diagnostics: do not suggest type name tweaks on type-inferred closure args

Fixes #111932

Author: notriddle

compiler/rustc_hir_typeck/src/check.rs

@@ -96,7 +96,19 @@ pub(super) fn check_fn<'a, 'tcx>(
         // for simple cases like `fn foo(x: Trait)`,
         // where we would error once on the parameter as a whole, and once on the binding `x`.
         if param.pat.simple_ident().is_none() && !params_can_be_unsized {
-            fcx.require_type_is_sized(param_ty, param.pat.span, traits::SizedArgumentType(ty_span));
+            fcx.require_type_is_sized(
+                param_ty,
+                param.pat.span,
+                // ty_span == binding_span iff this is a closure parameter with no type ascription,
+                // or if it's an implicit `self` parameter
+                traits::SizedArgumentType(
+                    if ty_span == Some(param.span) && tcx.is_closure(fn_def_id.into()) {
+                        None
+                    } else {
+                        ty_span
+                    },
+                ),
+            );
         }
 
         fcx.write_ty(param.hir_id, param_ty);

compiler/rustc_hir_typeck/src/gather_locals.rs

@@ -129,7 +129,17 @@ impl<'a, 'tcx> Visitor<'tcx> for GatherLocalsVisitor<'a, 'tcx> {
                     self.fcx.require_type_is_sized(
                         var_ty,
                         p.span,
-                        traits::SizedArgumentType(Some(ty_span)),
+                        // ty_span == ident.span iff this is a closure parameter with no type
+                        // ascription, or if it's an implicit `self` parameter
+                        traits::SizedArgumentType(
+                            if ty_span == ident.span
+                                && self.fcx.tcx.is_closure(self.fcx.body_id.into())
+                            {
+                                None
+                            } else {
+                                Some(ty_span)
+                            },
+                        ),
                     );
                 }
             } else {

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

@@ -2807,8 +2807,8 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                     err.help("unsized locals are gated as an unstable feature");
                 }
             }
-            ObligationCauseCode::SizedArgumentType(sp) => {
-                if let Some(span) = sp {
+            ObligationCauseCode::SizedArgumentType(ty_span) => {
+                if let Some(span) = ty_span {
                     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()

tests/ui/closures/issue-111932.rs

@@ -0,0 +1,9 @@
+trait Foo: std::fmt::Debug {}
+
+fn print_foos(foos: impl Iterator<Item = dyn Foo>) {
+    foos.for_each(|foo| { //~ ERROR [E0277]
+        println!("{:?}", foo); //~ ERROR [E0277]
+    });
+}
+
+fn main() {}

tests/ui/closures/issue-111932.stderr

@@ -0,0 +1,26 @@
+error[E0277]: the size for values of type `(dyn Foo + 'static)` cannot be known at compilation time
+  --> $DIR/issue-111932.rs:4:20
+   |
+LL |     foos.for_each(|foo| {
+   |                    ^^^ doesn't have a size known at compile-time
+   |
+   = help: the trait `Sized` is not implemented for `(dyn Foo + 'static)`
+   = note: all function arguments must have a statically known size
+   = help: unsized fn params are gated as an unstable feature
+
+error[E0277]: the size for values of type `dyn Foo` cannot be known at compilation time
+  --> $DIR/issue-111932.rs:5:26
+   |
+LL |         println!("{:?}", foo);
+   |                   ----   ^^^ doesn't have a size known at compile-time
+   |                   |
+   |                   required by a bound introduced by this call
+   |
+   = help: the trait `Sized` is not implemented for `dyn Foo`
+note: required by a bound in `core::fmt::rt::Argument::<'a>::new_debug`
+  --> $SRC_DIR/core/src/fmt/rt.rs:LL:COL
+   = note: this error originates in the macro `$crate::format_args_nl` which comes from the expansion of the macro `println` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0277`.

tests/ui/unsized-locals/issue-67981.stderr

@@ -5,10 +5,7 @@ LL |     let f: fn([u8]) = |_| {};
    |                        ^ doesn't have a size known at compile-time
    |
    = help: the trait `Sized` is not implemented for `[u8]`
-help: function arguments must have a statically known size, borrowed types always have a known size
-   |
-LL |     let f: fn([u8]) = |&_| {};
-   |                        +
+   = note: all function arguments must have a statically known size
 
 error: aborting due to previous error