fix(middle): emit error rather than delay bug when reaching limit

compiler/rustc_middle/messages.ftl

@@ -16,6 +16,10 @@ middle_limit_invalid =
     `limit` must be a non-negative integer
     .label = {$error_str}
 
+middle_recursion_limit_reached =
+    reached the recursion limit finding the struct tail for `{$ty}`
+    .help = consider increasing the recursion limit by adding a `#![recursion_limit = "{$suggested_limit}"]`
+
 middle_const_eval_non_int =
     constant evaluation of enum discriminant resulted in non-integer
 

compiler/rustc_middle/src/error.rs

@@ -49,6 +49,14 @@ pub struct LimitInvalid<'a> {
     pub error_str: &'a str,
 }
 
+#[derive(Diagnostic)]
+#[diag(middle_recursion_limit_reached)]
+#[help]
+pub struct RecursionLimitReached<'tcx> {
+    pub ty: Ty<'tcx>,
+    pub suggested_limit: rustc_session::Limit,
+}
+
 #[derive(Diagnostic)]
 #[diag(middle_const_eval_non_int)]
 pub struct ConstEvalNonIntError {

compiler/rustc_middle/src/ty/util.rs

@@ -19,7 +19,8 @@ use rustc_hir::def_id::{DefId, LocalDefId};
 use rustc_index::bit_set::GrowableBitSet;
 use rustc_index::vec::{Idx, IndexVec};
 use rustc_macros::HashStable;
-use rustc_span::{sym, DUMMY_SP};
+use rustc_session::Limit;
+use rustc_span::sym;
 use rustc_target::abi::{Integer, IntegerType, Size, TargetDataLayout};
 use rustc_target::spec::abi::Abi;
 use smallvec::SmallVec;
@@ -225,10 +226,13 @@ impl<'tcx> TyCtxt<'tcx> {
         let recursion_limit = self.recursion_limit();
         for iteration in 0.. {
             if !recursion_limit.value_within_limit(iteration) {
-                return self.ty_error_with_message(
-                    DUMMY_SP,
-                    &format!("reached the recursion limit finding the struct tail for {}", ty),
-                );
+                let suggested_limit = match recursion_limit {
+                    Limit(0) => Limit(2),
+                    limit => limit * 2,
+                };
+                let reported =
+                    self.sess.emit_err(crate::error::RecursionLimitReached { ty, suggested_limit });
+                return self.ty_error(reported);
             }
             match *ty.kind() {
                 ty::Adt(def, substs) => {

tests/ui/autoref-autoderef/issue-38940.rs

@@ -1,52 +0,0 @@
-// issue-38940: error printed twice for deref recursion limit exceeded
-// Test that the recursion limit can be changed. In this case, we have
-// deeply nested types that will fail the `Send` check by overflow
-// when the recursion limit is set very low.
-// compile-flags: -Zdeduplicate-diagnostics=yes
-
-#![allow(dead_code)]
-#![recursion_limit = "10"]
-macro_rules! link {
-    ($outer:ident, $inner:ident) => {
-        struct $outer($inner);
-        impl $outer {
-            fn new() -> $outer {
-                $outer($inner::new())
-            }
-        }
-        impl std::ops::Deref for $outer {
-            type Target = $inner;
-            fn deref(&self) -> &$inner {
-                &self.0
-            }
-        }
-    };
-}
-
-struct Bottom;
-
-impl Bottom {
-    fn new() -> Bottom {
-        Bottom
-    }
-}
-
-link!(Top, A);
-link!(A, B);
-link!(B, C);
-link!(C, D);
-link!(D, E);
-link!(E, F);
-link!(F, G);
-link!(G, H);
-link!(H, I);
-link!(I, J);
-link!(J, K);
-link!(K, Bottom);
-
-fn main() {
-    let t = Top::new();
-    let x: &Bottom = &t;
-    //~^ ERROR mismatched types
-    //~| ERROR reached the recursion limit while auto-dereferencing `J`
-}

tests/ui/autoref-autoderef/issue-38940.stderr

@@ -1,23 +0,0 @@
-error[E0055]: reached the recursion limit while auto-dereferencing `J`
-  --> $DIR/issue-38940.rs:49:22
-   |
-LL |     let x: &Bottom = &t;
-   |                      ^^ deref recursion limit reached
-   |
-   = help: consider increasing the recursion limit by adding a `#![recursion_limit = "20"]` attribute to your crate (`issue_38940`)
-
-error[E0308]: mismatched types
-  --> $DIR/issue-38940.rs:49:22
-   |
-LL |     let x: &Bottom = &t;
-   |            -------   ^^ expected `&Bottom`, found `&Top`
-   |            |
-   |            expected due to this
-   |
-   = note: expected reference `&Bottom`
-              found reference `&Top`
-
-error: aborting due to 2 previous errors
-
-Some errors have detailed explanations: E0055, E0308.
-For more information about an error, try `rustc --explain E0055`.

tests/ui/did_you_mean/recursion_limit_deref.stderr

@@ -1,3 +1,7 @@
+error: reached the recursion limit finding the struct tail for `Bottom`
+   |
+   = help: consider increasing the recursion limit by adding a `#![recursion_limit = "20"]`
+
 error[E0055]: reached the recursion limit while auto-dereferencing `J`
   --> $DIR/recursion_limit_deref.rs:51:22
    |
@@ -17,7 +21,7 @@ LL |     let x: &Bottom = &t;
    = note: expected reference `&Bottom`
               found reference `&Top`
 
-error: aborting due to 2 previous errors
+error: aborting due to 3 previous errors
 
 Some errors have detailed explanations: E0055, E0308.
 For more information about an error, try `rustc --explain E0055`.