Tweak output for bare dyn Trait in arguments

Fix #35825.

compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs

@@ -1618,7 +1618,7 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
         let trait_obj = if has_dyn { &snippet[4..] } else { &snippet };
         if only_never_return {
             // No return paths, probably using `panic!()` or similar.
-            // Suggest `-> T`, `-> impl Trait`, and if `Trait` is object safe, `-> Box<dyn Trait>`.
+            // Suggest `-> impl Trait`, and if `Trait` is object safe, `-> Box<dyn Trait>`.
             suggest_trait_object_return_type_alternatives(
                 err,
                 ret_ty.span,
@@ -2540,6 +2540,25 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
             }
             ObligationCauseCode::SizedArgumentType(sp) => {
                 if let Some(span) = sp {
+                    if let ty::PredicateKind::Clause(clause) = predicate.kind().skip_binder()
+                        && let ty::Clause::Trait(trait_pred) = clause
+                        && let ty::Dynamic(..) = trait_pred.self_ty().kind()
+                    {
+                        let span = if let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(span)
+                            && snippet.starts_with("dyn ")
+                        {
+                            let pos = snippet.len() - snippet[3..].trim_start().len();
+                            span.with_hi(span.lo() + BytePos(pos as u32))
+                        } else {
+                            span.shrink_to_lo()
+                        };
+                        err.span_suggestion_verbose(
+                            span,
+                            "you can use `impl Trait` as the argument type",
+                            "impl ".to_string(),
+                            Applicability::MaybeIncorrect,
+                        );
+                    }
                     err.span_suggestion_verbose(
                         span.shrink_to_lo(),
                         "function arguments must have a statically known size, borrowed types \
@@ -3580,13 +3599,6 @@ fn suggest_trait_object_return_type_alternatives(
     trait_obj: &str,
     is_object_safe: bool,
 ) {
-    err.span_suggestion(
-        ret_ty,
-        "use some type `T` that is `T: Sized` as the return type if all return paths have the \
-            same type",
-        "T",
-        Applicability::MaybeIncorrect,
-    );
     err.span_suggestion(
         ret_ty,
         &format!(

src/test/ui/feature-gates/feature-gate-unsized_fn_params.rs

@@ -1,5 +1,5 @@
+#![allow(unused, bare_trait_objects)]
 #[repr(align(256))]
-#[allow(dead_code)]
 struct A {
     v: u8,
 }
@@ -14,13 +14,17 @@ impl Foo for A {
     }
 }
 
-fn foo(x: dyn Foo) {
+fn foo(x: dyn Foo) { //~ ERROR [E0277]
-    //~^ ERROR [E0277]
     x.foo()
 }
 
+fn bar(x: Foo) { //~ ERROR [E0277]
+    x.foo()
+}
+
+fn qux(_: [()]) {} //~ ERROR [E0277]
+
 fn main() {
     let x: Box<dyn Foo> = Box::new(A { v: 22 });
-    foo(*x);
+    foo(*x); //~ ERROR [E0277]
-    //~^ ERROR [E0277]
 }

src/test/ui/feature-gates/feature-gate-unsized_fn_params.stderr

@@ -6,13 +6,47 @@ LL | fn foo(x: dyn Foo) {
    |
    = help: the trait `Sized` is not implemented for `(dyn Foo + 'static)`
    = help: unsized fn params are gated as an unstable feature
+help: you can use `impl Trait` as the argument type
+   |
+LL | fn foo(x: impl Foo) {
+   |           ~~~~
 help: function arguments must have a statically known size, borrowed types always have a known size
    |
 LL | fn foo(x: &dyn Foo) {
    |           +
 
 error[E0277]: the size for values of type `(dyn Foo + 'static)` cannot be known at compilation time
-  --> $DIR/feature-gate-unsized_fn_params.rs:24:9
+  --> $DIR/feature-gate-unsized_fn_params.rs:21:8
+   |
+LL | fn bar(x: Foo) {
+   |        ^ doesn't have a size known at compile-time
+   |
+   = help: the trait `Sized` is not implemented for `(dyn Foo + 'static)`
+   = help: unsized fn params are gated as an unstable feature
+help: you can use `impl Trait` as the argument type
+   |
+LL | fn bar(x: impl Foo) {
+   |           ++++
+help: function arguments must have a statically known size, borrowed types always have a known size
+   |
+LL | fn bar(x: &Foo) {
+   |           +
+
+error[E0277]: the size for values of type `[()]` cannot be known at compilation time
+  --> $DIR/feature-gate-unsized_fn_params.rs:25:8
+   |
+LL | fn qux(_: [()]) {}
+   |        ^ doesn't have a size known at compile-time
+   |
+   = help: the trait `Sized` is not implemented for `[()]`
+   = help: unsized fn params are gated as an unstable feature
+help: function arguments must have a statically known size, borrowed types always have a known size
+   |
+LL | fn qux(_: &[()]) {}
+   |           +
+
+error[E0277]: the size for values of type `(dyn Foo + 'static)` cannot be known at compilation time
+  --> $DIR/feature-gate-unsized_fn_params.rs:29:9
    |
 LL |     foo(*x);
    |         ^^ doesn't have a size known at compile-time
@@ -21,6 +55,6 @@ LL |     foo(*x);
    = note: all function arguments must have a statically known size
    = help: unsized fn params are gated as an unstable feature
 
-error: aborting due to 2 previous errors
+error: aborting due to 4 previous errors
 
 For more information about this error, try `rustc --explain E0277`.

src/test/ui/feature-gates/feature-gate-unsized_locals.stderr

@@ -6,6 +6,10 @@ LL | fn f(f: dyn FnOnce()) {}
    |
    = help: the trait `Sized` is not implemented for `(dyn FnOnce() + 'static)`
    = help: unsized fn params are gated as an unstable feature
+help: you can use `impl Trait` as the argument type
+   |
+LL | fn f(f: impl FnOnce()) {}
+   |         ~~~~
 help: function arguments must have a statically known size, borrowed types always have a known size
    |
 LL | fn f(f: &dyn FnOnce()) {}

src/test/ui/impl-trait/dyn-trait-return-should-be-impl-trait.stderr

@@ -70,10 +70,6 @@ error[E0746]: return type cannot have an unboxed trait object
 LL | fn bak() -> dyn Trait { unimplemented!() }
    |             ^^^^^^^^^ doesn't have a size known at compile-time
    |
-help: use some type `T` that is `T: Sized` as the return type if all return paths have the same type
-   |
-LL | fn bak() -> T { unimplemented!() }
-   |             ~
 help: use `impl Trait` as the return type if all return paths have the same type but you want to expose only the trait in the signature
    |
 LL | fn bak() -> impl Trait { unimplemented!() }

src/test/ui/issues/issue-18107.stderr

@@ -4,10 +4,6 @@ error[E0746]: return type cannot have an unboxed trait object
 LL |     dyn AbstractRenderer
    |     ^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
    |
-help: use some type `T` that is `T: Sized` as the return type if all return paths have the same type
-   |
-LL |     T
-   |
 help: use `impl AbstractRenderer` as the return type if all return paths have the same type but you want to expose only the trait in the signature
    |
 LL |     impl AbstractRenderer

src/test/ui/issues/issue-42312.stderr

@@ -23,6 +23,10 @@ LL | pub fn f(_: dyn ToString) {}
    |
    = help: the trait `Sized` is not implemented for `(dyn ToString + 'static)`
    = help: unsized fn params are gated as an unstable feature
+help: you can use `impl Trait` as the argument type
+   |
+LL | pub fn f(_: impl ToString) {}
+   |             ~~~~
 help: function arguments must have a statically known size, borrowed types always have a known size
    |
 LL | pub fn f(_: &dyn ToString) {}

src/test/ui/issues/issue-5883.stderr

@@ -6,6 +6,10 @@ LL |     r: dyn A + 'static
    |
    = help: the trait `Sized` is not implemented for `(dyn A + 'static)`
    = help: unsized fn params are gated as an unstable feature
+help: you can use `impl Trait` as the argument type
+   |
+LL |     r: impl A + 'static
+   |        ~~~~
 help: function arguments must have a statically known size, borrowed types always have a known size
    |
 LL |     r: &dyn A + 'static

src/test/ui/resolve/issue-5035-2.stderr

@@ -6,6 +6,10 @@ LL | fn foo(_x: K) {}
    |
    = help: the trait `Sized` is not implemented for `(dyn I + 'static)`
    = help: unsized fn params are gated as an unstable feature
+help: you can use `impl Trait` as the argument type
+   |
+LL | fn foo(_x: impl K) {}
+   |            ++++
 help: function arguments must have a statically known size, borrowed types always have a known size
    |
 LL | fn foo(_x: &K) {}

src/test/ui/traits/bound/not-on-bare-trait.stderr

@@ -20,6 +20,10 @@ LL | fn foo(_x: Foo + Send) {
    |
    = help: the trait `Sized` is not implemented for `(dyn Foo + Send + 'static)`
    = help: unsized fn params are gated as an unstable feature
+help: you can use `impl Trait` as the argument type
+   |
+LL | fn foo(_x: impl Foo + Send) {
+   |            ++++
 help: function arguments must have a statically known size, borrowed types always have a known size
    |
 LL | fn foo(_x: &Foo + Send) {