Reenable disabled early syntax gates as future-incompatibility lints

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -2,10 +2,10 @@ use rustc_ast as ast;
 use rustc_ast::visit::{self, AssocCtxt, FnCtxt, FnKind, Visitor};
 use rustc_ast::{AssocConstraint, AssocConstraintKind, NodeId};
 use rustc_ast::{PatKind, RangeEnd, VariantData};
-use rustc_errors::{struct_span_err, Applicability};
+use rustc_errors::{struct_span_err, Applicability, StashKey};
+use rustc_feature::Features;
 use rustc_feature::{AttributeGate, BuiltinAttribute, BUILTIN_ATTRIBUTE_MAP};
-use rustc_feature::{Features, GateIssue};
-use rustc_session::parse::{feature_err, feature_err_issue};
+use rustc_session::parse::{feature_err, feature_warn};
 use rustc_session::Session;
 use rustc_span::source_map::Spanned;
 use rustc_span::symbol::sym;
@@ -20,9 +20,7 @@ macro_rules! gate_feature_fn {
         let has_feature: bool = has_feature(visitor.features);
         debug!("gate_feature(feature = {:?}, span = {:?}); has? {}", name, span, has_feature);
         if !has_feature && !span.allows_unstable($name) {
-            feature_err_issue(&visitor.sess.parse_sess, name, span, GateIssue::Language, explain)
-                .help(help)
-                .emit();
+            feature_err(&visitor.sess.parse_sess, name, span, explain).help(help).emit();
         }
     }};
     ($visitor: expr, $has_feature: expr, $span: expr, $name: expr, $explain: expr) => {{
@@ -31,8 +29,19 @@ macro_rules! gate_feature_fn {
         let has_feature: bool = has_feature(visitor.features);
         debug!("gate_feature(feature = {:?}, span = {:?}); has? {}", name, span, has_feature);
         if !has_feature && !span.allows_unstable($name) {
-            feature_err_issue(&visitor.sess.parse_sess, name, span, GateIssue::Language, explain)
-                .emit();
+            feature_err(&visitor.sess.parse_sess, name, span, explain).emit();
+        }
+    }};
+    (future_incompatible; $visitor: expr, $has_feature: expr, $span: expr, $name: expr, $explain: expr) => {{
+        let (visitor, has_feature, span, name, explain) =
+            (&*$visitor, $has_feature, $span, $name, $explain);
+        let has_feature: bool = has_feature(visitor.features);
+        debug!(
+            "gate_feature(feature = {:?}, span = {:?}); has? {} (future_incompatible)",
+            name, span, has_feature
+        );
+        if !has_feature && !span.allows_unstable($name) {
+            feature_warn(&visitor.sess.parse_sess, name, span, explain);
         }
     }};
 }
@@ -44,6 +53,9 @@ macro_rules! gate_feature_post {
     ($visitor: expr, $feature: ident, $span: expr, $explain: expr) => {
         gate_feature_fn!($visitor, |x: &Features| x.$feature, $span, sym::$feature, $explain)
     };
+    (future_incompatible; $visitor: expr, $feature: ident, $span: expr, $explain: expr) => {
+        gate_feature_fn!(future_incompatible; $visitor, |x: &Features| x.$feature, $span, sym::$feature, $explain)
+    };
 }
 
 pub fn check_attribute(attr: &ast::Attribute, sess: &Session, features: &Features) {
@@ -588,11 +600,10 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
         {
             // When we encounter a statement of the form `foo: Ty = val;`, this will emit a type
             // ascription error, but the likely intention was to write a `let` statement. (#78907).
-            feature_err_issue(
+            feature_err(
                 &self.sess.parse_sess,
                 sym::type_ascription,
                 lhs.span,
-                GateIssue::Language,
                 "type ascription is experimental",
             ).span_suggestion_verbose(
                 lhs.span.shrink_to_lo(),
@@ -615,15 +626,22 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
                 );
             }
             ast::ExprKind::Type(..) => {
-                // To avoid noise about type ascription in common syntax errors, only emit if it
-                // is the *only* error.
                 if self.sess.parse_sess.span_diagnostic.err_count() == 0 {
+                    // To avoid noise about type ascription in common syntax errors,
+                    // only emit if it is the *only* error.
                     gate_feature_post!(
                         &self,
                         type_ascription,
                         e.span,
                         "type ascription is experimental"
                     );
+                } else {
+                    // And if it isn't, cancel the early-pass warning.
+                    self.sess
+                        .parse_sess
+                        .span_diagnostic
+                        .steal_diagnostic(e.span, StashKey::EarlySyntaxWarning)
+                        .map(|err| err.cancel());
                 }
             }
             ast::ExprKind::TryBlock(_) => {
@@ -789,14 +807,12 @@ pub fn check_crate(krate: &ast::Crate, sess: &Session) {
 
     // All uses of `gate_all!` below this point were added in #65742,
     // and subsequently disabled (with the non-early gating readded).
+    // We emit an early future-incompatible warning for these.
+    // New syntax gates should go above here to get a hard error gate.
     macro_rules! gate_all {
         ($gate:ident, $msg:literal) => {
-            // FIXME(eddyb) do something more useful than always
-            // disabling these uses of early feature-gatings.
-            if false {
-                for span in spans.get(&sym::$gate).unwrap_or(&vec![]) {
-                    gate_feature_post!(&visitor, $gate, *span, $msg);
-                }
+            for span in spans.get(&sym::$gate).unwrap_or(&vec![]) {
+                gate_feature_post!(future_incompatible; &visitor, $gate, *span, $msg);
             }
         };
     }
@@ -809,11 +825,7 @@ pub fn check_crate(krate: &ast::Crate, sess: &Session) {
     gate_all!(try_blocks, "`try` blocks are unstable");
     gate_all!(label_break_value, "labels on blocks are unstable");
     gate_all!(box_syntax, "box expression syntax is experimental; you can call `Box::new` instead");
-    // To avoid noise about type ascription in common syntax errors,
-    // only emit if it is the *only* error. (Also check it last.)
-    if sess.parse_sess.span_diagnostic.err_count() == 0 {
-        gate_all!(type_ascription, "type ascription is experimental");
-    }
+    gate_all!(type_ascription, "type ascription is experimental");
 
     visit::walk_crate(&mut visitor, krate);
 }

compiler/rustc_errors/src/lib.rs

@@ -458,6 +458,7 @@ struct HandlerInner {
 pub enum StashKey {
     ItemNoType,
     UnderscoreForArrayLengths,
+    EarlySyntaxWarning,
 }
 
 fn default_track_diagnostic(_: &Diagnostic) {}
@@ -625,19 +626,13 @@ impl Handler {
     /// Stash a given diagnostic with the given `Span` and `StashKey` as the key for later stealing.
     pub fn stash_diagnostic(&self, span: Span, key: StashKey, diag: Diagnostic) {
         let mut inner = self.inner.borrow_mut();
-        // FIXME(Centril, #69537): Consider reintroducing panic on overwriting a stashed diagnostic
-        // if/when we have a more robust macro-friendly replacement for `(span, key)` as a key.
-        // See the PR for a discussion.
-        inner.stashed_diagnostics.insert((span, key), diag);
+        inner.stash((span, key), diag);
     }
 
     /// Steal a previously stashed diagnostic with the given `Span` and `StashKey` as the key.
     pub fn steal_diagnostic(&self, span: Span, key: StashKey) -> Option<DiagnosticBuilder<'_, ()>> {
-        self.inner
-            .borrow_mut()
-            .stashed_diagnostics
-            .remove(&(span, key))
-            .map(|diag| DiagnosticBuilder::new_diagnostic(self, diag))
+        let mut inner = self.inner.borrow_mut();
+        inner.steal((span, key)).map(|diag| DiagnosticBuilder::new_diagnostic(self, diag))
     }
 
     /// Emit all stashed diagnostics.
@@ -1105,13 +1100,31 @@ impl HandlerInner {
 
     /// Emit all stashed diagnostics.
     fn emit_stashed_diagnostics(&mut self) -> Option<ErrorGuaranteed> {
+        let has_errors = self.has_errors();
         let diags = self.stashed_diagnostics.drain(..).map(|x| x.1).collect::<Vec<_>>();
         let mut reported = None;
         for mut diag in diags {
+            // Decrement the count tracking the stash; emitting will increment it.
             if diag.is_error() {
-                reported = Some(ErrorGuaranteed(()));
+                if matches!(diag.level, Level::Error { lint: true }) {
+                    self.lint_err_count -= 1;
+                } else {
+                    self.err_count -= 1;
+                }
+            } else {
+                if diag.is_force_warn() {
+                    self.warn_count -= 1;
+                } else {
+                    // Unless they're forced, don't flush stashed warnings when
+                    // there are errors, to avoid causing warning overload. The
+                    // stash would've been stolen already if it were important.
+                    if has_errors {
+                        continue;
+                    }
+                }
             }
-            self.emit_diagnostic(&mut diag);
+            let reported_this = self.emit_diagnostic(&mut diag);
+            reported = reported.or(reported_this);
         }
         reported
     }
@@ -1301,9 +1314,47 @@ impl HandlerInner {
         }
     }
 
+    fn stash(&mut self, key: (Span, StashKey), diagnostic: Diagnostic) {
+        // Track the diagnostic for counts, but don't panic-if-treat-err-as-bug
+        // yet; that happens when we actually emit the diagnostic.
+        if diagnostic.is_error() {
+            if matches!(diagnostic.level, Level::Error { lint: true }) {
+                self.lint_err_count += 1;
+            } else {
+                self.err_count += 1;
+            }
+        } else {
+            // Warnings are only automatically flushed if they're forced.
+            if diagnostic.is_force_warn() {
+                self.warn_count += 1;
+            }
+        }
+
+        // FIXME(Centril, #69537): Consider reintroducing panic on overwriting a stashed diagnostic
+        // if/when we have a more robust macro-friendly replacement for `(span, key)` as a key.
+        // See the PR for a discussion.
+        self.stashed_diagnostics.insert(key, diagnostic);
+    }
+
+    fn steal(&mut self, key: (Span, StashKey)) -> Option<Diagnostic> {
+        let diagnostic = self.stashed_diagnostics.remove(&key)?;
+        if diagnostic.is_error() {
+            if matches!(diagnostic.level, Level::Error { lint: true }) {
+                self.lint_err_count -= 1;
+            } else {
+                self.err_count -= 1;
+            }
+        } else {
+            if diagnostic.is_force_warn() {
+                self.warn_count -= 1;
+            }
+        }
+        Some(diagnostic)
+    }
+
     #[inline]
     fn err_count(&self) -> usize {
-        self.err_count + self.stashed_diagnostics.len()
+        self.err_count
     }
 
     fn has_errors(&self) -> bool {

compiler/rustc_lint_defs/src/builtin.rs

@@ -3212,6 +3212,56 @@ declare_lint! {
     };
 }
 
+declare_lint! {
+    /// The `unstable_syntax_pre_expansion` lint detects the use of unstable
+    /// syntax that is discarded during attribute expansion.
+    ///
+    /// ### Example
+    ///
+    /// ```rust
+    /// #[cfg(FALSE)]
+    /// macro foo() {}
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// The input to active attributes such as `#[cfg]` or procedural macro
+    /// attributes is required to be valid syntax. Previously, the compiler only
+    /// gated the use of unstable syntax features after resolving `#[cfg]` gates
+    /// and expanding procedural macros.
+    ///
+    /// To avoid relying on unstable syntax, move the use of unstable syntax
+    /// into a position where the compiler does not parse the syntax, such as a
+    /// functionlike macro.
+    ///
+    /// ```rust
+    /// # #![deny(unstable_syntax_pre_expansion)]
+    ///
+    /// macro_rules! identity {
+    ///    ( $($tokens:tt)* ) => { $($tokens)* }
+    /// }
+    ///
+    /// #[cfg(FALSE)]
+    /// identity! {
+    ///    macro foo() {}
+    /// }
+    /// ```
+    ///
+    /// This is a [future-incompatible] lint to transition this
+    /// to a hard error in the future. See [issue #65860] for more details.
+    ///
+    /// [issue #65860]: https://github.com/rust-lang/rust/issues/65860
+    /// [future-incompatible]: ../index.md#future-incompatible-lints
+    pub UNSTABLE_SYNTAX_PRE_EXPANSION,
+    Warn,
+    "unstable syntax can change at any point in the future, causing a hard error!",
+    @future_incompatible = FutureIncompatibleInfo {
+        reference: "issue #65860 <https://github.com/rust-lang/rust/issues/65860>",
+    };
+}
+
 declare_lint_pass! {
     /// Does nothing as a lint pass, but registers some `Lint`s
     /// that are used by other parts of the compiler.
@@ -3280,6 +3330,7 @@ declare_lint_pass! {
         POINTER_STRUCTURAL_MATCH,
         NONTRIVIAL_STRUCTURAL_MATCH,
         SOFT_UNSTABLE,
+        UNSTABLE_SYNTAX_PRE_EXPANSION,
         INLINE_NO_SANITIZE,
         BAD_ASM_STYLE,
         ASM_SUB_REGISTER,

compiler/rustc_session/src/parse.rs

@@ -2,15 +2,17 @@
 //! It also serves as an input to the parser itself.
 
 use crate::config::CheckCfg;
-use crate::lint::{BufferedEarlyLint, BuiltinLintDiagnostics, Lint, LintId};
+use crate::lint::{
+    builtin::UNSTABLE_SYNTAX_PRE_EXPANSION, BufferedEarlyLint, BuiltinLintDiagnostics, Lint, LintId,
+};
 use crate::SessionDiagnostic;
 use rustc_ast::node_id::NodeId;
 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
 use rustc_data_structures::sync::{Lock, Lrc};
 use rustc_errors::{emitter::SilentEmitter, ColorConfig, Handler};
 use rustc_errors::{
-    error_code, fallback_fluent_bundle, Applicability, Diagnostic, DiagnosticBuilder,
-    DiagnosticMessage, ErrorGuaranteed, MultiSpan,
+    error_code, fallback_fluent_bundle, Applicability, Diagnostic, DiagnosticBuilder, DiagnosticId,
+    DiagnosticMessage, ErrorGuaranteed, MultiSpan, StashKey,
 };
 use rustc_feature::{find_feature_issue, GateIssue, UnstableFeatures};
 use rustc_span::edition::Edition;
@@ -101,11 +103,58 @@ pub fn feature_err_issue<'a>(
     issue: GateIssue,
     explain: &str,
 ) -> DiagnosticBuilder<'a, ErrorGuaranteed> {
+    let span = span.into();
+
+    // Cancel an earlier warning for this same error, if it exists.
+    if let Some(span) = span.primary_span() {
+        sess.span_diagnostic
+            .steal_diagnostic(span, StashKey::EarlySyntaxWarning)
+            .map(|err| err.cancel());
+    }
+
     let mut err = sess.span_diagnostic.struct_span_err_with_code(span, explain, error_code!(E0658));
     add_feature_diagnostics_for_issue(&mut err, sess, feature, issue);
     err
 }
 
+/// Construct a future incompatibility diagnostic for a feature gate.
+///
+/// This diagnostic is only a warning and *does not cause compilation to fail*.
+pub fn feature_warn<'a>(sess: &'a ParseSess, feature: Symbol, span: Span, explain: &str) {
+    feature_warn_issue(sess, feature, span, GateIssue::Language, explain);
+}
+
+/// Construct a future incompatibility diagnostic for a feature gate.
+///
+/// This diagnostic is only a warning and *does not cause compilation to fail*.
+///
+/// This variant allows you to control whether it is a library or language feature.
+/// Almost always, you want to use this for a language feature. If so, prefer `feature_warn`.
+pub fn feature_warn_issue<'a>(
+    sess: &'a ParseSess,
+    feature: Symbol,
+    span: Span,
+    issue: GateIssue,
+    explain: &str,
+) {
+    let mut err = sess.span_diagnostic.struct_span_warn(span, explain);
+    add_feature_diagnostics_for_issue(&mut err, sess, feature, issue);
+
+    // Decorate this as a future-incompatibility lint as in rustc_middle::lint::struct_lint_level
+    let lint = UNSTABLE_SYNTAX_PRE_EXPANSION;
+    let future_incompatible = lint.future_incompatible.as_ref().unwrap();
+    err.code(DiagnosticId::Lint {
+        name: lint.name_lower(),
+        has_future_breakage: false,
+        is_force_warn: false,
+    });
+    err.warn(lint.desc);
+    err.note(format!("for more information, see {}", future_incompatible.reference));
+
+    // A later feature_err call can steal and cancel this warning.
+    err.stash(span, StashKey::EarlySyntaxWarning);
+}
+
 /// Adds the diagnostics for a feature to an existing error.
 pub fn add_feature_diagnostics<'a>(err: &mut Diagnostic, sess: &'a ParseSess, feature: Symbol) {
     add_feature_diagnostics_for_issue(err, sess, feature, GateIssue::Language);

src/test/ui/feature-gates/soft-syntax-gates-with-errors.rs

@@ -0,0 +1,30 @@
+// check-fail
+// This file is used to test the behavior of the early-pass syntax warnings.
+// If macro syntax is stabilized, replace with a different unstable syntax.
+
+macro a() {}
+//~^ ERROR: `macro` is experimental
+
+#[cfg(FALSE)]
+macro b() {}
+
+macro_rules! identity {
+    ($($x:tt)*) => ($($x)*);
+}
+
+identity! {
+    macro c() {}
+    //~^ ERROR: `macro` is experimental
+}
+
+#[cfg(FALSE)]
+identity! {
+    macro d() {} // No error
+}
+
+identity! {
+    #[cfg(FALSE)]
+    macro e() {}
+}
+
+fn main() {}

src/test/ui/feature-gates/soft-syntax-gates-with-errors.stderr

@@ -0,0 +1,21 @@
+error[E0658]: `macro` is experimental
+  --> $DIR/soft-syntax-gates-with-errors.rs:5:1
+   |
+LL | macro a() {}
+   | ^^^^^^^^^^^^
+   |
+   = note: see issue #39412 <https://github.com/rust-lang/rust/issues/39412> for more information
+   = help: add `#![feature(decl_macro)]` to the crate attributes to enable
+
+error[E0658]: `macro` is experimental
+  --> $DIR/soft-syntax-gates-with-errors.rs:16:5
+   |
+LL |     macro c() {}
+   |     ^^^^^^^^^^^^
+   |
+   = note: see issue #39412 <https://github.com/rust-lang/rust/issues/39412> for more information
+   = help: add `#![feature(decl_macro)]` to the crate attributes to enable
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0658`.

src/test/ui/feature-gates/soft-syntax-gates-without-errors.rs

@@ -0,0 +1,26 @@
+// check-pass
+// This file is used to test the behavior of the early-pass syntax warnings.
+// If macro syntax is stabilized, replace with a different unstable syntax.
+
+#[cfg(FALSE)]
+macro b() {}
+//~^ WARN: `macro` is experimental
+//~| WARN: unstable syntax
+
+macro_rules! identity {
+    ($($x:tt)*) => ($($x)*);
+}
+
+#[cfg(FALSE)]
+identity! {
+    macro d() {} // No error
+}
+
+identity! {
+    #[cfg(FALSE)]
+    macro e() {}
+    //~^ WARN: `macro` is experimental
+    //~| WARN: unstable syntax
+}
+
+fn main() {}

src/test/ui/feature-gates/soft-syntax-gates-without-errors.stderr

@@ -0,0 +1,24 @@
+warning: `macro` is experimental
+  --> $DIR/soft-syntax-gates-without-errors.rs:6:1
+   |
+LL | macro b() {}
+   | ^^^^^^^^^^^^
+   |
+   = note: see issue #39412 <https://github.com/rust-lang/rust/issues/39412> for more information
+   = help: add `#![feature(decl_macro)]` to the crate attributes to enable
+   = warning: unstable syntax can change at any point in the future, causing a hard error!
+   = note: for more information, see issue #65860 <https://github.com/rust-lang/rust/issues/65860>
+
+warning: `macro` is experimental
+  --> $DIR/soft-syntax-gates-without-errors.rs:21:5
+   |
+LL |     macro e() {}
+   |     ^^^^^^^^^^^^
+   |
+   = note: see issue #39412 <https://github.com/rust-lang/rust/issues/39412> for more information
+   = help: add `#![feature(decl_macro)]` to the crate attributes to enable
+   = warning: unstable syntax can change at any point in the future, causing a hard error!
+   = note: for more information, see issue #65860 <https://github.com/rust-lang/rust/issues/65860>
+
+warning: 2 warnings emitted
+

src/test/ui/macros/stringify.rs

@@ -3,11 +3,18 @@
 // compile-flags: --test
 
 #![feature(async_closure)]
+#![feature(box_patterns)]
+#![feature(box_syntax)]
 #![feature(const_trait_impl)]
+#![feature(decl_macro)]
 #![feature(generators)]
 #![feature(half_open_range_patterns)]
+#![feature(label_break_value)]
 #![feature(more_qualified_paths)]
 #![feature(raw_ref_op)]
+#![feature(trait_alias)]
+#![feature(try_blocks)]
+#![feature(type_ascription)]
 #![deny(unused_macros)]
 
 macro_rules! stringify_block {

src/test/ui/or-patterns/or-patterns-syntactic-pass.rs

@@ -7,7 +7,7 @@ fn main() {}
 
 // Test the `pat` macro fragment parser:
 macro_rules! accept_pat {
-    ($p:pat) => {}
+    ($p:pat) => {};
 }
 
 accept_pat!((p | q));
@@ -21,28 +21,28 @@ accept_pat!([p | q]);
 #[cfg(FALSE)]
 fn or_patterns() {
     // Top level of `let`:
-    let (| A | B);
+    let (A | B);
     let (A | B);
     let (A | B): u8;
     let (A | B) = 0;
     let (A | B): u8 = 0;
 
     // Top level of `for`:
-    for | A | B in 0 {}
+    for A | B in 0 {}
     for A | B in 0 {}
 
     // Top level of `while`:
-    while let | A | B = 0 {}
+    while let A | B = 0 {}
     while let A | B = 0 {}
 
     // Top level of `if`:
-    if let | A | B = 0 {}
+    if let A | B = 0 {}
     if let A | B = 0 {}
 
     // Top level of `match` arms:
     match 0 {
-        | A | B => {},
-        A | B => {},
+        A | B => {}
+        A | B => {}
     }
 
     // Functions:
@@ -68,6 +68,8 @@ fn or_patterns() {
 
     // These bind as `(prefix p) | q` as opposed to `prefix (p | q)`:
     let (box 0 | 1); // Unstable; we *can* change the precedence if we want.
+                     //~^ WARN box pattern syntax is experimental
+                     //~| WARN unstable syntax
     let (&0 | 1);
     let (&mut 0 | 1);
     let (x @ 0 | 1);

src/test/ui/or-patterns/or-patterns-syntactic-pass.stderr

@@ -0,0 +1,13 @@
+warning: box pattern syntax is experimental
+  --> $DIR/or-patterns-syntactic-pass.rs:70:10
+   |
+LL |     let (box 0 | 1); // Unstable; we *can* change the precedence if we want.
+   |          ^^^^^
+   |
+   = note: see issue #29641 <https://github.com/rust-lang/rust/issues/29641> for more information
+   = help: add `#![feature(box_patterns)]` to the crate attributes to enable
+   = warning: unstable syntax can change at any point in the future, causing a hard error!
+   = note: for more information, see issue #65860 <https://github.com/rust-lang/rust/issues/65860>
+
+warning: 1 warning emitted
+

src/test/ui/parser/constraints-before-generic-args-syntactic-pass.rs

@@ -3,7 +3,11 @@
 #[cfg(FALSE)]
 fn syntax() {
     foo::<T = u8, T: Ord, String>();
+    //~^ WARN associated type bounds are unstable
+    //~| WARN unstable syntax
     foo::<T = u8, 'a, T: Ord>();
+    //~^ WARN associated type bounds are unstable
+    //~| WARN unstable syntax
 }
 
 fn main() {}

src/test/ui/parser/constraints-before-generic-args-syntactic-pass.stderr

@@ -0,0 +1,24 @@
+warning: associated type bounds are unstable
+  --> $DIR/constraints-before-generic-args-syntactic-pass.rs:5:19
+   |
+LL |     foo::<T = u8, T: Ord, String>();
+   |                   ^^^^^^
+   |
+   = note: see issue #52662 <https://github.com/rust-lang/rust/issues/52662> for more information
+   = help: add `#![feature(associated_type_bounds)]` to the crate attributes to enable
+   = warning: unstable syntax can change at any point in the future, causing a hard error!
+   = note: for more information, see issue #65860 <https://github.com/rust-lang/rust/issues/65860>
+
+warning: associated type bounds are unstable
+  --> $DIR/constraints-before-generic-args-syntactic-pass.rs:8:23
+   |
+LL |     foo::<T = u8, 'a, T: Ord>();
+   |                       ^^^^^^
+   |
+   = note: see issue #52662 <https://github.com/rust-lang/rust/issues/52662> for more information
+   = help: add `#![feature(associated_type_bounds)]` to the crate attributes to enable
+   = warning: unstable syntax can change at any point in the future, causing a hard error!
+   = note: for more information, see issue #65860 <https://github.com/rust-lang/rust/issues/65860>
+
+warning: 2 warnings emitted
+

src/test/ui/pattern/rest-pat-syntactic.rs

@@ -19,6 +19,8 @@ fn rest_patterns() {
 
     // Box patterns:
     let box ..;
+    //~^ WARN box pattern syntax is experimental
+    //~| WARN unstable syntax
 
     // In or-patterns:
     match x {
@@ -57,7 +59,7 @@ fn rest_patterns() {
         .. |
         [
             (
-                box ..,
+                box .., //~ WARN box pattern syntax is experimental
                 &(..),
                 &mut ..,
                 x @ ..
@@ -67,4 +69,5 @@ fn rest_patterns() {
         ref mut x @ ..
         => {}
     }
+    //~| WARN unstable syntax
 }

src/test/ui/pattern/rest-pat-syntactic.stderr

@@ -0,0 +1,24 @@
+warning: box pattern syntax is experimental
+  --> $DIR/rest-pat-syntactic.rs:21:9
+   |
+LL |     let box ..;
+   |         ^^^^^^
+   |
+   = note: see issue #29641 <https://github.com/rust-lang/rust/issues/29641> for more information
+   = help: add `#![feature(box_patterns)]` to the crate attributes to enable
+   = warning: unstable syntax can change at any point in the future, causing a hard error!
+   = note: for more information, see issue #65860 <https://github.com/rust-lang/rust/issues/65860>
+
+warning: box pattern syntax is experimental
+  --> $DIR/rest-pat-syntactic.rs:62:17
+   |
+LL |                 box ..,
+   |                 ^^^^^^
+   |
+   = note: see issue #29641 <https://github.com/rust-lang/rust/issues/29641> for more information
+   = help: add `#![feature(box_patterns)]` to the crate attributes to enable
+   = warning: unstable syntax can change at any point in the future, causing a hard error!
+   = note: for more information, see issue #65860 <https://github.com/rust-lang/rust/issues/65860>
+
+warning: 2 warnings emitted
+

src/test/ui/suggestions/many-type-ascription.rs

@@ -0,0 +1,4 @@
+fn main() {
+    let _ = 0: i32; //~ ERROR: type ascription is experimental
+    let _ = 0: i32; // (error only emitted once)
+}

src/test/ui/suggestions/many-type-ascription.stderr

@@ -0,0 +1,12 @@
+error[E0658]: type ascription is experimental
+  --> $DIR/many-type-ascription.rs:2:13
+   |
+LL |     let _ = 0: i32;
+   |             ^^^^^^
+   |
+   = note: see issue #23416 <https://github.com/rust-lang/rust/issues/23416> for more information
+   = help: add `#![feature(type_ascription)]` to the crate attributes to enable
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0658`.

src/test/ui/suggestions/type-ascription-and-other-error.rs

@@ -0,0 +1,6 @@
+fn main() {
+    not rust; //~ ERROR
+    let _ = 0: i32; // (error hidden by existing error)
+    #[cfg(FALSE)]
+    let _ = 0: i32; // (warning hidden by existing error)
+}

src/test/ui/suggestions/type-ascription-and-other-error.stderr

@@ -0,0 +1,8 @@
+error: expected one of `!`, `.`, `::`, `;`, `?`, `{`, `}`, or an operator, found `rust`
+  --> $DIR/type-ascription-and-other-error.rs:2:9
+   |
+LL |     not rust;
+   |         ^^^^ expected one of 8 possible tokens
+
+error: aborting due to previous error
+