Make pre-expansion gate slightly less obnoxious

Author: varkor

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -630,7 +630,11 @@ pub fn check_crate(krate: &ast::Crate, sess: &Session) {
     gate_all!(const_trait_impl, "const trait impls are experimental");
     gate_all!(half_open_range_patterns, "half-open range patterns are unstable");
     gate_all!(inline_const, "inline-const is experimental");
-    gate_all!(destructuring_assignment, "destructuring assignments are unstable");
+    if parse_sess.span_diagnostic.err_count() == 0 {
+        // Errors for `destructuring_assignment` can get quite noisy, especially where `_` is
+        // involved, so we only emit errors where there are no other parsing errors.
+        gate_all!(destructuring_assignment, "destructuring assignments are unstable");
+    }
 
     // All uses of `gate_all!` below this point were added in #65742,
     // and subsequently disabled (with the non-early gating readded).

src/test/ui/cross/cross-file-errors/main.rs

@@ -4,4 +4,5 @@ mod underscore;
 fn main() {
     underscore!();
     //~^ ERROR expected expression, found reserved identifier `_`
+    //~^^ ERROR destructuring assignments are unstable
 }

src/test/ui/cross/cross-file-errors/main.stderr

@@ -1,3 +1,18 @@
+error[E0658]: destructuring assignments are unstable
+  --> $DIR/underscore.rs:8:9
+   |
+LL |         _
+   |         ^
+   | 
+  ::: $DIR/main.rs:5:5
+   |
+LL |     underscore!();
+   |     -------------- in this macro invocation
+   |
+   = note: see issue #71126 <https://github.com/rust-lang/rust/issues/71126> for more information
+   = help: add `#![feature(destructuring_assignment)]` to the crate attributes to enable
+   = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
+
 error: expected expression, found reserved identifier `_`
   --> $DIR/underscore.rs:8:9
    |
@@ -11,5 +26,6 @@ LL |     underscore!();
    |
    = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
 
-error: aborting due to previous error
+error: aborting due to 2 previous errors
 
+For more information about this error, try `rustc --explain E0658`.

src/test/ui/suggestions/fn-or-tuple-struct-with-underscore-args.rs

@@ -10,10 +10,16 @@ fn main() {
     let _: usize = foo(_, _);
     //~^ ERROR expected expression
     //~| ERROR expected expression
+    //~| ERROR destructuring assignments are unstable
+    //~| ERROR destructuring assignments are unstable
     let _: S = S(_, _);
     //~^ ERROR expected expression
     //~| ERROR expected expression
+    //~| ERROR destructuring assignments are unstable
+    //~| ERROR destructuring assignments are unstable
     let _: usize = T::baz(_, _);
     //~^ ERROR expected expression
     //~| ERROR expected expression
+    //~| ERROR destructuring assignments are unstable
+    //~| ERROR destructuring assignments are unstable
 }

src/test/ui/suggestions/fn-or-tuple-struct-with-underscore-args.stderr

@@ -1,3 +1,57 @@
+error[E0658]: destructuring assignments are unstable
+  --> $DIR/fn-or-tuple-struct-with-underscore-args.rs:10:24
+   |
+LL |     let _: usize = foo(_, _);
+   |                        ^
+   |
+   = note: see issue #71126 <https://github.com/rust-lang/rust/issues/71126> for more information
+   = help: add `#![feature(destructuring_assignment)]` to the crate attributes to enable
+
+error[E0658]: destructuring assignments are unstable
+  --> $DIR/fn-or-tuple-struct-with-underscore-args.rs:10:27
+   |
+LL |     let _: usize = foo(_, _);
+   |                           ^
+   |
+   = note: see issue #71126 <https://github.com/rust-lang/rust/issues/71126> for more information
+   = help: add `#![feature(destructuring_assignment)]` to the crate attributes to enable
+
+error[E0658]: destructuring assignments are unstable
+  --> $DIR/fn-or-tuple-struct-with-underscore-args.rs:15:18
+   |
+LL |     let _: S = S(_, _);
+   |                  ^
+   |
+   = note: see issue #71126 <https://github.com/rust-lang/rust/issues/71126> for more information
+   = help: add `#![feature(destructuring_assignment)]` to the crate attributes to enable
+
+error[E0658]: destructuring assignments are unstable
+  --> $DIR/fn-or-tuple-struct-with-underscore-args.rs:15:21
+   |
+LL |     let _: S = S(_, _);
+   |                     ^
+   |
+   = note: see issue #71126 <https://github.com/rust-lang/rust/issues/71126> for more information
+   = help: add `#![feature(destructuring_assignment)]` to the crate attributes to enable
+
+error[E0658]: destructuring assignments are unstable
+  --> $DIR/fn-or-tuple-struct-with-underscore-args.rs:20:27
+   |
+LL |     let _: usize = T::baz(_, _);
+   |                           ^
+   |
+   = note: see issue #71126 <https://github.com/rust-lang/rust/issues/71126> for more information
+   = help: add `#![feature(destructuring_assignment)]` to the crate attributes to enable
+
+error[E0658]: destructuring assignments are unstable
+  --> $DIR/fn-or-tuple-struct-with-underscore-args.rs:20:30
+   |
+LL |     let _: usize = T::baz(_, _);
+   |                              ^
+   |
+   = note: see issue #71126 <https://github.com/rust-lang/rust/issues/71126> for more information
+   = help: add `#![feature(destructuring_assignment)]` to the crate attributes to enable
+
 error: expected expression, found reserved identifier `_`
   --> $DIR/fn-or-tuple-struct-with-underscore-args.rs:10:24
    |
@@ -11,28 +65,29 @@ LL |     let _: usize = foo(_, _);
    |                           ^ expected expression
 
 error: expected expression, found reserved identifier `_`
-  --> $DIR/fn-or-tuple-struct-with-underscore-args.rs:13:18
+  --> $DIR/fn-or-tuple-struct-with-underscore-args.rs:15:18
    |
 LL |     let _: S = S(_, _);
    |                  ^ expected expression
 
 error: expected expression, found reserved identifier `_`
-  --> $DIR/fn-or-tuple-struct-with-underscore-args.rs:13:21
+  --> $DIR/fn-or-tuple-struct-with-underscore-args.rs:15:21
    |
 LL |     let _: S = S(_, _);
    |                     ^ expected expression
 
 error: expected expression, found reserved identifier `_`
-  --> $DIR/fn-or-tuple-struct-with-underscore-args.rs:16:27
+  --> $DIR/fn-or-tuple-struct-with-underscore-args.rs:20:27
    |
 LL |     let _: usize = T::baz(_, _);
    |                           ^ expected expression
 
 error: expected expression, found reserved identifier `_`
-  --> $DIR/fn-or-tuple-struct-with-underscore-args.rs:16:30
+  --> $DIR/fn-or-tuple-struct-with-underscore-args.rs:20:30
    |
 LL |     let _: usize = T::baz(_, _);
    |                              ^ expected expression
 
-error: aborting due to 6 previous errors
+error: aborting due to 12 previous errors
 
+For more information about this error, try `rustc --explain E0658`.