Improve error message for #[repr(align=x)]

src/libsyntax/attr.rs

@@ -1045,6 +1045,30 @@ pub fn find_repr_attrs(diagnostic: &Handler, attr: &Attribute) -> Vec<ReprAttr>
                         span_err!(diagnostic, item.span, E0589,
                                   "invalid `repr(align)` attribute: {}", literal_error);
                     }
+                } else {
+                    if let Some(meta_item) = item.meta_item() {
+                        if meta_item.ident.name == "align" {
+                            if let MetaItemKind::NameValue(ref value) = meta_item.node {
+                                recognised = true;
+                                let mut err = struct_span_err!(diagnostic, item.span, E0693,
+                                    "incorrect `repr(align)` attribute format");
+                                match value.node {
+                                    ast::LitKind::Int(int, ast::LitIntType::Unsuffixed) => {
+                                        err.span_suggestion(item.span,
+                                                            "use parentheses instead",
+                                                            format!("align({})", int));
+                                    }
+                                    ast::LitKind::Str(s, _) => {
+                                        err.span_suggestion(item.span,
+                                                            "use parentheses instead",
+                                                            format!("align({})", s));
+                                    }
+                                    _ => {}
+                                }
+                                err.emit();
+                            }
+                        }
+                    }
                 }
                 if !recognised {
                     // Not a word we recognize

src/libsyntax/diagnostic_list.rs

@@ -324,4 +324,5 @@ register_diagnostics! {
     E0589, // invalid `repr(align)` attribute
     E0629, // missing 'feature' (rustc_const_unstable)
     E0630, // rustc_const_unstable attribute must be paired with stable/unstable attribute
+    E0693, // incorrect `repr(align)` attribute format
 }

src/test/ui/repr-align-assign.rs

@@ -0,0 +1,17 @@
+// 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.
+
+#[repr(align=8)] //~ ERROR incorrect `repr(align)` attribute format
+struct A(u64);
+
+#[repr(align="8")] //~ ERROR incorrect `repr(align)` attribute format
+struct B(u64);
+
+fn main() {}

src/test/ui/repr-align-assign.stderr

@@ -0,0 +1,15 @@
+error[E0693]: incorrect `repr(align)` attribute format
+  --> $DIR/repr-align-assign.rs:11:8
+   |
+LL | #[repr(align=8)] //~ ERROR incorrect `repr(align)` attribute format
+   |        ^^^^^^^ help: use parentheses instead: `align(8)`
+
+error[E0693]: incorrect `repr(align)` attribute format
+  --> $DIR/repr-align-assign.rs:14:8
+   |
+LL | #[repr(align="8")] //~ ERROR incorrect `repr(align)` attribute format
+   |        ^^^^^^^^^ help: use parentheses instead: `align(8)`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0693`.