Structured suggestion for &mut dyn Iterator when possible

compiler/rustc_hir_typeck/src/method/mod.rs

@@ -57,7 +57,12 @@ pub enum MethodError<'tcx> {
     PrivateMatch(DefKind, DefId, Vec<DefId>),
 
     // Found a `Self: Sized` bound where `Self` is a trait object.
-    IllegalSizedBound(Vec<DefId>, bool, Span),
+    IllegalSizedBound {
+        candidates: Vec<DefId>,
+        needs_mut: bool,
+        bound_span: Span,
+        self_expr: &'tcx hir::Expr<'tcx>,
+    },
 
     // Found a match, but the return type is wrong
     BadReturnType,
@@ -112,7 +117,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             Err(NoMatch(..)) => false,
             Err(Ambiguity(..)) => true,
             Err(PrivateMatch(..)) => allow_private,
-            Err(IllegalSizedBound(..)) => true,
+            Err(IllegalSizedBound { .. }) => true,
             Err(BadReturnType) => bug!("no return type expectations but got BadReturnType"),
         }
     }
@@ -236,7 +241,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     _ => Vec::new(),
                 };
 
-            return Err(IllegalSizedBound(candidates, needs_mut, span));
+            return Err(IllegalSizedBound { candidates, needs_mut, bound_span: span, self_expr });
         }
 
         Ok(result.callee)

compiler/rustc_hir_typeck/src/method/suggest.rs

@@ -176,10 +176,18 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 err.emit();
             }
 
-            MethodError::IllegalSizedBound(candidates, needs_mut, bound_span) => {
-                let msg = format!("the `{}` method cannot be invoked on a trait object", item_name);
+            MethodError::IllegalSizedBound { candidates, needs_mut, bound_span, self_expr } => {
+                let msg = if needs_mut {
+                    with_forced_trimmed_paths!(format!(
+                        "the `{item_name}` method cannot be invoked on `{rcvr_ty}`"
+                    ))
+                } else {
+                    format!("the `{item_name}` method cannot be invoked on a trait object")
+                };
                 let mut err = self.sess().struct_span_err(span, &msg);
-                err.span_label(bound_span, "this has a `Sized` requirement");
+                if !needs_mut {
+                    err.span_label(bound_span, "this has a `Sized` requirement");
+                }
                 if !candidates.is_empty() {
                     let help = format!(
                         "{an}other candidate{s} {were} found in the following trait{s}, perhaps \
@@ -197,7 +205,32 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                             *region,
                             ty::TypeAndMut { ty: *t_type, mutbl: mutability.invert() },
                         );
-                        err.note(&format!("you need `{}` instead of `{}`", trait_type, rcvr_ty));
+                        let msg = format!("you need `{}` instead of `{}`", trait_type, rcvr_ty);
+                        let mut kind = &self_expr.kind;
+                        while let hir::ExprKind::AddrOf(_, _, expr)
+                        | hir::ExprKind::Unary(hir::UnOp::Deref, expr) = kind
+                        {
+                            kind = &expr.kind;
+                        }
+                        if let hir::ExprKind::Path(hir::QPath::Resolved(None, path)) = kind
+                            && let hir::def::Res::Local(hir_id) = path.res
+                            && let Some(hir::Node::Pat(b)) = self.tcx.hir().find(hir_id)
+                            && let Some(hir::Node::Param(p)) = self.tcx.hir().find_parent(b.hir_id)
+                            && let Some(node) = self.tcx.hir().find_parent(p.hir_id)
+                            && let Some(decl) = node.fn_decl()
+                            && let Some(ty) = decl.inputs.iter().find(|ty| ty.span == p.ty_span)
+                            && let hir::TyKind::Ref(_, mut_ty) = &ty.kind
+                            && let hir::Mutability::Not = mut_ty.mutbl
+                        {
+                            err.span_suggestion_verbose(
+                                mut_ty.ty.span.shrink_to_lo(),
+                                &msg,
+                                "mut ",
+                                Applicability::MachineApplicable,
+                            );
+                        } else {
+                            err.help(&msg);
+                        }
                     }
                 }
                 err.emit();

src/test/ui/illegal-sized-bound/mutability-mismatch-arg.fixed

@@ -0,0 +1,9 @@
+// run-rustfix
+fn test(t: &mut dyn Iterator<Item=&u64>) -> u64 {
+     *t.min().unwrap() //~ ERROR the `min` method cannot be invoked on
+}
+
+fn main() {
+     let array = [0u64];
+     test(&mut array.iter());
+}

src/test/ui/illegal-sized-bound/mutability-mismatch-arg.rs

@@ -0,0 +1,9 @@
+// run-rustfix
+fn test(t: &dyn Iterator<Item=&u64>) -> u64 {
+     *t.min().unwrap() //~ ERROR the `min` method cannot be invoked on
+}
+
+fn main() {
+     let array = [0u64];
+     test(&mut array.iter());
+}

src/test/ui/illegal-sized-bound/mutability-mismatch-arg.stderr

@@ -0,0 +1,13 @@
+error: the `min` method cannot be invoked on `&dyn Iterator<Item = &u64>`
+  --> $DIR/mutability-mismatch-arg.rs:3:9
+   |
+LL |      *t.min().unwrap()
+   |         ^^^
+   |
+help: you need `&mut dyn Iterator<Item = &u64>` instead of `&dyn Iterator<Item = &u64>`
+   |
+LL | fn test(t: &mut dyn Iterator<Item=&u64>) -> u64 {
+   |             +++
+
+error: aborting due to previous error
+

src/test/ui/illegal-sized-bound/mutability-mismatch.rs

@@ -4,7 +4,6 @@ pub trait MutTrait {
     fn function(&mut self)
     where
         Self: Sized;
-    //~^ this has a `Sized` requirement
 }
 
 impl MutTrait for MutType {
@@ -17,7 +16,6 @@ pub trait Trait {
     fn function(&self)
     where
         Self: Sized;
-    //~^ this has a `Sized` requirement
 }
 
 impl Trait for Type {
@@ -26,9 +24,9 @@ impl Trait for Type {
 
 fn main() {
     (&MutType as &dyn MutTrait).function();
-    //~^ ERROR the `function` method cannot be invoked on a trait object
-    //~| NOTE you need `&mut dyn MutTrait` instead of `&dyn MutTrait`
+    //~^ ERROR the `function` method cannot be invoked on `&dyn MutTrait`
+    //~| HELP you need `&mut dyn MutTrait` instead of `&dyn MutTrait`
     (&mut Type as &mut dyn Trait).function();
-    //~^ ERROR the `function` method cannot be invoked on a trait object
-    //~| NOTE you need `&dyn Trait` instead of `&mut dyn Trait`
+    //~^ ERROR the `function` method cannot be invoked on `&mut dyn Trait`
+    //~| HELP you need `&dyn Trait` instead of `&mut dyn Trait`
 }

src/test/ui/illegal-sized-bound/mutability-mismatch.stderr

@@ -1,24 +1,18 @@
-error: the `function` method cannot be invoked on a trait object
-  --> $DIR/mutability-mismatch.rs:28:33
+error: the `function` method cannot be invoked on `&dyn MutTrait`
+  --> $DIR/mutability-mismatch.rs:26:33
    |
-LL |         Self: Sized;
-   |               ----- this has a `Sized` requirement
-...
 LL |     (&MutType as &dyn MutTrait).function();
    |                                 ^^^^^^^^
    |
-   = note: you need `&mut dyn MutTrait` instead of `&dyn MutTrait`
+   = help: you need `&mut dyn MutTrait` instead of `&dyn MutTrait`
 
-error: the `function` method cannot be invoked on a trait object
-  --> $DIR/mutability-mismatch.rs:31:35
+error: the `function` method cannot be invoked on `&mut dyn Trait`
+  --> $DIR/mutability-mismatch.rs:29:35
    |
-LL |         Self: Sized;
-   |               ----- this has a `Sized` requirement
-...
 LL |     (&mut Type as &mut dyn Trait).function();
    |                                   ^^^^^^^^
    |
-   = note: you need `&dyn Trait` instead of `&mut dyn Trait`
+   = help: you need `&dyn Trait` instead of `&mut dyn Trait`
 
 error: aborting due to 2 previous errors
 

src/test/ui/suggestions/imm-ref-trait-object.rs

@@ -1,5 +1,5 @@
 fn test(t: &dyn Iterator<Item=&u64>) -> u64 {
-     t.min().unwrap() //~ ERROR the `min` method cannot be invoked on a trait object
+     t.min().unwrap() //~ ERROR the `min` method cannot be invoked on `&dyn Iterator<Item = &u64>`
 }
 
 fn main() {

src/test/ui/suggestions/imm-ref-trait-object.stderr

@@ -1,13 +1,13 @@
-error: the `min` method cannot be invoked on a trait object
+error: the `min` method cannot be invoked on `&dyn Iterator<Item = &u64>`
   --> $DIR/imm-ref-trait-object.rs:2:8
    |
 LL |      t.min().unwrap()
    |        ^^^
-  --> $SRC_DIR/core/src/iter/traits/iterator.rs:LL:COL
    |
-   = note: this has a `Sized` requirement
+help: you need `&mut dyn Iterator<Item = &u64>` instead of `&dyn Iterator<Item = &u64>`
    |
-   = note: you need `&mut dyn Iterator<Item = &u64>` instead of `&dyn Iterator<Item = &u64>`
+LL | fn test(t: &mut dyn Iterator<Item=&u64>) -> u64 {
+   |             +++
 
 error: aborting due to previous error