Better recover static/const type in type_of

When the type is an inferred placeholder or an error, we used to infer
the type for an error message and then return `TyKind::Error`.
Now we return the proper type, which helps follow-up code do more
checking.

The primary motivation for this was fixing the const-eval mutability
assertion bug, because this commit forwards the inferred type
to that assertion, but it is also just nicer in general.

compiler/rustc_hir_analysis/src/collect/type_of.rs

@@ -571,8 +571,7 @@ fn infer_placeholder_type<'a>(
     // then the user may have written e.g. `const A = 42;`.
     // In this case, the parser has stashed a diagnostic for
     // us to improve in typeck so we do that now.
-    let guar = tcx
-        .dcx()
+    tcx.dcx()
         .try_steal_modify_and_emit_err(span, StashKey::ItemNoType, |err| {
             if !ty.references_error() {
                 // Only suggest adding `:` if it was missing (and suggested by parsing diagnostic).
@@ -619,7 +618,16 @@ fn infer_placeholder_type<'a>(
             }
             diag.emit()
         });
-    Ty::new_error(tcx, guar)
+
+    // Typeck returns regions as erased. We can't deal with erased regions here though, so we
+    // turn them into `&'static`, which is *generally* correct for statics and consts.
+    // Assoc consts can reference generic lifetimes from the parent generics, but treating them
+    // as static is unlikely to cause issues.
+    let ty = tcx.fold_regions(ty, |region, _| match region.kind() {
+        ty::ReErased => tcx.lifetimes.re_static,
+        _ => region,
+    });
+    ty
 }
 
 fn check_feature_inherent_assoc_ty(tcx: TyCtxt<'_>, span: Span) {

tests/ui/generic-const-items/assoc-const-missing-type.rs

@@ -13,6 +13,7 @@ impl Trait for () {
     //~^ ERROR missing type for `const` item
     //~| ERROR mismatched types
     //~| ERROR mismatched types
+    //~| ERROR implemented const `K` has an incompatible type for trait
     const Q = "";
     //~^ ERROR missing type for `const` item
     //~| ERROR lifetime parameters or bounds on const `Q` do not match the trait declaration

tests/ui/generic-const-items/assoc-const-missing-type.stderr

@@ -15,8 +15,24 @@ error: missing type for `const` item
 LL |     const K<T> = ();
    |               ^ help: provide a type for the associated constant: `()`
 
+error[E0326]: implemented const `K` has an incompatible type for trait
+  --> $DIR/assoc-const-missing-type.rs:12:15
+   |
+LL |     const K<T> = ();
+   |             - ^ expected type parameter `T`, found `()`
+   |             |
+   |             expected this type parameter
+   |
+note: type in trait
+  --> $DIR/assoc-const-missing-type.rs:7:17
+   |
+LL |     const K<T>: T;
+   |                 ^
+   = note: expected type parameter `T`
+                   found unit type `()`
+
 error[E0195]: lifetime parameters or bounds on const `Q` do not match the trait declaration
-  --> $DIR/assoc-const-missing-type.rs:16:12
+  --> $DIR/assoc-const-missing-type.rs:17:12
    |
 LL |     const Q<'a>: &'a str;
    |            ---- lifetimes in impl do not match this const in trait
@@ -25,7 +41,7 @@ LL |     const Q = "";
    |            ^ lifetimes do not match const in trait
 
 error: missing type for `const` item
-  --> $DIR/assoc-const-missing-type.rs:16:12
+  --> $DIR/assoc-const-missing-type.rs:17:12
    |
 LL |     const Q = "";
    |            ^ help: provide a type for the associated constant: `: &str`
@@ -42,7 +58,7 @@ LL |     const K<T> = ();
                    found unit type `()`
    = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
 
-error: aborting due to 5 previous errors
+error: aborting due to 6 previous errors
 
-Some errors have detailed explanations: E0195, E0308.
+Some errors have detailed explanations: E0195, E0308, E0326.
 For more information about an error, try `rustc --explain E0195`.

tests/ui/impl-trait/issues/issue-86642.rs

@@ -1,5 +1,7 @@
 static x: impl Fn(&str) -> Result<&str, ()> = move |source| {
-    //~^ `impl Trait` is not allowed in static types
+    //~^ ERROR `impl Trait` is not allowed in static types
+    //~| ERROR cycle detected when computing type of `x`
+    //~| ERROR the placeholder `_` is not allowed within types on item signatures for static variables
     let res = (move |source| Ok(source))(source);
     let res = res.or((move |source| Ok(source))(source));
     res

tests/ui/impl-trait/issues/issue-86642.stderr

@@ -6,6 +6,37 @@ LL | static x: impl Fn(&str) -> Result<&str, ()> = move |source| {
    |
    = note: `impl Trait` is only allowed in arguments and return types of functions and methods
 
-error: aborting due to 1 previous error
+error[E0391]: cycle detected when computing type of `x`
+  --> $DIR/issue-86642.rs:1:1
+   |
+LL | static x: impl Fn(&str) -> Result<&str, ()> = move |source| {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+note: ...which requires type-checking `x`...
+  --> $DIR/issue-86642.rs:1:1
+   |
+LL | static x: impl Fn(&str) -> Result<&str, ()> = move |source| {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+note: ...which requires computing the opaque types defined by `x`...
+  --> $DIR/issue-86642.rs:1:1
+   |
+LL | static x: impl Fn(&str) -> Result<&str, ()> = move |source| {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   = note: ...which again requires computing type of `x`, completing the cycle
+note: cycle used when checking that `x` is well-formed
+  --> $DIR/issue-86642.rs:1:1
+   |
+LL | static x: impl Fn(&str) -> Result<&str, ()> = move |source| {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   = note: see https://rustc-dev-guide.rust-lang.org/overview.html#queries and https://rustc-dev-guide.rust-lang.org/query.html for more information
+
+error[E0121]: the placeholder `_` is not allowed within types on item signatures for static variables
+  --> $DIR/issue-86642.rs:1:11
+   |
+LL | static x: impl Fn(&str) -> Result<&str, ()> = move |source| {
+   |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not allowed in type signatures
+
+error: aborting due to 3 previous errors
 
-For more information about this error, try `rustc --explain E0562`.
+Some errors have detailed explanations: E0121, E0391, E0562.
+For more information about an error, try `rustc --explain E0121`.

tests/ui/typeck/typeck_type_placeholder_item.stderr

@@ -583,7 +583,10 @@ error[E0121]: the placeholder `_` is not allowed within types on item signatures
   --> $DIR/typeck_type_placeholder_item.rs:209:14
    |
 LL |     const D: _ = 42;
-   |              ^ not allowed in type signatures
+   |              ^
+   |              |
+   |              not allowed in type signatures
+   |              help: replace with the correct type: `i32`
 
 error[E0046]: not all trait items implemented, missing: `F`
   --> $DIR/typeck_type_placeholder_item.rs:200:1