Emit error for pattern arguments in trait methods

src/librustc_passes/diagnostics.rs

@@ -261,6 +261,27 @@ let result = loop { // ok!
 ```
 "##,
 
+E0642: r##"
+Trait methods currently cannot take patterns as arguments.
+
+Example of erroneous code:
+
+```compile_fail,E0642
+trait Foo {
+    fn foo((x, y): (i32, i32)); // error: patterns aren't allowed
+                                //        in trait methods
+}
+```
+
+You can instead use a single name for the argument:
+
+```
+trait Foo {
+    fn foo(x_and_y: (i32, i32)); // ok!
+}
+```
+"##,
+
 E0695: r##"
 A `break` statement without a label appeared inside a labeled block.
 
@@ -306,7 +327,6 @@ register_diagnostics! {
     E0561, // patterns aren't allowed in function pointer types
     E0567, // auto traits can not have generic parameters
     E0568, // auto traits can not have super traits
-    E0642, // patterns aren't allowed in methods without bodies
     E0666, // nested `impl Trait` is illegal
     E0667, // `impl Trait` in projections
     E0696, // `continue` pointing to a labeled block

src/libsyntax/lib.rs

@@ -26,6 +26,7 @@
 #![feature(slice_sort_by_cached_key)]
 #![feature(str_escape)]
 #![feature(unicode_internals)]
+#![feature(catch_expr)]
 
 #![recursion_limit="256"]
 

src/libsyntax/parse/parser.rs

@@ -44,7 +44,7 @@ use ast::{RangeEnd, RangeSyntax};
 use {ast, attr};
 use codemap::{self, CodeMap, Spanned, respan};
 use syntax_pos::{self, Span, MultiSpan, BytePos, FileName, edition::Edition};
-use errors::{self, Applicability, DiagnosticBuilder};
+use errors::{self, Applicability, DiagnosticBuilder, DiagnosticId};
 use parse::{self, SeqSep, classify, token};
 use parse::lexer::TokenAndSpan;
 use parse::lexer::comments::{doc_comment_style, strip_doc_comment_decoration};
@@ -1371,7 +1371,7 @@ impl<'a> Parser<'a> {
             let ident = self.parse_ident()?;
             let mut generics = self.parse_generics()?;
 
-            let d = self.parse_fn_decl_with_self(|p: &mut Parser<'a>|{
+            let d = self.parse_fn_decl_with_self(|p: &mut Parser<'a>| {
                 // This is somewhat dubious; We don't want to allow
                 // argument names to be left off if there is a
                 // definition...
@@ -1753,21 +1753,59 @@ impl<'a> Parser<'a> {
             (pat, self.parse_ty()?)
         } else {
             debug!("parse_arg_general ident_to_pat");
-            let ident = Ident::new(keywords::Invalid.name(), self.prev_span);
-            let ty = self.parse_ty()?;
-            let pat = P(Pat {
-                id: ast::DUMMY_NODE_ID,
-                node: PatKind::Ident(BindingMode::ByValue(Mutability::Immutable), ident, None),
-                span: ty.span,
-            });
-            (pat, ty)
+
+            let parser_snapshot_before_pat = self.clone();
+
+            // We're going to try parsing the argument as a pattern (even though it's not
+            // allowed). This way we can provide better errors to the user.
+            let pat_arg: PResult<'a, _> = do catch {
+                let pat = self.parse_pat()?;
+                self.expect(&token::Colon)?;
+                (pat, self.parse_ty()?)
+            };
+
+            match pat_arg {
+                Ok((pat, ty)) => {
+                    let mut err = self.diagnostic().struct_span_err_with_code(
+                        pat.span,
+                        "patterns aren't allowed in methods without bodies",
+                        DiagnosticId::Error("E0642".into()),
+                    );
+                    err.span_suggestion_short_with_applicability(
+                        pat.span,
+                        "give this argument a name or use an underscore to ignore it",
+                        "_".to_owned(),
+                        Applicability::MachineApplicable,
+                    );
+                    err.emit();
+                    // Pretend the pattern is `_`, to avoid duplicate errors from AST validation.
+                    let pat = P(Pat {
+                        node: PatKind::Wild,
+                        span: pat.span,
+                        id: ast::DUMMY_NODE_ID
+                    });
+                    (pat, ty)
+                }
+                Err(mut err) => {
+                    err.cancel();
+                    // Recover from attempting to parse the argument as a pattern. This means
+                    // the type is alone, with no name, e.g. `fn foo(u32)`.
+                    mem::replace(self, parser_snapshot_before_pat);
+                    debug!("parse_arg_general ident_to_pat");
+                    let ident = Ident::new(keywords::Invalid.name(), self.prev_span);
+                    let ty = self.parse_ty()?;
+                    let pat = P(Pat {
+                        id: ast::DUMMY_NODE_ID,
+                        node: PatKind::Ident(
+                            BindingMode::ByValue(Mutability::Immutable), ident, None),
+                        span: ty.span,
+                    });
+                    (pat, ty)
+                }
+            }
         };
 
-        Ok(Arg {
-            ty,
-            pat,
-            id: ast::DUMMY_NODE_ID,
-        })
+        Ok(Arg { ty, pat, id: ast::DUMMY_NODE_ID })
     }
 
     /// Parse a single function argument

src/test/ui/E0642.rs

@@ -0,0 +1,24 @@
+// 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.
+
+#[derive(Clone, Copy)]
+struct S;
+
+trait T {
+    fn foo((x, y): (i32, i32)); //~ ERROR patterns aren't allowed in methods without bodies
+
+    fn bar((x, y): (i32, i32)) {} //~ ERROR patterns aren't allowed in methods without bodies
+
+    fn f(&ident: &S) {} // ok
+    fn g(&&ident: &&S) {} // ok
+    fn h(mut ident: S) {} // ok
+}
+
+fn main() {}

src/test/ui/E0642.stderr

@@ -0,0 +1,23 @@
+error[E0642]: patterns aren't allowed in methods without bodies
+  --> $DIR/E0642.rs:15:12
+   |
+LL |     fn foo((x, y): (i32, i32)); //~ ERROR patterns aren't allowed in methods without bodies
+   |            ^^^^^^
+help: give this argument a name or use an underscore to ignore it
+   |
+LL |     fn foo(_: (i32, i32)); //~ ERROR patterns aren't allowed in methods without bodies
+   |            ^
+
+error[E0642]: patterns aren't allowed in methods without bodies
+  --> $DIR/E0642.rs:17:12
+   |
+LL |     fn bar((x, y): (i32, i32)) {} //~ ERROR patterns aren't allowed in methods without bodies
+   |            ^^^^^^
+help: give this argument a name or use an underscore to ignore it
+   |
+LL |     fn bar(_: (i32, i32)) {} //~ ERROR patterns aren't allowed in methods without bodies
+   |            ^
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0642`.