 2018
+
+    while let Range { start: _, end: _ } = true..true || false { }
+    //~^ WARN error in 2018
+
+    if let true = false && false { }
+    //~^ WARN error in 2018
+
+    while let true = (1 == 2) && false { }
+    //~^ WARN error in 2018
+
+    // The following cases are not an error as parenthesis are used to
+    // clarify intent:
+
+    if let Range { start: _, end: _ } = true..(true || false) { }
+
+    if let Range { start: _, end: _ } = true..(true && false) { }
+
+    while let Range { start: _, end: _ } = true..(true || false) { }
+
+    while let Range { start: _, end: _ } = true..(true && false) { }
+}

src/test/ui/rfc-2497-if-let-chains/syntax-ambiguity-2015.stderr

@@ -0,0 +1,48 @@
+warning: ambigious use of `&&`
+  --> $DIR/syntax-ambiguity-2015.rs:22:47
+   |
+LL |     if let Range { start: _, end: _ } = true..true && false { }
+   |                                               ^^^^^^^^^^^^^ help: consider adding parenthesis: `(true && false)`
+   |
+   = note: This will be a error in Rust 2018 until the `let_chains` feature is stabilized.
+
+warning: ambigious use of `||`
+  --> $DIR/syntax-ambiguity-2015.rs:25:47
+   |
+LL |     if let Range { start: _, end: _ } = true..true || false { }
+   |                                               ^^^^^^^^^^^^^ help: consider adding parenthesis: `(true || false)`
+   |
+   = note: This will be a error in Rust 2018 until the `let_chains` feature is stabilized.
+
+warning: ambigious use of `&&`
+  --> $DIR/syntax-ambiguity-2015.rs:28:50
+   |
+LL |     while let Range { start: _, end: _ } = true..true && false { }
+   |                                                  ^^^^^^^^^^^^^ help: consider adding parenthesis: `(true && false)`
+   |
+   = note: This will be a error in Rust 2018 until the `let_chains` feature is stabilized.
+
+warning: ambigious use of `||`
+  --> $DIR/syntax-ambiguity-2015.rs:31:50
+   |
+LL |     while let Range { start: _, end: _ } = true..true || false { }
+   |                                                  ^^^^^^^^^^^^^ help: consider adding parenthesis: `(true || false)`
+   |
+   = note: This will be a error in Rust 2018 until the `let_chains` feature is stabilized.
+
+warning: ambigious use of `&&`
+  --> $DIR/syntax-ambiguity-2015.rs:34:19
+   |
+LL |     if let true = false && false { }
+   |                   ^^^^^^^^^^^^^^ help: consider adding parenthesis: `(false && false)`
+   |
+   = note: This will be a error in Rust 2018 until the `let_chains` feature is stabilized.
+
+warning: ambigious use of `&&`
+  --> $DIR/syntax-ambiguity-2015.rs:37:22
+   |
+LL |     while let true = (1 == 2) && false { }
+   |                      ^^^^^^^^^^^^^^^^^ help: consider adding parenthesis: `((1 == 2) && false)`
+   |
+   = note: This will be a error in Rust 2018 until the `let_chains` feature is stabilized.
+

src/test/ui/rfc-2497-if-let-chains/syntax-ambiguity-2018.rs

@@ -0,0 +1,49 @@
+// Copyright 2016 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.
+
+// edition:2018
+
+// Enabling `ireffutable_let_patterns` isn't necessary for what this tests, but it makes coming up
+// with examples easier.
+#![feature(irrefutable_let_patterns)]
+
+#[allow(irrefutable_let_patterns)]
+fn main() {
+    use std::ops::Range;
+
+    if let Range { start: _, end: _ } = true..true && false { }
+    //~^ ERROR ambigious use of `&&`
+
+    if let Range { start: _, end: _ } = true..true || false { }
+    //~^ ERROR ambigious use of `||`
+
+    while let Range { start: _, end: _ } = true..true && false { }
+    //~^ ERROR ambigious use of `&&`
+
+    while let Range { start: _, end: _ } = true..true || false { }
+    //~^ ERROR ambigious use of `||`
+
+    if let true = false && false { }
+    //~^ ERROR ambigious use of `&&`
+
+    while let true = (1 == 2) && false { }
+    //~^ ERROR ambigious use of `&&`
+
+    // The following cases are not an error as parenthesis are used to
+    // clarify intent:
+
+    if let Range { start: _, end: _ } = true..(true || false) { }
+
+    if let Range { start: _, end: _ } = true..(true && false) { }
+
+    while let Range { start: _, end: _ } = true..(true || false) { }
+
+    while let Range { start: _, end: _ } = true..(true && false) { }
+}

src/test/ui/rfc-2497-if-let-chains/syntax-ambiguity-2018.stderr

@@ -0,0 +1,50 @@
+error: ambigious use of `&&`
+  --> $DIR/syntax-ambiguity-2018.rs:21:47
+   |
+LL |     if let Range { start: _, end: _ } = true..true && false { }
+   |                                               ^^^^^^^^^^^^^ help: consider adding parenthesis: `(true && false)`
+   |
+   = note: This will be a error until the `let_chains` feature is stabilized.
+
+error: ambigious use of `||`
+  --> $DIR/syntax-ambiguity-2018.rs:24:47
+   |
+LL |     if let Range { start: _, end: _ } = true..true || false { }
+   |                                               ^^^^^^^^^^^^^ help: consider adding parenthesis: `(true || false)`
+   |
+   = note: This will be a error until the `let_chains` feature is stabilized.
+
+error: ambigious use of `&&`
+  --> $DIR/syntax-ambiguity-2018.rs:27:50
+   |
+LL |     while let Range { start: _, end: _ } = true..true && false { }
+   |                                                  ^^^^^^^^^^^^^ help: consider adding parenthesis: `(true && false)`
+   |
+   = note: This will be a error until the `let_chains` feature is stabilized.
+
+error: ambigious use of `||`
+  --> $DIR/syntax-ambiguity-2018.rs:30:50
+   |
+LL |     while let Range { start: _, end: _ } = true..true || false { }
+   |                                                  ^^^^^^^^^^^^^ help: consider adding parenthesis: `(true || false)`
+   |
+   = note: This will be a error until the `let_chains` feature is stabilized.
+
+error: ambigious use of `&&`
+  --> $DIR/syntax-ambiguity-2018.rs:33:19
+   |
+LL |     if let true = false && false { }
+   |                   ^^^^^^^^^^^^^^ help: consider adding parenthesis: `(false && false)`
+   |
+   = note: This will be a error until the `let_chains` feature is stabilized.
+
+error: ambigious use of `&&`
+  --> $DIR/syntax-ambiguity-2018.rs:36:22
+   |
+LL |     while let true = (1 == 2) && false { }
+   |                      ^^^^^^^^^^^^^^^^^ help: consider adding parenthesis: `((1 == 2) && false)`
+   |
+   = note: This will be a error until the `let_chains` feature is stabilized.
+
+error: aborting due to 6 previous errors
+