Add sanity check for inconsistent soft usage on soft-unstable items

It doesn't make sense for something to be partially soft, and allowing inconsistent softness
markers would risk an item accidentally becoming properly unstable as its features stabilize.

Author: dianne

compiler/rustc_attr/messages.ftl

@@ -100,6 +100,9 @@ attr_rustc_const_stable_indirect_pairing =
 attr_rustc_promotable_pairing =
     `rustc_promotable` attribute must be paired with either a `rustc_const_unstable` or a `rustc_const_stable` attribute
 
+attr_soft_inconsistent =
+    `soft` must be present on either none or all of an item's `unstable` attributes
+
 attr_soft_no_args =
     `soft` should not have any arguments
 

compiler/rustc_attr/src/builtin.rs

@@ -502,10 +502,17 @@ fn add_level(
                         .emit_err(session_diagnostics::MultipleUnstableReasons { span: attr.span });
                 }
             }
-            // Make the unstability soft if any unstable attributes are marked 'soft'; if an
-            // unstable item is allowed in stable rust, another attribute shouldn't break that.
-            // FIXME(dianne): should there be a check that all unstables are soft if any are?
-            *is_soft |= new_soft;
+            // If any unstable attributes are marked 'soft', all should be. This keeps soft-unstable
+            // items from accidentally being made properly unstable as attributes are removed.
+            if *is_soft != new_soft {
+                let spans = stab_spans
+                    .iter()
+                    .filter(|(stab, _)| stab.is_unstable())
+                    .map(|&(_, sp)| sp)
+                    .chain([attr.span])
+                    .collect();
+                sess.dcx().emit_err(session_diagnostics::SoftInconsistent { spans });
+            }
         }
         // an item with some stable and some unstable features is unstable
         (Some(Unstable { .. }), Stable { .. }) => {}

compiler/rustc_attr/src/session_diagnostics.rs

@@ -407,6 +407,13 @@ pub(crate) struct SoftNoArgs {
     pub span: Span,
 }
 
+#[derive(Diagnostic)]
+#[diag(attr_soft_inconsistent)]
+pub(crate) struct SoftInconsistent {
+    #[primary_span]
+    pub spans: Vec<Span>,
+}
+
 #[derive(Diagnostic)]
 #[diag(attr_unknown_version_literal)]
 pub(crate) struct UnknownVersionLiteral {

tests/ui/stability-attribute/multiple-stability-attribute-sanity.rs

@@ -13,4 +13,12 @@ fn f1() { }
 #[unstable(feature = "c", issue = "none", reason = "reason 3")] //~ ERROR multiple reasons provided for unstability
 fn f2() { }
 
+#[unstable(feature = "a", issue = "none")] //~ ERROR `soft` must be present on either none or all of an item's `unstable` attributes
+#[unstable(feature = "b", issue = "none", soft)]
+fn f3() { }
+
+#[unstable(feature = "a", issue = "none", soft)] //~ ERROR `soft` must be present on either none or all of an item's `unstable` attributes
+#[unstable(feature = "b", issue = "none")]
+fn f4() { }
+
 fn main() { }

tests/ui/stability-attribute/multiple-stability-attribute-sanity.stderr

@@ -16,5 +16,21 @@ error: multiple reasons provided for unstability
 LL | #[unstable(feature = "c", issue = "none", reason = "reason 3")]
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: aborting due to 3 previous errors
+error: `soft` must be present on either none or all of an item's `unstable` attributes
+  --> $DIR/multiple-stability-attribute-sanity.rs:16:1
+   |
+LL | #[unstable(feature = "a", issue = "none")]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+LL | #[unstable(feature = "b", issue = "none", soft)]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: `soft` must be present on either none or all of an item's `unstable` attributes
+  --> $DIR/multiple-stability-attribute-sanity.rs:20:1
+   |
+LL | #[unstable(feature = "a", issue = "none", soft)]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+LL | #[unstable(feature = "b", issue = "none")]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 5 previous errors