delegation: emit error when self type is not specified and accessed

compiler/rustc_hir_analysis/src/delegation.rs

@@ -14,6 +14,7 @@ use rustc_middle::ty::{
 use rustc_span::{ErrorGuaranteed, Span, kw};
 
 use crate::collect::ItemCtxt;
+use crate::errors::DelegationSelfTypeNotSpecified;
 use crate::hir_ty_lowering::HirTyLowerer;
 
 type RemapTable = FxHashMap<u32, u32>;
@@ -284,6 +285,12 @@ fn get_delegation_self_ty_or_err(tcx: TyCtxt<'_>, delegation_id: LocalDefId) ->
             ctx.lower_ty(tcx.hir_node(id).expect_ty())
         })
         .unwrap_or_else(|| {
+            // It is possible to attempt to get self type when it is used in signature
+            // (i.e., `fn default() -> Self`), so emit error here in addition to possible
+            // `mismatched types` error (see #156388).
+            let err = DelegationSelfTypeNotSpecified { span: tcx.def_span(delegation_id) };
+            tcx.dcx().emit_err(err);
+
             Ty::new_error_with_message(
                 tcx,
                 tcx.def_span(delegation_id),

compiler/rustc_hir_analysis/src/errors.rs

@@ -1670,6 +1670,14 @@ pub(crate) struct UnsupportedDelegation<'a> {
     pub callee_span: Span,
 }
 
+#[derive(Diagnostic)]
+#[diag("delegation self type is not specified")]
+#[help("consider explicitly specifying self type: `reuse </* Type */ as Trait>::function`")]
+pub(crate) struct DelegationSelfTypeNotSpecified {
+    #[primary_span]
+    pub span: Span,
+}
+
 #[derive(Diagnostic)]
 #[diag("method should be `async` or return a future, but it is synchronous")]
 pub(crate) struct MethodShouldReturnFuture {

tests/ui/delegation/generics/free-to-trait-static-reuse.rs

@@ -1,3 +1,5 @@
+//@ compile-flags: -Z deduplicate-diagnostics=yes
+
 #![feature(fn_delegation)]
 
 trait Bound<T> {}
@@ -19,12 +21,16 @@ reuse <usize as Trait>::static_method::<'static, Vec<i32>, false> as bar2;
 
 reuse Trait::static_method as error { self - 123 }
 //~^ ERROR: type annotations needed
+//~| ERROR: delegation self type is not specified
 reuse Trait::<'static, i32, 123>::static_method as error2;
 //~^ ERROR: type annotations needed
+//~| ERROR: delegation self type is not specified
 reuse Trait::<'static, i32, 123>::static_method::<'static, String, false> as error3;
 //~^ ERROR: type annotations needed
+//~| ERROR: delegation self type is not specified
 reuse Trait::static_method::<'static, Vec<i32>, false> as error4 { self + 4 }
 //~^ ERROR: type annotations needed
+//~| ERROR: delegation self type is not specified
 
 reuse <String as Trait>::static_method as error5;
 //~^ ERROR: the trait bound `String: Trait<'a, T, X>` is not satisfied

tests/ui/delegation/generics/free-to-trait-static-reuse.stderr

@@ -1,21 +1,53 @@
+error: delegation self type is not specified
+  --> $DIR/free-to-trait-static-reuse.rs:22:14
+   |
+LL | reuse Trait::static_method as error { self - 123 }
+   |              ^^^^^^^^^^^^^
+   |
+   = help: consider explicitly specifying self type: `reuse </* Type */ as Trait>::function`
+
+error: delegation self type is not specified
+  --> $DIR/free-to-trait-static-reuse.rs:25:35
+   |
+LL | reuse Trait::<'static, i32, 123>::static_method as error2;
+   |                                   ^^^^^^^^^^^^^
+   |
+   = help: consider explicitly specifying self type: `reuse </* Type */ as Trait>::function`
+
+error: delegation self type is not specified
+  --> $DIR/free-to-trait-static-reuse.rs:28:35
+   |
+LL | reuse Trait::<'static, i32, 123>::static_method::<'static, String, false> as error3;
+   |                                   ^^^^^^^^^^^^^
+   |
+   = help: consider explicitly specifying self type: `reuse </* Type */ as Trait>::function`
+
+error: delegation self type is not specified
+  --> $DIR/free-to-trait-static-reuse.rs:31:14
+   |
+LL | reuse Trait::static_method::<'static, Vec<i32>, false> as error4 { self + 4 }
+   |              ^^^^^^^^^^^^^
+   |
+   = help: consider explicitly specifying self type: `reuse </* Type */ as Trait>::function`
+
 error[E0277]: the trait bound `Struct: Bound<T>` is not satisfied
-  --> $DIR/free-to-trait-static-reuse.rs:33:49
+  --> $DIR/free-to-trait-static-reuse.rs:39:49
    |
 LL | impl<'a, T, const X: usize> Trait<'a, T, X> for Struct {}
    |                                                 ^^^^^^ unsatisfied trait bound
    |
 help: the trait `Bound<T>` is not implemented for `Struct`
-  --> $DIR/free-to-trait-static-reuse.rs:32:1
+  --> $DIR/free-to-trait-static-reuse.rs:38:1
    |
 LL | struct Struct;
    | ^^^^^^^^^^^^^
 help: the trait `Bound<T>` is implemented for `usize`
-  --> $DIR/free-to-trait-static-reuse.rs:13:1
+  --> $DIR/free-to-trait-static-reuse.rs:15:1
    |
 LL | impl<T> Bound<T> for usize {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^
 note: required by a bound in `Trait`
-  --> $DIR/free-to-trait-static-reuse.rs:7:11
+  --> $DIR/free-to-trait-static-reuse.rs:9:11
    |
 LL | trait Trait<'a, T, const X: usize>
    |       ----- required by a bound in this trait
@@ -24,13 +56,13 @@ LL |     Self: Bound<T>,
    |           ^^^^^^^^ required by this bound in `Trait`
 
 error[E0283]: type annotations needed
-  --> $DIR/free-to-trait-static-reuse.rs:20:14
+  --> $DIR/free-to-trait-static-reuse.rs:22:14
    |
 LL | reuse Trait::static_method as error { self - 123 }
    |              ^^^^^^^^^^^^^ cannot infer type
    |
 note: multiple `impl`s satisfying `_: Trait<'a, T, X>` found
-  --> $DIR/free-to-trait-static-reuse.rs:12:1
+  --> $DIR/free-to-trait-static-reuse.rs:14:1
    |
 LL | impl<'a, T, const X: usize> Trait<'a, T, X> for usize {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -39,13 +71,13 @@ LL | impl<'a, T, const X: usize> Trait<'a, T, X> for Struct {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error[E0283]: type annotations needed
-  --> $DIR/free-to-trait-static-reuse.rs:22:35
+  --> $DIR/free-to-trait-static-reuse.rs:25:35
    |
 LL | reuse Trait::<'static, i32, 123>::static_method as error2;
    |                                   ^^^^^^^^^^^^^ cannot infer type
    |
 note: multiple `impl`s satisfying `_: Trait<'static, i32, 123>` found
-  --> $DIR/free-to-trait-static-reuse.rs:12:1
+  --> $DIR/free-to-trait-static-reuse.rs:14:1
    |
 LL | impl<'a, T, const X: usize> Trait<'a, T, X> for usize {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -54,13 +86,13 @@ LL | impl<'a, T, const X: usize> Trait<'a, T, X> for Struct {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error[E0283]: type annotations needed
-  --> $DIR/free-to-trait-static-reuse.rs:24:35
+  --> $DIR/free-to-trait-static-reuse.rs:28:35
    |
 LL | reuse Trait::<'static, i32, 123>::static_method::<'static, String, false> as error3;
    |                                   ^^^^^^^^^^^^^ cannot infer type
    |
 note: multiple `impl`s satisfying `_: Trait<'static, i32, 123>` found
-  --> $DIR/free-to-trait-static-reuse.rs:12:1
+  --> $DIR/free-to-trait-static-reuse.rs:14:1
    |
 LL | impl<'a, T, const X: usize> Trait<'a, T, X> for usize {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -69,13 +101,13 @@ LL | impl<'a, T, const X: usize> Trait<'a, T, X> for Struct {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error[E0283]: type annotations needed
-  --> $DIR/free-to-trait-static-reuse.rs:26:14
+  --> $DIR/free-to-trait-static-reuse.rs:31:14
    |
 LL | reuse Trait::static_method::<'static, Vec<i32>, false> as error4 { self + 4 }
    |              ^^^^^^^^^^^^^ cannot infer type
    |
 note: multiple `impl`s satisfying `_: Trait<'a, T, X>` found
-  --> $DIR/free-to-trait-static-reuse.rs:12:1
+  --> $DIR/free-to-trait-static-reuse.rs:14:1
    |
 LL | impl<'a, T, const X: usize> Trait<'a, T, X> for usize {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -84,13 +116,13 @@ LL | impl<'a, T, const X: usize> Trait<'a, T, X> for Struct {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error[E0277]: the trait bound `String: Trait<'a, T, X>` is not satisfied
-  --> $DIR/free-to-trait-static-reuse.rs:29:8
+  --> $DIR/free-to-trait-static-reuse.rs:35:8
    |
 LL | reuse <String as Trait>::static_method as error5;
    |        ^^^^^^ the trait `Trait<'a, T, X>` is not implemented for `String`
    |
 help: the following other types implement trait `Trait<'a, T, X>`
-  --> $DIR/free-to-trait-static-reuse.rs:12:1
+  --> $DIR/free-to-trait-static-reuse.rs:14:1
    |
 LL | impl<'a, T, const X: usize> Trait<'a, T, X> for usize {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `usize`
@@ -99,31 +131,31 @@ LL | impl<'a, T, const X: usize> Trait<'a, T, X> for Struct {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `Struct`
 
 error[E0277]: the trait bound `Struct: Bound<T>` is not satisfied
-  --> $DIR/free-to-trait-static-reuse.rs:36:8
+  --> $DIR/free-to-trait-static-reuse.rs:42:8
    |
 LL | reuse <Struct as Trait>::static_method as error6;
    |        ^^^^^^ unsatisfied trait bound
    |
 help: the trait `Bound<T>` is not implemented for `Struct`
-  --> $DIR/free-to-trait-static-reuse.rs:32:1
+  --> $DIR/free-to-trait-static-reuse.rs:38:1
    |
 LL | struct Struct;
    | ^^^^^^^^^^^^^
 help: the trait `Bound<T>` is implemented for `usize`
-  --> $DIR/free-to-trait-static-reuse.rs:13:1
+  --> $DIR/free-to-trait-static-reuse.rs:15:1
    |
 LL | impl<T> Bound<T> for usize {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^
 note: required by a bound in `Trait::static_method`
-  --> $DIR/free-to-trait-static-reuse.rs:7:11
+  --> $DIR/free-to-trait-static-reuse.rs:9:11
    |
 LL |     Self: Bound<T>,
    |           ^^^^^^^^ required by this bound in `Trait::static_method`
 LL | {
 LL |     fn static_method<'c: 'c, U, const B: bool>(x: usize) {}
    |        ------------- required by a bound in this associated function
 
-error: aborting due to 7 previous errors
+error: aborting due to 11 previous errors
 
 Some errors have detailed explanations: E0277, E0283.
 For more information about an error, try `rustc --explain E0277`.

tests/ui/delegation/self-ty-ice-156388.rs

@@ -0,0 +1,8 @@
+//@ compile-flags: -Z deduplicate-diagnostics=yes
+
+#![feature(fn_delegation)]
+
+reuse Default::default;
+//~^ ERROR: delegation self type is not specified
+
+fn main() {}

tests/ui/delegation/self-ty-ice-156388.stderr

@@ -0,0 +1,10 @@
+error: delegation self type is not specified
+  --> $DIR/self-ty-ice-156388.rs:5:16
+   |
+LL | reuse Default::default;
+   |                ^^^^^^^
+   |
+   = help: consider explicitly specifying self type: `reuse </* Type */ as Trait>::function`
+
+error: aborting due to 1 previous error
+

tests/ui/delegation/target-expr.rs

@@ -1,3 +1,5 @@
+//@ compile-flags: -Z deduplicate-diagnostics=yes
+
 #![feature(fn_delegation)]
 
 trait Trait {
@@ -14,6 +16,7 @@ fn foo(x: i32) -> i32 { x }
 fn bar<T: Default>(_: T) {
     reuse Trait::static_method {
     //~^ ERROR mismatched types
+    //~| ERROR: delegation self type is not specified
         let _ = T::Default();
         //~^ ERROR can't use generic parameters from outer item
     }

tests/ui/delegation/target-expr.stderr

@@ -1,26 +1,26 @@
 error[E0401]: can't use generic parameters from outer item
-  --> $DIR/target-expr.rs:17:17
+  --> $DIR/target-expr.rs:20:17
    |
 LL | fn bar<T: Default>(_: T) {
    |        - type parameter from outer item
 LL |     reuse Trait::static_method {
    |                  ------------- generic parameter used in this inner delegated function
-LL |
+...
 LL |         let _ = T::Default();
    |                 ^ use of generic parameter from outer item
    |
    = note: nested items are independent from their parent item for everything except for privacy and name resolution
 
 error[E0434]: can't capture dynamic environment in a fn item
-  --> $DIR/target-expr.rs:25:17
+  --> $DIR/target-expr.rs:28:17
    |
 LL |         let x = y;
    |                 ^
    |
    = help: use the `|| { ... }` closure form instead
 
 error[E0424]: expected value, found module `self`
-  --> $DIR/target-expr.rs:32:5
+  --> $DIR/target-expr.rs:35:5
    |
 LL | fn main() {
    |    ---- this function can't have a `self` parameter
@@ -29,29 +29,38 @@ LL |     self.0;
    |     ^^^^ `self` value is a keyword only available in methods with a `self` parameter
 
 error[E0425]: cannot find value `x` in this scope
-  --> $DIR/target-expr.rs:34:13
+  --> $DIR/target-expr.rs:37:13
    |
 LL |     let z = x;
    |             ^
    |
 help: the binding `x` is available in a different scope in the same function
-  --> $DIR/target-expr.rs:25:13
+  --> $DIR/target-expr.rs:28:13
    |
 LL |         let x = y;
    |             ^
 
+error: delegation self type is not specified
+  --> $DIR/target-expr.rs:17:18
+   |
+LL |     reuse Trait::static_method {
+   |                  ^^^^^^^^^^^^^
+   |
+   = help: consider explicitly specifying self type: `reuse </* Type */ as Trait>::function`
+
 error[E0308]: mismatched types
-  --> $DIR/target-expr.rs:15:32
+  --> $DIR/target-expr.rs:17:32
    |
 LL |       reuse Trait::static_method {
    |  ________________________________^
 LL | |
+LL | |
 LL | |         let _ = T::Default();
 LL | |
 LL | |     }
    | |_____^ expected `i32`, found `()`
 
-error: aborting due to 5 previous errors
+error: aborting due to 6 previous errors
 
 Some errors have detailed explanations: E0308, E0401, E0424, E0425, E0434.
 For more information about an error, try `rustc --explain E0308`.