Add sanity checks for reason and soft on unstable attributes

`reason` must appear at most once: it's the reason for the item being unstable, rather than a
particular feature. This simplifies diagnostic formatting.

`soft` must either be on all or no unstable attributes: 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.

compiler/rustc_attr/messages.ftl

@@ -85,6 +85,9 @@ attr_multiple_item =
 attr_multiple_stability_levels =
     multiple stability levels for feature `{$feature}`
 
+attr_multiple_unstable_reasons =
+    multiple reasons provided for unstability
+
 attr_non_ident_feature =
     'feature' is not an identifier
 
@@ -97,6 +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

@@ -493,11 +493,21 @@ fn add_level(
                 (_, UnstableReason::None) => {}
                 (reason @ UnstableReason::None, _) => *reason = new_reason,
                 _ => {
-                    // TODO: sanity check for only one reason
+                    sess.dcx()
+                        .emit_err(session_diagnostics::MultipleUnstableReasons { span: attr.span });
                 }
             }
-            // TODO: sanity check for is_soft consistency
-            *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

@@ -83,6 +83,13 @@ pub(crate) struct MultipleStabilityLevels {
     pub feature: Symbol,
 }
 
+#[derive(Diagnostic)]
+#[diag(attr_multiple_unstable_reasons)]
+pub(crate) struct MultipleUnstableReasons {
+    #[primary_span]
+    pub span: Span,
+}
+
 #[derive(Diagnostic)]
 #[diag(attr_invalid_issue_string, code = E0545)]
 pub(crate) struct InvalidIssueString {
@@ -400,6 +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/mixed-levels.stderr

@@ -13,9 +13,7 @@ error: `const_unstable_fn` is not yet stable as a const fn
 LL | const USE_UNSTABLE: () = mixed_levels::const_unstable_fn();
    |                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
-   = note: use of unstable library feature `unstable_c`
    = help: add `#![feature(unstable_c)]` to the crate attributes to enable
-   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
 
 error: aborting due to 2 previous errors
 

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

@@ -0,0 +1,24 @@
+//! Checks that multiple stability attributes are used correctly together
+
+#![feature(staged_api)]
+
+#![stable(feature = "stable_test_feature", since = "1.0.0")]
+
+#[unstable(feature = "a", issue = "none", reason = "reason 1")]
+#[unstable(feature = "b", issue = "none", reason = "reason 2")] //~ ERROR multiple reasons provided for unstability
+fn f1() { }
+
+#[unstable(feature = "a", issue = "none", reason = "reason 1")]
+#[unstable(feature = "b", issue = "none", reason = "reason 2")] //~ ERROR multiple reasons provided for unstability
+#[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

@@ -0,0 +1,36 @@
+error: multiple reasons provided for unstability
+  --> $DIR/multiple-stability-attribute-sanity.rs:8:1
+   |
+LL | #[unstable(feature = "b", issue = "none", reason = "reason 2")]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: multiple reasons provided for unstability
+  --> $DIR/multiple-stability-attribute-sanity.rs:12:1
+   |
+LL | #[unstable(feature = "b", issue = "none", reason = "reason 2")]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: multiple reasons provided for unstability
+  --> $DIR/multiple-stability-attribute-sanity.rs:13:1
+   |
+LL | #[unstable(feature = "c", issue = "none", reason = "reason 3")]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+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
+