suggest iter_mut() where trying to modify elements from .iter()

compiler/rustc_borrowck/src/diagnostics/mutability_errors.rs

@@ -1,9 +1,10 @@
+use hir::ExprKind;
 use rustc_errors::{Applicability, Diagnostic, DiagnosticBuilder, ErrorGuaranteed};
 use rustc_hir as hir;
 use rustc_hir::intravisit::Visitor;
 use rustc_hir::Node;
 use rustc_middle::mir::{Mutability, Place, PlaceRef, ProjectionElem};
-use rustc_middle::ty::{self, Ty, TyCtxt};
+use rustc_middle::ty::{self, InstanceDef, Ty, TyCtxt};
 use rustc_middle::{
     hir::place::PlaceBase,
     mir::{self, BindingForm, Local, LocalDecl, LocalInfo, LocalKind, Location},
@@ -491,6 +492,7 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
                             ),
                         );
 
+                        self.suggest_using_iter_mut(&mut err);
                         self.suggest_make_local_mut(&mut err, local, name);
                     }
                     _ => {
@@ -953,6 +955,44 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
         }
     }
 
+    fn suggest_using_iter_mut(&self, err: &mut DiagnosticBuilder<'_, ErrorGuaranteed>) {
+        let source = self.body.source;
+        let hir = self.infcx.tcx.hir();
+        if let InstanceDef::Item(def_id) = source.instance
+            && let Some(Node::Expr(hir::Expr { hir_id, kind, ..})) = hir.get_if_local(def_id)
+            && let ExprKind::Closure(closure) = kind && closure.movability == None
+            && let Some(Node::Expr(expr)) = hir.find_parent(*hir_id) {
+                let mut cur_expr = expr;
+                while let ExprKind::MethodCall(path_segment, recv, _, _) = cur_expr.kind {
+                    if path_segment.ident.name == sym::iter {
+                        // check `_ty` has `iter_mut` method
+                        let res = self
+                            .infcx
+                            .tcx
+                            .typeck(path_segment.hir_id.owner.def_id)
+                            .type_dependent_def_id(cur_expr.hir_id)
+                            .and_then(|def_id| self.infcx.tcx.impl_of_method(def_id))
+                            .map(|def_id| self.infcx.tcx.associated_items(def_id))
+                            .map(|assoc_items| {
+                                assoc_items.filter_by_name_unhygienic(sym::iter_mut).peekable()
+                            });
+
+                        if let Some(mut res) = res && res.peek().is_some() {
+                            err.span_suggestion_verbose(
+                                path_segment.ident.span,
+                                "you may want to use `iter_mut` here",
+                                "iter_mut",
+                                Applicability::MaybeIncorrect,
+                            );
+                        }
+                        break;
+                    } else {
+                        cur_expr = recv;
+                    }
+                }
+            }
+    }
+
     fn suggest_make_local_mut(
         &self,
         err: &mut DiagnosticBuilder<'_, ErrorGuaranteed>,

tests/ui/borrowck/issue-115259-suggest-iter-mut.fixed

@@ -0,0 +1,20 @@
+// run-rustfix
+#![allow(unused_mut)]
+#![allow(dead_code)]
+
+pub trait Layer {
+    fn process(&mut self) -> u32;
+}
+
+pub struct State {
+    layers: Vec<Box<dyn Layer>>,
+}
+
+impl State {
+    pub fn process(&mut self) -> u32 {
+        self.layers.iter_mut().fold(0, |result, mut layer| result + layer.process())
+        //~^ ERROR cannot borrow `**layer` as mutable, as it is behind a `&` reference
+    }
+}
+
+fn main() {}

tests/ui/borrowck/issue-115259-suggest-iter-mut.rs

@@ -0,0 +1,20 @@
+// run-rustfix
+#![allow(unused_mut)]
+#![allow(dead_code)]
+
+pub trait Layer {
+    fn process(&mut self) -> u32;
+}
+
+pub struct State {
+    layers: Vec<Box<dyn Layer>>,
+}
+
+impl State {
+    pub fn process(&mut self) -> u32 {
+        self.layers.iter().fold(0, |result, mut layer| result + layer.process())
+        //~^ ERROR cannot borrow `**layer` as mutable, as it is behind a `&` reference
+    }
+}
+
+fn main() {}

tests/ui/borrowck/issue-115259-suggest-iter-mut.stderr

@@ -0,0 +1,16 @@
+error[E0596]: cannot borrow `**layer` as mutable, as it is behind a `&` reference
+  --> $DIR/issue-115259-suggest-iter-mut.rs:15:65
+   |
+LL |         self.layers.iter().fold(0, |result, mut layer| result + layer.process())
+   |                                             ---------           ^^^^^ `layer` is a `&` reference, so the data it refers to cannot be borrowed as mutable
+   |                                             |
+   |                                             consider changing this binding's type to be: `&mut Box<dyn Layer>`
+   |
+help: you may want to use `iter_mut` here
+   |
+LL |         self.layers.iter_mut().fold(0, |result, mut layer| result + layer.process())
+   |                     ~~~~~~~~
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0596`.

tests/ui/borrowck/issue-62387-suggest-iter-mut-2.fixed

@@ -0,0 +1,36 @@
+// run-rustfix
+#![allow(unused_mut)]
+#![allow(dead_code)]
+use std::path::PathBuf;
+
+#[derive(Clone)]
+struct Container {
+    things: Vec<PathBuf>,
+}
+
+impl Container {
+    fn things(&mut self) -> &[PathBuf] {
+        &self.things
+    }
+}
+
+// contains containers
+struct ContainerContainer {
+    contained: Vec<Container>,
+}
+
+impl ContainerContainer {
+    fn contained(&self) -> &[Container] {
+        &self.contained
+    }
+
+    fn all_the_things(&mut self) -> &[PathBuf] {
+        let mut vec = self.contained.clone();
+        let _a =
+            vec.iter_mut().flat_map(|container| container.things()).cloned().collect::<Vec<PathBuf>>();
+        //~^ ERROR cannot borrow `*container` as mutable, as it is behind a `&` reference
+        unimplemented!();
+    }
+}
+
+fn main() {}

tests/ui/borrowck/issue-62387-suggest-iter-mut-2.rs

@@ -0,0 +1,36 @@
+// run-rustfix
+#![allow(unused_mut)]
+#![allow(dead_code)]
+use std::path::PathBuf;
+
+#[derive(Clone)]
+struct Container {
+    things: Vec<PathBuf>,
+}
+
+impl Container {
+    fn things(&mut self) -> &[PathBuf] {
+        &self.things
+    }
+}
+
+// contains containers
+struct ContainerContainer {
+    contained: Vec<Container>,
+}
+
+impl ContainerContainer {
+    fn contained(&self) -> &[Container] {
+        &self.contained
+    }
+
+    fn all_the_things(&mut self) -> &[PathBuf] {
+        let mut vec = self.contained.clone();
+        let _a =
+            vec.iter().flat_map(|container| container.things()).cloned().collect::<Vec<PathBuf>>();
+        //~^ ERROR cannot borrow `*container` as mutable, as it is behind a `&` reference
+        unimplemented!();
+    }
+}
+
+fn main() {}

tests/ui/borrowck/issue-62387-suggest-iter-mut-2.stderr

@@ -0,0 +1,16 @@
+error[E0596]: cannot borrow `*container` as mutable, as it is behind a `&` reference
+  --> $DIR/issue-62387-suggest-iter-mut-2.rs:30:45
+   |
+LL |             vec.iter().flat_map(|container| container.things()).cloned().collect::<Vec<PathBuf>>();
+   |                                  ---------  ^^^^^^^^^ `container` is a `&` reference, so the data it refers to cannot be borrowed as mutable
+   |                                  |
+   |                                  consider changing this binding's type to be: `&mut Container`
+   |
+help: you may want to use `iter_mut` here
+   |
+LL |             vec.iter_mut().flat_map(|container| container.things()).cloned().collect::<Vec<PathBuf>>();
+   |                 ~~~~~~~~
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0596`.

tests/ui/borrowck/issue-62387-suggest-iter-mut.fixed

@@ -0,0 +1,30 @@
+// run-rustfix
+#![allow(unused_mut)]
+#![allow(dead_code)]
+
+#[derive(Debug)]
+struct A {
+    a: i32,
+}
+
+impl A {
+    fn double(&mut self) {
+        self.a += self.a
+    }
+}
+
+fn baz() {
+    let mut v = [A { a: 4 }];
+    v.iter_mut().for_each(|a| a.double());
+    //~^ ERROR cannot borrow `*a` as mutable, as it is behind a `&` reference
+    println!("{:?}", v);
+}
+
+fn bar() {
+    let mut v = [A { a: 4 }];
+    v.iter_mut().rev().rev().for_each(|a| a.double());
+    //~^ ERROR cannot borrow `*a` as mutable, as it is behind a `&` reference
+    println!("{:?}", v);
+}
+
+fn main() {}

tests/ui/borrowck/issue-62387-suggest-iter-mut.rs

@@ -0,0 +1,30 @@
+// run-rustfix
+#![allow(unused_mut)]
+#![allow(dead_code)]
+
+#[derive(Debug)]
+struct A {
+    a: i32,
+}
+
+impl A {
+    fn double(&mut self) {
+        self.a += self.a
+    }
+}
+
+fn baz() {
+    let mut v = [A { a: 4 }];
+    v.iter().for_each(|a| a.double());
+    //~^ ERROR cannot borrow `*a` as mutable, as it is behind a `&` reference
+    println!("{:?}", v);
+}
+
+fn bar() {
+    let mut v = [A { a: 4 }];
+    v.iter().rev().rev().for_each(|a| a.double());
+    //~^ ERROR cannot borrow `*a` as mutable, as it is behind a `&` reference
+    println!("{:?}", v);
+}
+
+fn main() {}

tests/ui/borrowck/issue-62387-suggest-iter-mut.stderr

@@ -0,0 +1,29 @@
+error[E0596]: cannot borrow `*a` as mutable, as it is behind a `&` reference
+  --> $DIR/issue-62387-suggest-iter-mut.rs:18:27
+   |
+LL |     v.iter().for_each(|a| a.double());
+   |                        -  ^ `a` is a `&` reference, so the data it refers to cannot be borrowed as mutable
+   |                        |
+   |                        consider changing this binding's type to be: `&mut A`
+   |
+help: you may want to use `iter_mut` here
+   |
+LL |     v.iter_mut().for_each(|a| a.double());
+   |       ~~~~~~~~
+
+error[E0596]: cannot borrow `*a` as mutable, as it is behind a `&` reference
+  --> $DIR/issue-62387-suggest-iter-mut.rs:25:39
+   |
+LL |     v.iter().rev().rev().for_each(|a| a.double());
+   |                                    -  ^ `a` is a `&` reference, so the data it refers to cannot be borrowed as mutable
+   |                                    |
+   |                                    consider changing this binding's type to be: `&mut A`
+   |
+help: you may want to use `iter_mut` here
+   |
+LL |     v.iter_mut().rev().rev().for_each(|a| a.double());
+   |       ~~~~~~~~
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0596`.