Rollup merge of #83174 - camelid:borrow-help, r=oli-obk

Suggest using a temporary variable to fix borrowck errors

Fixes #77834.

In Rust, nesting method calls with both require `&mut` access to `self`
produces a borrow-check error:

    error[E0499]: cannot borrow `*self` as mutable more than once at a time
     --> src/lib.rs:7:14
      |
    7 |     self.foo(self.bar());
      |     ---------^^^^^^^^^^-
      |     |    |   |
      |     |    |   second mutable borrow occurs here
      |     |    first borrow later used by call
      |     first mutable borrow occurs here

That's because Rust has a left-to-right evaluation order, and the method
receiver is passed first. Thus, the argument to the method cannot then
mutate `self`.

There's an easy solution to this error: just extract a local variable
for the inner argument:

    let tmp = self.bar();
    self.foo(tmp);

However, the error doesn't give any suggestion of how to solve the
problem. As a result, new users may assume that it's impossible to
express their code correctly and get stuck.

This commit adds a (non-structured) suggestion to extract a local
variable for the inner argument to solve the error. The suggestion uses
heuristics that eliminate most false positives, though there are a few
false negatives (cases where the suggestion should be emitted but is
not). Those other cases can be implemented in a future change.

7 files changed, 181 insertions, 0 deletions

diff --git a/src/test/ui/borrowck/suggest-local-var-double-mut.rs b/src/test/ui/borrowck/suggest-local-var-double-mut.rs
new file mode 100644
index 00000000000..d5996ba68be
--- /dev/null
+++ b/src/test/ui/borrowck/suggest-local-var-double-mut.rs
@@ -0,0 +1,27 @@
+// See issue #77834.
+
+#![crate_type = "lib"]
+
+mod method_syntax {
+    struct Foo;
+
+    impl Foo {
+        fn foo(&mut self, _: f32) -> i32 { todo!() }
+        fn bar(&mut self) -> f32 { todo!() }
+        fn baz(&mut self) {
+            self.foo(self.bar()); //~ ERROR
+        }
+    }
+}
+
+mod fully_qualified_syntax {
+    struct Foo;
+
+    impl Foo {
+        fn foo(&mut self, _: f32) -> i32 { todo!() }
+        fn bar(&mut self) -> f32 { todo!() }
+        fn baz(&mut self) {
+            Self::foo(self, Self::bar(self)); //~ ERROR
+        }
+    }
+}
diff --git a/src/test/ui/borrowck/suggest-local-var-double-mut.stderr b/src/test/ui/borrowck/suggest-local-var-double-mut.stderr
new file mode 100644
index 00000000000..3a43c18a7ed
--- /dev/null
+++ b/src/test/ui/borrowck/suggest-local-var-double-mut.stderr
@@ -0,0 +1,44 @@
+error[E0499]: cannot borrow `*self` as mutable more than once at a time
+  --> $DIR/suggest-local-var-double-mut.rs:12:22
+   |
+LL |             self.foo(self.bar());
+   |             ---------^^^^^^^^^^-
+   |             |    |   |
+   |             |    |   second mutable borrow occurs here
+   |             |    first borrow later used by call
+   |             first mutable borrow occurs here
+   |
+help: try adding a local storing this argument...
+  --> $DIR/suggest-local-var-double-mut.rs:12:22
+   |
+LL |             self.foo(self.bar());
+   |                      ^^^^^^^^^^
+help: ...and then using that local as the argument to this call
+  --> $DIR/suggest-local-var-double-mut.rs:12:13
+   |
+LL |             self.foo(self.bar());
+   |             ^^^^^^^^^^^^^^^^^^^^
+
+error[E0499]: cannot borrow `*self` as mutable more than once at a time
+  --> $DIR/suggest-local-var-double-mut.rs:24:39
+   |
+LL |             Self::foo(self, Self::bar(self));
+   |             --------- ----            ^^^^ second mutable borrow occurs here
+   |             |         |
+   |             |         first mutable borrow occurs here
+   |             first borrow later used by call
+   |
+help: try adding a local storing this argument...
+  --> $DIR/suggest-local-var-double-mut.rs:24:29
+   |
+LL |             Self::foo(self, Self::bar(self));
+   |                             ^^^^^^^^^^^^^^^
+help: ...and then using that local as the argument to this call
+  --> $DIR/suggest-local-var-double-mut.rs:24:13
+   |
+LL |             Self::foo(self, Self::bar(self));
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0499`.
diff --git a/src/test/ui/borrowck/suggest-local-var-imm-and-mut.nll.stderr b/src/test/ui/borrowck/suggest-local-var-imm-and-mut.nll.stderr
new file mode 100644
index 00000000000..2ba0b6b28aa
--- /dev/null
+++ b/src/test/ui/borrowck/suggest-local-var-imm-and-mut.nll.stderr
@@ -0,0 +1,33 @@
+error[E0502]: cannot borrow `*self` as mutable because it is also borrowed as immutable
+  --> $DIR/suggest-local-var-imm-and-mut.rs:12:22
+   |
+LL |             self.foo(self.bar());
+   |             ---------^^^^^^^^^^-
+   |             |    |   |
+   |             |    |   mutable borrow occurs here
+   |             |    immutable borrow later used by call
+   |             immutable borrow occurs here
+   |
+help: try adding a local storing this argument...
+  --> $DIR/suggest-local-var-imm-and-mut.rs:12:22
+   |
+LL |             self.foo(self.bar());
+   |                      ^^^^^^^^^^
+help: ...and then using that local as the argument to this call
+  --> $DIR/suggest-local-var-imm-and-mut.rs:12:13
+   |
+LL |             self.foo(self.bar());
+   |             ^^^^^^^^^^^^^^^^^^^^
+
+error[E0502]: cannot borrow `*self` as mutable because it is also borrowed as immutable
+  --> $DIR/suggest-local-var-imm-and-mut.rs:24:39
+   |
+LL |             Self::foo(self, Self::bar(self));
+   |             --------- ----            ^^^^ mutable borrow occurs here
+   |             |         |
+   |             |         immutable borrow occurs here
+   |             immutable borrow later used by call
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0502`.
diff --git a/src/test/ui/borrowck/suggest-local-var-imm-and-mut.rs b/src/test/ui/borrowck/suggest-local-var-imm-and-mut.rs
new file mode 100644
index 00000000000..bf167ba79f3
--- /dev/null
+++ b/src/test/ui/borrowck/suggest-local-var-imm-and-mut.rs
@@ -0,0 +1,27 @@
+// See issue #77834.
+
+#![crate_type = "lib"]
+
+mod method_syntax {
+    struct Foo;
+
+    impl Foo {
+        fn foo(&self, _: f32) -> i32 { todo!() }
+        fn bar(&mut self) -> f32 { todo!() }
+        fn baz(&mut self) {
+            self.foo(self.bar()); //~ ERROR
+        }
+    }
+}
+
+mod fully_qualified_syntax {
+    struct Foo;
+
+    impl Foo {
+        fn foo(&self, _: f32) -> i32 { todo!() }
+        fn bar(&mut self) -> f32 { todo!() }
+        fn baz(&mut self) {
+            Self::foo(self, Self::bar(self)); //~ ERROR
+        }
+    }
+}
diff --git a/src/test/ui/borrowck/suggest-local-var-imm-and-mut.stderr b/src/test/ui/borrowck/suggest-local-var-imm-and-mut.stderr
new file mode 100644
index 00000000000..eb934e7b72b
--- /dev/null
+++ b/src/test/ui/borrowck/suggest-local-var-imm-and-mut.stderr
@@ -0,0 +1,22 @@
+error[E0502]: cannot borrow `*self` as mutable because it is also borrowed as immutable
+  --> $DIR/suggest-local-var-imm-and-mut.rs:12:22
+   |
+LL |             self.foo(self.bar());
+   |             ---------^^^^^^^^^^-
+   |             |    |   |
+   |             |    |   mutable borrow occurs here
+   |             |    immutable borrow later used by call
+   |             immutable borrow occurs here
+
+error[E0502]: cannot borrow `*self` as mutable because it is also borrowed as immutable
+  --> $DIR/suggest-local-var-imm-and-mut.rs:24:29
+   |
+LL |             Self::foo(self, Self::bar(self));
+   |             --------- ----  ^^^^^^^^^^^^^^^ mutable borrow occurs here
+   |             |         |
+   |             |         immutable borrow occurs here
+   |             immutable borrow later used by call
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0502`.
diff --git a/src/test/ui/borrowck/two-phase-cannot-nest-mut-self-calls.stderr b/src/test/ui/borrowck/two-phase-cannot-nest-mut-self-calls.stderr
index a89bb941532..85c7159952f 100644
--- a/src/test/ui/borrowck/two-phase-cannot-nest-mut-self-calls.stderr
+++ b/src/test/ui/borrowck/two-phase-cannot-nest-mut-self-calls.stderr
@@ -13,6 +13,23 @@ LL | |
 LL | |         0
 LL | |     });
    | |______- immutable borrow occurs here
+   |
+help: try adding a local storing this argument...
+  --> $DIR/two-phase-cannot-nest-mut-self-calls.rs:16:9
+   |
+LL |         vec.push(2);
+   |         ^^^^^^^^^^^
+help: ...and then using that local as the argument to this call
+  --> $DIR/two-phase-cannot-nest-mut-self-calls.rs:14:5
+   |
+LL | /     vec.get({
+LL | |
+LL | |         vec.push(2);
+LL | |
+LL | |
+LL | |         0
+LL | |     });
+   | |______^
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/codemap_tests/one_line.stderr b/src/test/ui/codemap_tests/one_line.stderr
index 1ee612184de..6fe6e26135b 100644
--- a/src/test/ui/codemap_tests/one_line.stderr
+++ b/src/test/ui/codemap_tests/one_line.stderr
@@ -7,6 +7,17 @@ LL |     v.push(v.pop().unwrap());
    |     | |    second mutable borrow occurs here
    |     | first borrow later used by call
    |     first mutable borrow occurs here
+   |
+help: try adding a local storing this argument...
+  --> $DIR/one_line.rs:3:12
+   |
+LL |     v.push(v.pop().unwrap());
+   |            ^^^^^^^
+help: ...and then using that local as the argument to this call
+  --> $DIR/one_line.rs:3:5
+   |
+LL |     v.push(v.pop().unwrap());
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: aborting due to previous error