Accept inconsistent macro resolution for #147319 workaround

Author: Voultapher

compiler/rustc_resolve/src/macros.rs

@@ -838,10 +838,19 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
                                  res: Res| {
             if let Some(initial_res) = initial_res {
                 if res != initial_res {
-                    // Make sure compilation does not succeed if preferred macro resolution
-                    // has changed after the macro had been expanded. In theory all such
-                    // situations should be reported as errors, so this is a bug.
-                    this.dcx().span_delayed_bug(span, "inconsistent resolution for a macro");
+                    // Delayed versions of the #147319 workaround are inconsistent, but not really
+                    // problematic and already seen as a bug.
+                    // FIXME: Remove with lang team approval.
+                    if !this
+                        .ambiguity_errors
+                        .iter()
+                        .all(|ambiguity_error| ambiguity_error.warning.is_some())
+                    {
+                        // Make sure compilation does not succeed if preferred macro resolution
+                        // has changed after the macro had been expanded. In theory all such
+                        // situations should be reported as errors, so this is a bug.
+                        this.dcx().span_delayed_bug(span, "inconsistent resolution for a macro");
+                    }
                 }
             } else if this.tcx.dcx().has_errors().is_none() && this.privacy_errors.is_empty() {
                 // It's possible that the macro was unresolved (indeterminate) and silently

tests/ui/imports/ambiguous-panic-re-emit.rs

@@ -0,0 +1,24 @@
+#![crate_type = "lib"]
+#![no_std]
+
+macro_rules! re_emit {
+    ($($i:item)*) => ($($i)*)
+}
+
+// By re-emitting the prelude import via a macro, we run into the delayed bugs code path.
+re_emit! {
+    extern crate std;
+    use std::prelude::v1::*;
+}
+
+#[allow(unused)]
+fn xx() {
+    panic!();
+    //~^ WARNING `panic` is ambiguous
+    //~| WARNING this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+
+    // We can't deny the above lint, or else it *won't* run into the problematic issue of *not*
+    // having reported an error. So we crate a dummy error.
+    let _ = unknown_item;
+    //~^ ERROR: cannot find value `unknown_item`
+}

tests/ui/imports/ambiguous-panic-re-emit.stderr

@@ -0,0 +1,50 @@
+error[E0425]: cannot find value `unknown_item` in this scope
+  --> $DIR/ambiguous-panic-re-emit.rs:22:13
+   |
+LL |     let _ = unknown_item;
+   |             ^^^^^^^^^^^^ not found in this scope
+
+warning: `panic` is ambiguous
+  --> $DIR/ambiguous-panic-re-emit.rs:16:5
+   |
+LL |     panic!();
+   |     ^^^^^ ambiguous name
+   |
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #147319 <https://github.com/rust-lang/rust/issues/147319>
+   = note: ambiguous because of a conflict between a name from a glob import and an outer scope during import or macro resolution
+note: `panic` could refer to the macro imported here
+  --> $DIR/ambiguous-panic-re-emit.rs:11:9
+   |
+LL |     use std::prelude::v1::*;
+   |         ^^^^^^^^^^^^^^^^^^^
+   = help: consider adding an explicit import of `panic` to disambiguate
+   = help: or use `crate::panic` to refer to this macro unambiguously
+note: `panic` could also refer to a macro from prelude
+  --> $SRC_DIR/core/src/prelude/mod.rs:LL:COL
+   = note: `#[warn(ambiguous_panic_imports)]` (part of `#[warn(future_incompatible)]`) on by default
+
+error: aborting due to 1 previous error; 1 warning emitted
+
+For more information about this error, try `rustc --explain E0425`.
+Future incompatibility report: Future breakage diagnostic:
+warning: `panic` is ambiguous
+  --> $DIR/ambiguous-panic-re-emit.rs:16:5
+   |
+LL |     panic!();
+   |     ^^^^^ ambiguous name
+   |
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #147319 <https://github.com/rust-lang/rust/issues/147319>
+   = note: ambiguous because of a conflict between a name from a glob import and an outer scope during import or macro resolution
+note: `panic` could refer to the macro imported here
+  --> $DIR/ambiguous-panic-re-emit.rs:11:9
+   |
+LL |     use std::prelude::v1::*;
+   |         ^^^^^^^^^^^^^^^^^^^
+   = help: consider adding an explicit import of `panic` to disambiguate
+   = help: or use `crate::panic` to refer to this macro unambiguously
+note: `panic` could also refer to a macro from prelude
+  --> $SRC_DIR/core/src/prelude/mod.rs:LL:COL
+   = note: `#[warn(ambiguous_panic_imports)]` (part of `#[warn(future_incompatible)]`) on by default
+