Auto merge of #51275 - pnkfelix:nll-diagnostics-revise-check-access-permissions, r=nikomatsakis

NLL diagnostics: revise `fn check_access_permissions`

NLL: revise `fn check_access_permissions` so that its (still branchy) shares more code paths between the different cases, and also provide more diagnostics in more cases (though the added diagnostics still do not always meet the quality bar established by AST-borrowck)

----

Transcribing "checklist" suggested by Niko, except I am rendering it as a table to make it clear that I do not regard every item in the list to be a "must have" for landing this PR.

goal | does this PR do it?
-----|------------------------------
no suggestions for `ref mut` |  yes
suggestions for direct local assignment (`{ let x = 3; x = 4; }`) | yes (see commits at end)
suggestions for direct field assignment (`{ let x = (3, 4); x.0 = 5; }` | yes (see commits at end)
suggestions for upvars (`let x = 3; let c = \|\| { &mut x; }`) | yes

Note that I added support for a couple of rows via changes that are not strictly part of `fn check_access_permissions`. If desired I can remove those commits from this PR and leave them for a later PR.

Fix #51031
Fix #51032
(bug #51191 needs a little more investigation before closing.)
Fix #51578

7 files changed, 59 insertions, 18 deletions

diff --git a/src/test/ui/span/borrowck-borrow-overloaded-auto-deref-mut.nll.stderr b/src/test/ui/span/borrowck-borrow-overloaded-auto-deref-mut.nll.stderr
index 172828b9a40..3282fbba6c5 100644
--- a/src/test/ui/span/borrowck-borrow-overloaded-auto-deref-mut.nll.stderr
+++ b/src/test/ui/span/borrowck-borrow-overloaded-auto-deref-mut.nll.stderr
@@ -1,50 +1,66 @@
 error[E0596]: cannot borrow immutable item `x` as mutable
   --> $DIR/borrowck-borrow-overloaded-auto-deref-mut.rs:63:24
    |
+LL | fn deref_mut_field1(x: Own<Point>) {
+   |                     - help: consider changing this to be mutable: `mut x`
 LL |     let __isize = &mut x.y; //~ ERROR cannot borrow
    |                        ^ cannot borrow as mutable
 
 error[E0596]: cannot borrow immutable item `*x` as mutable
   --> $DIR/borrowck-borrow-overloaded-auto-deref-mut.rs:75:10
    |
+LL | fn deref_extend_mut_field1(x: &Own<Point>) -> &mut isize {
+   |                               ----------- help: consider changing this to be a mutable reference: `&mut Own<Point>`
 LL |     &mut x.y //~ ERROR cannot borrow
-   |          ^ cannot borrow as mutable
+   |          ^ `x` is a `&` reference, so the data it refers to cannot be borrowed as mutable
 
 error[E0596]: cannot borrow immutable item `x` as mutable
   --> $DIR/borrowck-borrow-overloaded-auto-deref-mut.rs:98:5
    |
+LL | fn assign_field1<'a>(x: Own<Point>) {
+   |                      - help: consider changing this to be mutable: `mut x`
 LL |     x.y = 3; //~ ERROR cannot borrow
    |     ^ cannot borrow as mutable
 
 error[E0596]: cannot borrow immutable item `*x` as mutable
   --> $DIR/borrowck-borrow-overloaded-auto-deref-mut.rs:102:5
    |
+LL | fn assign_field2<'a>(x: &'a Own<Point>) {
+   |                         -------------- help: consider changing this to be a mutable reference: `&mut Own<Point>`
 LL |     x.y = 3; //~ ERROR cannot borrow
-   |     ^ cannot borrow as mutable
+   |     ^ `x` is a `&` reference, so the data it refers to cannot be borrowed as mutable
 
 error[E0596]: cannot borrow immutable item `x` as mutable
   --> $DIR/borrowck-borrow-overloaded-auto-deref-mut.rs:119:5
    |
+LL | fn deref_mut_method1(x: Own<Point>) {
+   |                      - help: consider changing this to be mutable: `mut x`
 LL |     x.set(0, 0); //~ ERROR cannot borrow
    |     ^ cannot borrow as mutable
 
 error[E0596]: cannot borrow immutable item `*x` as mutable
   --> $DIR/borrowck-borrow-overloaded-auto-deref-mut.rs:131:5
    |
+LL | fn deref_extend_mut_method1(x: &Own<Point>) -> &mut isize {
+   |                                ----------- help: consider changing this to be a mutable reference: `&mut Own<Point>`
 LL |     x.y_mut() //~ ERROR cannot borrow
-   |     ^ cannot borrow as mutable
+   |     ^ `x` is a `&` reference, so the data it refers to cannot be borrowed as mutable
 
 error[E0596]: cannot borrow immutable item `x` as mutable
   --> $DIR/borrowck-borrow-overloaded-auto-deref-mut.rs:139:6
    |
+LL | fn assign_method1<'a>(x: Own<Point>) {
+   |                       - help: consider changing this to be mutable: `mut x`
 LL |     *x.y_mut() = 3; //~ ERROR cannot borrow
    |      ^ cannot borrow as mutable
 
 error[E0596]: cannot borrow immutable item `*x` as mutable
   --> $DIR/borrowck-borrow-overloaded-auto-deref-mut.rs:143:6
    |
+LL | fn assign_method2<'a>(x: &'a Own<Point>) {
+   |                          -------------- help: consider changing this to be a mutable reference: `&mut Own<Point>`
 LL |     *x.y_mut() = 3; //~ ERROR cannot borrow
-   |      ^ cannot borrow as mutable
+   |      ^ `x` is a `&` reference, so the data it refers to cannot be borrowed as mutable
 
 error: aborting due to 8 previous errors
 
diff --git a/src/test/ui/span/borrowck-borrow-overloaded-deref-mut.nll.stderr b/src/test/ui/span/borrowck-borrow-overloaded-deref-mut.nll.stderr
index 24abe85de76..0b1bfd8cee6 100644
--- a/src/test/ui/span/borrowck-borrow-overloaded-deref-mut.nll.stderr
+++ b/src/test/ui/span/borrowck-borrow-overloaded-deref-mut.nll.stderr
@@ -1,26 +1,34 @@
 error[E0596]: cannot borrow immutable item `x` as mutable
   --> $DIR/borrowck-borrow-overloaded-deref-mut.rs:39:25
    |
+LL | fn deref_mut1(x: Own<isize>) {
+   |               - help: consider changing this to be mutable: `mut x`
 LL |     let __isize = &mut *x; //~ ERROR cannot borrow
    |                         ^ cannot borrow as mutable
 
 error[E0596]: cannot borrow immutable item `*x` as mutable
   --> $DIR/borrowck-borrow-overloaded-deref-mut.rs:51:11
    |
+LL | fn deref_extend_mut1<'a>(x: &'a Own<isize>) -> &'a mut isize {
+   |                             -------------- help: consider changing this to be a mutable reference: `&mut Own<isize>`
 LL |     &mut **x //~ ERROR cannot borrow
-   |           ^^ cannot borrow as mutable
+   |           ^^ `x` is a `&` reference, so the data it refers to cannot be borrowed as mutable
 
 error[E0596]: cannot borrow immutable item `x` as mutable
   --> $DIR/borrowck-borrow-overloaded-deref-mut.rs:59:6
    |
+LL | fn assign1<'a>(x: Own<isize>) {
+   |                - help: consider changing this to be mutable: `mut x`
 LL |     *x = 3; //~ ERROR cannot borrow
    |      ^ cannot borrow as mutable
 
 error[E0596]: cannot borrow immutable item `*x` as mutable
   --> $DIR/borrowck-borrow-overloaded-deref-mut.rs:63:6
    |
+LL | fn assign2<'a>(x: &'a Own<isize>) {
+   |                   -------------- help: consider changing this to be a mutable reference: `&mut Own<isize>`
 LL |     **x = 3; //~ ERROR cannot borrow
-   |      ^^ cannot borrow as mutable
+   |      ^^ `x` is a `&` reference, so the data it refers to cannot be borrowed as mutable
 
 error: aborting due to 4 previous errors
 
diff --git a/src/test/ui/span/borrowck-call-is-borrow-issue-12224.nll.stderr b/src/test/ui/span/borrowck-call-is-borrow-issue-12224.nll.stderr
index 26e9ea4dc0b..c4bdef21de4 100644
--- a/src/test/ui/span/borrowck-call-is-borrow-issue-12224.nll.stderr
+++ b/src/test/ui/span/borrowck-call-is-borrow-issue-12224.nll.stderr
@@ -15,16 +15,18 @@ LL | |     }));
 error[E0596]: cannot borrow immutable item `*f` as mutable
   --> $DIR/borrowck-call-is-borrow-issue-12224.rs:35:5
    |
+LL | fn test2<F>(f: &F) where F: FnMut() {
+   |                -- help: consider changing this to be a mutable reference: `&mut F`
 LL |     (*f)();
-   |     ^^^^ cannot borrow as mutable
+   |     ^^^^ `f` is a `&` reference, so the data it refers to cannot be borrowed as mutable
 
 error[E0596]: cannot borrow immutable item `*f.f` as mutable
   --> $DIR/borrowck-call-is-borrow-issue-12224.rs:44:5
    |
+LL | fn test4(f: &Test) {
+   |             ----- help: consider changing this to be a mutable reference: `&mut Test<'_>`
 LL |     f.f.call_mut(())
-   |     ^^^ cannot borrow as mutable
-   |
-   = note: the value which is causing this path not to be mutable is...: `*f`
+   |     ^^^ `f` is a `&` reference, so the data it refers to cannot be borrowed as mutable
 
 error[E0507]: cannot move out of borrowed content
   --> $DIR/borrowck-call-is-borrow-issue-12224.rs:66:13
diff --git a/src/test/ui/span/borrowck-call-method-from-mut-aliasable.nll.stderr b/src/test/ui/span/borrowck-call-method-from-mut-aliasable.nll.stderr
index 43934bf4aee..0bc614589e3 100644
--- a/src/test/ui/span/borrowck-call-method-from-mut-aliasable.nll.stderr
+++ b/src/test/ui/span/borrowck-call-method-from-mut-aliasable.nll.stderr
@@ -1,8 +1,11 @@
 error[E0596]: cannot borrow immutable item `*x` as mutable
   --> $DIR/borrowck-call-method-from-mut-aliasable.rs:27:5
    |
+LL | fn b(x: &Foo) {
+   |         ---- help: consider changing this to be a mutable reference: `&mut Foo`
+LL |     x.f();
 LL |     x.h(); //~ ERROR cannot borrow
-   |     ^ cannot borrow as mutable
+   |     ^ `x` is a `&` reference, so the data it refers to cannot be borrowed as mutable
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/span/borrowck-fn-in-const-b.nll.stderr b/src/test/ui/span/borrowck-fn-in-const-b.nll.stderr
index d3c6fd66599..1cb2c92833c 100644
--- a/src/test/ui/span/borrowck-fn-in-const-b.nll.stderr
+++ b/src/test/ui/span/borrowck-fn-in-const-b.nll.stderr
@@ -1,8 +1,10 @@
 error[E0596]: cannot borrow immutable item `*x` as mutable
   --> $DIR/borrowck-fn-in-const-b.rs:17:9
    |
+LL |     fn broken(x: &Vec<String>) {
+   |                  ------------ help: consider changing this to be a mutable reference: `&mut std::vec::Vec<std::string::String>`
 LL |         x.push(format!("this is broken"));
-   |         ^ cannot borrow as mutable
+   |         ^ `x` is a `&` reference, so the data it refers to cannot be borrowed as mutable
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/span/borrowck-object-mutability.nll.stderr b/src/test/ui/span/borrowck-object-mutability.nll.stderr
index 9b5e084bd37..cf615eed556 100644
--- a/src/test/ui/span/borrowck-object-mutability.nll.stderr
+++ b/src/test/ui/span/borrowck-object-mutability.nll.stderr
@@ -1,16 +1,20 @@
 error[E0596]: cannot borrow immutable item `*x` as mutable
   --> $DIR/borrowck-object-mutability.rs:19:5
    |
+LL | fn borrowed_receiver(x: &Foo) {
+   |                         ---- help: consider changing this to be a mutable reference: `&mut Foo`
+LL |     x.borrowed();
 LL |     x.borrowed_mut(); //~ ERROR cannot borrow
-   |     ^ cannot borrow as mutable
+   |     ^ `x` is a `&` reference, so the data it refers to cannot be borrowed as mutable
 
 error[E0596]: cannot borrow immutable item `*x` as mutable
   --> $DIR/borrowck-object-mutability.rs:29:5
    |
+LL | fn owned_receiver(x: Box<Foo>) {
+   |                   - help: consider changing this to be mutable: `mut x`
+LL |     x.borrowed();
 LL |     x.borrowed_mut(); //~ ERROR cannot borrow
    |     ^ cannot borrow as mutable
-   |
-   = note: the value which is causing this path not to be mutable is...: `x`
 
 error: aborting due to 2 previous errors
 
diff --git a/src/test/ui/span/mut-arg-hint.nll.stderr b/src/test/ui/span/mut-arg-hint.nll.stderr
index 8e1cb9720e2..cd19059fdae 100644
--- a/src/test/ui/span/mut-arg-hint.nll.stderr
+++ b/src/test/ui/span/mut-arg-hint.nll.stderr
@@ -1,20 +1,26 @@
 error[E0596]: cannot borrow immutable item `*a` as mutable
   --> $DIR/mut-arg-hint.rs:13:9
    |
+LL |     fn foo(mut a: &String) {
+   |                   ------- help: consider changing this to be a mutable reference: `&mut std::string::String`
 LL |         a.push_str("bar"); //~ ERROR cannot borrow immutable borrowed content
-   |         ^ cannot borrow as mutable
+   |         ^ `a` is a `&` reference, so the data it refers to cannot be borrowed as mutable
 
 error[E0596]: cannot borrow immutable item `*a` as mutable
   --> $DIR/mut-arg-hint.rs:18:5
    |
+LL | pub fn foo<'a>(mut a: &'a String) {
+   |                       ---------- help: consider changing this to be a mutable reference: `&mut std::string::String`
 LL |     a.push_str("foo"); //~ ERROR cannot borrow immutable borrowed content
-   |     ^ cannot borrow as mutable
+   |     ^ `a` is a `&` reference, so the data it refers to cannot be borrowed as mutable
 
 error[E0596]: cannot borrow immutable item `*a` as mutable
   --> $DIR/mut-arg-hint.rs:25:9
    |
+LL |     pub fn foo(mut a: &String) {
+   |                       ------- help: consider changing this to be a mutable reference: `&mut std::string::String`
 LL |         a.push_str("foo"); //~ ERROR cannot borrow immutable borrowed content
-   |         ^ cannot borrow as mutable
+   |         ^ `a` is a `&` reference, so the data it refers to cannot be borrowed as mutable
 
 error: aborting due to 3 previous errors