reject invalid external doc attributes

Also, provide a suggestion for the correct syntax.

src/libsyntax/ext/expand.rs

@@ -8,7 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-use ast::{self, Block, Ident, NodeId, PatKind, Path};
+use ast::{self, Block, Ident, LitKind, NodeId, PatKind, Path};
 use ast::{MacStmtStyle, StmtKind, ItemKind};
 use attr::{self, HasAttrs};
 use source_map::{ExpnInfo, MacroBang, MacroAttribute, dummy_spanned, respan};
@@ -1549,7 +1549,35 @@ impl<'a, 'b> Folder for InvocationCollector<'a, 'b> {
                         }
                     }
                 } else {
-                    items.push(noop_fold_meta_list_item(it, self));
+                    let mut err = self.cx.struct_span_err(
+                        it.span,
+                        &format!("expected path to external documentation"),
+                    );
+
+                    // Check if the user erroneously used `doc(include(...))` syntax.
+                    let literal = it.meta_item_list().and_then(|list| {
+                        if list.len() == 1 {
+                            list[0].literal().map(|literal| &literal.node)
+                        } else {
+                            None
+                        }
+                    });
+
+                    let (path, applicability) = match &literal {
+                        Some(LitKind::Str(path, ..)) => {
+                            (path.to_string(), Applicability::MachineApplicable)
+                        }
+                        _ => (String::from("<path>"), Applicability::HasPlaceholders),
+                    };
+
+                    err.span_suggestion_with_applicability(
+                        it.span,
+                        "provide a file path with `=`",
+                        format!("include = \"{}\"", path),
+                        applicability,
+                    );
+
+                    err.emit();
                 }
             }
 

src/test/ui/extern/external-doc-error.rs

@@ -5,4 +5,24 @@
 #[doc(include = "not-a-file.md")] //~ ERROR: couldn't read
 pub struct SomeStruct;
 
+#[doc(include)]
+pub struct MissingPath; //~^ ERROR expected path
+                        //~| HELP provide a file path with `=`
+                        //~| SUGGESTION include = "<path>"
+
+#[doc(include("../README.md"))]
+pub struct InvalidPathSyntax; //~^ ERROR expected path
+                              //~| HELP provide a file path with `=`
+                              //~| SUGGESTION include = "../README.md"
+
+#[doc(include = 123)]
+pub struct InvalidPathType; //~^ ERROR expected path
+                            //~| HELP provide a file path with `=`
+                            //~| SUGGESTION include = "<path>"
+
+#[doc(include(123))]
+pub struct InvalidPathSyntaxAndType; //~^ ERROR expected path
+                                     //~| HELP provide a file path with `=`
+                                     //~| SUGGESTION include = "<path>"
+
 fn main() {}

src/test/ui/extern/external-doc-error.stderr

@@ -4,5 +4,29 @@ error: couldn't read $DIR/not-a-file.md: $FILE_NOT_FOUND_MSG (os error 2)
 LL | #[doc(include = "not-a-file.md")] //~ ERROR: couldn't read
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: aborting due to previous error
+error: expected path to external documentation
+  --> $DIR/external-doc-error.rs:8:7
+   |
+LL | #[doc(include)]
+   |       ^^^^^^^ help: provide a file path with `=`: `include = "<path>"`
+
+error: expected path to external documentation
+  --> $DIR/external-doc-error.rs:13:7
+   |
+LL | #[doc(include("../README.md"))]
+   |       ^^^^^^^^^^^^^^^^^^^^^^^ help: provide a file path with `=`: `include = "../README.md"`
+
+error: expected path to external documentation
+  --> $DIR/external-doc-error.rs:18:7
+   |
+LL | #[doc(include = 123)]
+   |       ^^^^^^^^^^^^^ help: provide a file path with `=`: `include = "<path>"`
+
+error: expected path to external documentation
+  --> $DIR/external-doc-error.rs:23:7
+   |
+LL | #[doc(include(123))]
+   |       ^^^^^^^^^^^^ help: provide a file path with `=`: `include = "<path>"`
+
+error: aborting due to 5 previous errors