Add old borrowck examples

Author: WaffleLapkin

code/examples/borrowck_1.rs

@@ -0,0 +1,13 @@
+fn func(a: &'static u32) {
+    dbg!(a);
+}
+
+fn accepts_func(f: fn(&u32), data: &u32) {
+    f(data);
+}
+
+fn main() {
+    //  &'static u32 can be shortened to any
+    // lifetime borrow so func should be fine
+    accepts_func(func, &274);
+}

code/examples/borrowck_2.rs

@@ -0,0 +1,9 @@
+fn main() {
+    consume_fn(identity);
+}
+
+fn identity(x: &u32) -> &u32 {
+    x
+}
+
+fn consume_fn<T>(_: impl FnOnce(&u32) -> T) {}

code/examples/borrowck_3.rs

@@ -0,0 +1,11 @@
+fn foo(a: &mut u32) -> u32 {
+    let mut b = || &mut *a;
+
+    *b() = 12;
+    *b() = 73;
+    *a
+}
+
+fn main() {
+    foo(&mut 292);
+}

code/examples/borrowck_4.rs

@@ -0,0 +1,13 @@
+#[derive(Debug)]
+struct Foo<'a>(&'a mut u32);
+
+fn main() {
+    let mut data = 10;
+    let mut foo = Foo(&mut data);
+    mutate(&mut foo);
+    dbg!(foo);
+}
+
+fn mutate<'a>(f: &'a mut Foo<'a>) {
+    *f.0 += 1;
+}

code/examples/borrowck_5.rs

@@ -0,0 +1,12 @@
+use std::collections::HashMap;
+
+fn get_or_insert_1234(map: &mut HashMap<u32, u64>) -> &mut u64 {
+    if let Some(v) = map.get_mut(&0) {
+        return v;
+    }
+
+    map.insert(0, 1234);
+    map.get_mut(&0).unwrap()
+}
+
+fn main() {}

code/examples/stderr/borrowck_1.stderr

@@ -0,0 +1,18 @@
+error[E0308]: mismatched types
+  --> examples/borrowck_1.rs:12:18
+   |
+12 |     accepts_func(func, &274);
+   |     ------------ ^^^^ one type is more general than the other
+   |     |
+   |     arguments to this function are incorrect
+   |
+   = note: expected fn pointer `for<'a> fn(&'a _)`
+                 found fn item `fn(&'static _) {func}`
+note: function defined here
+  --> examples/borrowck_1.rs:5:4
+   |
+5  | fn accepts_func(f: fn(&u32), data: &u32) {
+   |    ^^^^^^^^^^^^ -----------
+
+For more information about this error, try `rustc --explain E0308`.
+error: could not compile `code` (example "borrowck_1") due to 1 previous error

code/examples/stderr/borrowck_2.stderr

@@ -0,0 +1,16 @@
+error[E0308]: mismatched types
+ --> examples/borrowck_2.rs:2:5
+  |
+2 |     consume_fn(identity);
+  |     ^^^^^^^^^^^^^^^^^^^^ one type is more general than the other
+  |
+  = note: expected reference `&_`
+             found reference `&_`
+note: the lifetime requirement is introduced here
+ --> examples/borrowck_2.rs:9:42
+  |
+9 | fn consume_fn<T>(_: impl FnOnce(&u32) -> T) {}
+  |                                          ^
+
+For more information about this error, try `rustc --explain E0308`.
+error: could not compile `code` (example "borrowck_2") due to 1 previous error

code/examples/stderr/borrowck_3.stderr

@@ -0,0 +1,16 @@
+error: captured variable cannot escape `FnMut` closure body
+ --> examples/borrowck_3.rs:2:20
+  |
+1 | fn foo(a: &mut u32) -> u32 {
+  |        - variable defined here
+2 |     let mut b = || &mut *a;
+  |                  - ^^^^^^-
+  |                  | |     |
+  |                  | |     variable captured here
+  |                  | returns a reference to a captured variable which escapes the closure body
+  |                  inferred to be a `FnMut` closure
+  |
+  = note: `FnMut` closures only have access to their captured variables while they are executing...
+  = note: ...therefore, they cannot allow references to captured variables to escape
+
+error: could not compile `code` (example "borrowck_3") due to 1 previous error

code/examples/stderr/borrowck_4.stderr

@@ -0,0 +1,21 @@
+error[E0505]: cannot move out of `foo` because it is borrowed
+ --> examples/borrowck_4.rs:8:5
+  |
+6 |     let mut foo = Foo(&mut data);
+  |         ------- binding `foo` declared here
+7 |     mutate(&mut foo);
+  |            -------- borrow of `foo` occurs here
+8 |     dbg!(foo);
+  |     ^^^^^^^^^
+  |     |
+  |     move out of `foo` occurs here
+  |     borrow later used here
+  |
+  = note: this error originates in the macro `dbg` (in Nightly builds, run with -Z macro-backtrace for more info)
+help: consider cloning the value if the performance cost is acceptable
+  |
+7 |     mutate(&mut foo).clone();
+  |                     ++++++++
+
+For more information about this error, try `rustc --explain E0505`.
+error: could not compile `code` (example "borrowck_4") due to 1 previous error

code/examples/stderr/borrowck_5.stderr

@@ -0,0 +1,28 @@
+error[E0499]: cannot borrow `*map` as mutable more than once at a time
+ --> examples/borrowck_5.rs:8:5
+  |
+3 | fn get_or_insert_1234(map: &mut HashMap<u32, u64>) -> &mut u64 {
+  |                            - let's call the lifetime of this reference `'1`
+4 |     if let Some(v) = map.get_mut(&0) {
+  |                      --- first mutable borrow occurs here
+5 |         return v;
+  |                - returning this value requires that `*map` is borrowed for `'1`
+...
+8 |     map.insert(0, 1234);
+  |     ^^^ second mutable borrow occurs here
+
+error[E0499]: cannot borrow `*map` as mutable more than once at a time
+ --> examples/borrowck_5.rs:9:5
+  |
+3 | fn get_or_insert_1234(map: &mut HashMap<u32, u64>) -> &mut u64 {
+  |                            - let's call the lifetime of this reference `'1`
+4 |     if let Some(v) = map.get_mut(&0) {
+  |                      --- first mutable borrow occurs here
+5 |         return v;
+  |                - returning this value requires that `*map` is borrowed for `'1`
+...
+9 |     map.get_mut(&0).unwrap()
+  |     ^^^ second mutable borrow occurs here
+
+For more information about this error, try `rustc --explain E0499`.
+error: could not compile `code` (example "borrowck_5") due to 2 previous errors