make `#[unwind]` attribute specify expectations more clearly

You can now choose between the following:

- `#[unwind(allowed)]`
- `#[unwind(aborts)]`

Per rust-lang/rust#48251, the default is `#[unwind(allowed)]`, though
I think we should change this eventually.

3 files changed, 73 insertions, 1 deletions

diff --git a/src/libsyntax/attr.rs b/src/libsyntax/attr.rs
index d18d6f5e6bd..d0822b69aa6 100644
--- a/src/libsyntax/attr.rs
+++ b/src/libsyntax/attr.rs
@@ -565,6 +565,51 @@ pub fn find_inline_attr(diagnostic: Option<&Handler>, attrs: &[Attribute]) -> In
     })
 }
 
+#[derive(Copy, Clone, PartialEq)]
+pub enum UnwindAttr {
+    Allowed,
+    Aborts,
+}
+
+/// Determine what `#[unwind]` attribute is present in `attrs`, if any.
+pub fn find_unwind_attr(diagnostic: Option<&Handler>, attrs: &[Attribute]) -> Option<UnwindAttr> {
+    let syntax_error = |attr: &Attribute| {
+        mark_used(attr);
+        diagnostic.map(|d| {
+            span_err!(d, attr.span, E0633, "malformed `#[unwind]` attribute");
+        });
+        None
+    };
+
+    attrs.iter().fold(None, |ia, attr| {
+        if attr.path != "unwind" {
+            return ia;
+        }
+        let meta = match attr.meta() {
+            Some(meta) => meta.node,
+            None => return ia,
+        };
+        match meta {
+            MetaItemKind::Word => {
+                syntax_error(attr)
+            }
+            MetaItemKind::List(ref items) => {
+                mark_used(attr);
+                if items.len() != 1 {
+                    syntax_error(attr)
+                } else if list_contains_name(&items[..], "allowed") {
+                    Some(UnwindAttr::Allowed)
+                } else if list_contains_name(&items[..], "aborts") {
+                    Some(UnwindAttr::Aborts)
+                } else {
+                    syntax_error(attr)
+                }
+            }
+            _ => ia,
+        }
+    })
+}
+
 /// True if `#[inline]` or `#[inline(always)]` is present in `attrs`.
 pub fn requests_inline(attrs: &[Attribute]) -> bool {
     match find_inline_attr(None, attrs) {
diff --git a/src/libsyntax/diagnostic_list.rs b/src/libsyntax/diagnostic_list.rs
index d841281e485..84ab0336f16 100644
--- a/src/libsyntax/diagnostic_list.rs
+++ b/src/libsyntax/diagnostic_list.rs
@@ -342,6 +342,33 @@ fn main() {
 ```
 "##,
 
+E0633: r##"
+The `unwind` attribute was malformed.
+
+Erroneous code example:
+
+```ignore (compile_fail not working here; see Issue #43707)
+#[unwind()] // error: expected one argument
+pub extern fn something() {}
+
+fn main() {}
+```
+
+The `#[unwind]` attribute should be used as follows:
+
+- `#[unwind(aborts)]` -- specifies that if a non-Rust ABI function
+  should abort the process if it attempts to unwind. This is the safer
+  and preferred option.
+
+- `#[unwind(allowed)]` -- specifies that a non-Rust ABI function
+  should be allowed to unwind. This can easily result in Undefined
+  Behavior (UB), so be careful.
+
+NB. The default behavior here is "allowed", but this is unspecified
+and likely to change in the future.
+
+"##,
+
 }
 
 register_diagnostics! {
diff --git a/src/libsyntax/feature_gate.rs b/src/libsyntax/feature_gate.rs
index 3b137f9570a..f1d0a70a22c 100644
--- a/src/libsyntax/feature_gate.rs
+++ b/src/libsyntax/feature_gate.rs
@@ -233,7 +233,7 @@ declare_features! (
     // allow `extern "platform-intrinsic" { ... }`
     (active, platform_intrinsics, "1.4.0", Some(27731)),
 
-    // allow `#[unwind]`
+    // allow `#[unwind(..)]`
     // rust runtime internal
     (active, unwind_attributes, "1.4.0", None),