Improve errors for recursive type aliases

Author: camelid

compiler/rustc_middle/src/query/mod.rs

@@ -116,7 +116,18 @@ rustc_queries! {
 
     /// Records the type of every item.
     query type_of(key: DefId) -> Ty<'tcx> {
-        desc { |tcx| "computing type of `{}`", tcx.def_path_str(key) }
+        desc { |tcx|
+            "{action} `{path}`",
+            action = {
+                use rustc_hir::def::DefKind;
+                match tcx.def_kind(key) {
+                    DefKind::TyAlias => "expanding type alias",
+                    DefKind::TraitAlias => "expanding trait alias",
+                    _ => "computing type of",
+                }
+            },
+            path = tcx.def_path_str(key),
+        }
         cache_on_disk_if { key.is_local() }
     }
 

compiler/rustc_query_system/src/query/job.rs

@@ -591,10 +591,14 @@ pub(crate) fn report_cycle<'a>(
         err.span_note(span, &format!("...which requires {}...", query.description));
     }
 
-    err.note(&format!(
-        "...which again requires {}, completing the cycle",
-        stack[0].query.description
-    ));
+    if stack.len() == 1 {
+        err.note(&format!("...which immediately requires {} again", stack[0].query.description));
+    } else {
+        err.note(&format!(
+            "...which again requires {}, completing the cycle",
+            stack[0].query.description
+        ));
+    }
 
     if let Some((span, query)) = usage {
         err.span_note(fix_span(span, &query), &format!("cycle used when {}", query.description));

src/test/ui/associated-type-bounds/ambiguous-associated-type2.stderr

@@ -4,7 +4,7 @@ error[E0391]: cycle detected when computing the super traits of `Baz` with assoc
 LL | trait Baz: Foo + Bar<Self::Item> {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
-   = note: ...which again requires computing the super traits of `Baz` with associated type name `Item`, completing the cycle
+   = note: ...which immediately requires computing the super traits of `Baz` with associated type name `Item` again
 note: cycle used when computing the super traits of `Baz`
   --> $DIR/ambiguous-associated-type2.rs:7:1
    |

src/test/ui/coherence/coherence-inherited-assoc-ty-cycle-err.stderr

@@ -14,7 +14,7 @@ error[E0391]: cycle detected when building specialization graph of trait `Trait`
 LL | trait Trait<T> { type Assoc; }
    | ^^^^^^^^^^^^^^
    |
-   = note: ...which again requires building specialization graph of trait `Trait`, completing the cycle
+   = note: ...which immediately requires building specialization graph of trait `Trait` again
 note: cycle used when coherence checking all impls of trait `Trait`
   --> $DIR/coherence-inherited-assoc-ty-cycle-err.rs:9:1
    |

src/test/ui/cycle-trait/cycle-trait-default-type-trait.stderr

@@ -4,7 +4,7 @@ error[E0391]: cycle detected when computing type of `Foo::X`
 LL | trait Foo<X = Box<dyn Foo>> {
    |                       ^^^
    |
-   = note: ...which again requires computing type of `Foo::X`, completing the cycle
+   = note: ...which immediately requires computing type of `Foo::X` again
 note: cycle used when collecting item types in top-level module
   --> $DIR/cycle-trait-default-type-trait.rs:4:1
    |
@@ -17,7 +17,7 @@ error[E0391]: cycle detected when computing type of `Foo::X`
 LL | trait Foo<X = Box<dyn Foo>> {
    |                       ^^^
    |
-   = note: ...which again requires computing type of `Foo::X`, completing the cycle
+   = note: ...which immediately requires computing type of `Foo::X` again
 note: cycle used when collecting item types in top-level module
   --> $DIR/cycle-trait-default-type-trait.rs:4:1
    |

src/test/ui/infinite/infinite-struct.stderr

@@ -18,7 +18,7 @@ error[E0391]: cycle detected when computing drop-check constraints for `Take`
 LL | struct Take(Take);
    | ^^^^^^^^^^^^^^^^^^
    |
-   = note: ...which again requires computing drop-check constraints for `Take`, completing the cycle
+   = note: ...which immediately requires computing drop-check constraints for `Take` again
    = note: cycle used when computing dropck types for `Canonical { max_universe: U0, variables: [], value: ParamEnvAnd { param_env: ParamEnv { caller_bounds: [], reveal: UserFacing }, value: Take } }`
 
 error: aborting due to 2 previous errors

src/test/ui/infinite/infinite-tag-type-recursion.stderr

@@ -17,7 +17,7 @@ error[E0391]: cycle detected when computing drop-check constraints for `MList`
 LL | enum MList { Cons(isize, MList), Nil }
    | ^^^^^^^^^^
    |
-   = note: ...which again requires computing drop-check constraints for `MList`, completing the cycle
+   = note: ...which immediately requires computing drop-check constraints for `MList` again
    = note: cycle used when computing dropck types for `Canonical { max_universe: U0, variables: [], value: ParamEnvAnd { param_env: ParamEnv { caller_bounds: [], reveal: UserFacing }, value: MList } }`
 
 error: aborting due to 2 previous errors

src/test/ui/infinite/infinite-vec-type-recursion.stderr

@@ -1,10 +1,10 @@
-error[E0391]: cycle detected when computing type of `X`
+error[E0391]: cycle detected when expanding type alias `X`
   --> $DIR/infinite-vec-type-recursion.rs:1:14
    |
 LL | type X = Vec<X>;
    |              ^
    |
-   = note: ...which again requires computing type of `X`, completing the cycle
+   = note: ...which immediately requires expanding type alias `X` again
 note: cycle used when collecting item types in top-level module
   --> $DIR/infinite-vec-type-recursion.rs:1:1
    |

src/test/ui/issues/issue-20772.stderr

@@ -6,7 +6,7 @@ LL | |
 LL | | {}
    | |__^
    |
-   = note: ...which again requires computing the super traits of `T` with associated type name `Item`, completing the cycle
+   = note: ...which immediately requires computing the super traits of `T` with associated type name `Item` again
 note: cycle used when computing the super traits of `T`
   --> $DIR/issue-20772.rs:1:1
    |

src/test/ui/issues/issue-20825.stderr

@@ -4,7 +4,7 @@ error[E0391]: cycle detected when computing the super traits of `Processor` with
 LL | pub trait Processor: Subscriber<Input = Self::Input> {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
-   = note: ...which again requires computing the super traits of `Processor` with associated type name `Input`, completing the cycle
+   = note: ...which immediately requires computing the super traits of `Processor` with associated type name `Input` again
 note: cycle used when computing the super traits of `Processor`
   --> $DIR/issue-20825.rs:5:1
    |