Rollup merge of rust-lang#106005 - LeSeulArtichaut:if-let-guard-borro…

…wck-test, r=Nilstrieb

Test the borrowck behavior of if-let guards

Add some tests to make sure that if-let guards behave the same as if guards with respect to borrow-checking. Most of them are a naive adaptation, replacing an `if` guard with `if let Some(())`.
This includes regression tests for notable issues that arose for if guards (rust-lang#24535, rust-lang#27282, rust-lang#29723, rust-lang#31287) as suggested in rust-lang#51114 (comment).

cc `@pnkfelix` are there any other tests that you would want to see?
cc tracking issue rust-lang#51114

src/test/ui/borrowck/borrowck-drop-from-guard.rs

@@ -1,3 +1,5 @@
+#![feature(if_let_guard)]
+
 fn foo(_:String) {}
 
 fn main()
@@ -8,4 +10,11 @@ fn main()
         Some(_) => {}
         None => { foo(my_str); } //~ ERROR [E0382]
     }
+
+    let my_str = "hello".to_owned();
+    match Some(42) {
+        Some(_) if let Some(()) = { drop(my_str); None } => {}
+        Some(_) => {}
+        None => { foo(my_str); } //~ ERROR [E0382]
+    }
 }

src/test/ui/borrowck/borrowck-drop-from-guard.stderr

@@ -1,5 +1,5 @@
 error[E0382]: use of moved value: `my_str`
-  --> $DIR/borrowck-drop-from-guard.rs:9:23
+  --> $DIR/borrowck-drop-from-guard.rs:11:23
    |
 LL |     let my_str = "hello".to_owned();
    |         ------ move occurs because `my_str` has type `String`, which does not implement the `Copy` trait
@@ -15,6 +15,23 @@ help: consider cloning the value if the performance cost is acceptable
 LL |         Some(_) if { drop(my_str.clone()); false } => {}
    |                                 ++++++++
 
-error: aborting due to previous error
+error[E0382]: use of moved value: `my_str`
+  --> $DIR/borrowck-drop-from-guard.rs:18:23
+   |
+LL |     let my_str = "hello".to_owned();
+   |         ------ move occurs because `my_str` has type `String`, which does not implement the `Copy` trait
+LL |     match Some(42) {
+LL |         Some(_) if let Some(()) = { drop(my_str); None } => {}
+   |                                          ------ value moved here
+LL |         Some(_) => {}
+LL |         None => { foo(my_str); }
+   |                       ^^^^^^ value used here after move
+   |
+help: consider cloning the value if the performance cost is acceptable
+   |
+LL |         Some(_) if let Some(()) = { drop(my_str.clone()); None } => {}
+   |                                                ++++++++
+
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0382`.

src/test/ui/borrowck/borrowck-mutate-in-guard.rs

@@ -1,9 +1,11 @@
+#![feature(if_let_guard)]
+
 enum Enum<'a> {
     A(&'a isize),
     B(bool),
 }
 
-fn foo() -> isize {
+fn if_guard() -> isize {
     let mut n = 42;
     let mut x = Enum::A(&mut n);
     match x {
@@ -16,6 +18,17 @@ fn foo() -> isize {
     }
 }
 
-fn main() {
-    foo();
+fn if_let_guard() -> isize {
+    let mut n = 42;
+    let mut x = Enum::A(&mut n);
+    match x {
+        Enum::A(_) if let Some(()) = { x = Enum::B(false); None } => 1,
+        //~^ ERROR cannot assign `x` in match guard
+        Enum::A(_) if let Some(()) = { let y = &mut x; *y = Enum::B(false); None } => 1,
+        //~^ ERROR cannot mutably borrow `x` in match guard
+        Enum::A(p) => *p,
+        Enum::B(_) => 2,
+    }
 }
+
+fn main() {}

src/test/ui/borrowck/borrowck-mutate-in-guard.stderr

@@ -1,20 +1,37 @@
 error[E0510]: cannot assign `x` in match guard
-  --> $DIR/borrowck-mutate-in-guard.rs:10:25
+  --> $DIR/borrowck-mutate-in-guard.rs:12:25
    |
 LL |     match x {
    |           - value is immutable in match guard
 LL |         Enum::A(_) if { x = Enum::B(false); false } => 1,
    |                         ^^^^^^^^^^^^^^^^^^ cannot assign
 
 error[E0510]: cannot mutably borrow `x` in match guard
-  --> $DIR/borrowck-mutate-in-guard.rs:12:33
+  --> $DIR/borrowck-mutate-in-guard.rs:14:33
    |
 LL |     match x {
    |           - value is immutable in match guard
 ...
 LL |         Enum::A(_) if { let y = &mut x; *y = Enum::B(false); false } => 1,
    |                                 ^^^^^^ cannot mutably borrow
 
-error: aborting due to 2 previous errors
+error[E0510]: cannot assign `x` in match guard
+  --> $DIR/borrowck-mutate-in-guard.rs:25:40
+   |
+LL |     match x {
+   |           - value is immutable in match guard
+LL |         Enum::A(_) if let Some(()) = { x = Enum::B(false); None } => 1,
+   |                                        ^^^^^^^^^^^^^^^^^^ cannot assign
+
+error[E0510]: cannot mutably borrow `x` in match guard
+  --> $DIR/borrowck-mutate-in-guard.rs:27:48
+   |
+LL |     match x {
+   |           - value is immutable in match guard
+...
+LL |         Enum::A(_) if let Some(()) = { let y = &mut x; *y = Enum::B(false); None } => 1,
+   |                                                ^^^^^^ cannot mutably borrow
+
+error: aborting due to 4 previous errors
 
 For more information about this error, try `rustc --explain E0510`.

src/test/ui/borrowck/issue-31287-drop-in-guard.rs

@@ -1,8 +1,15 @@
+#![feature(if_let_guard)]
+
 fn main() {
     let a = Some("...".to_owned());
     let b = match a {
         Some(_) if { drop(a); false } => None,
         x => x, //~ ERROR use of moved value: `a`
     };
-    println!("{:?}", b);
+
+    let a = Some("...".to_owned());
+    let b = match a {
+        Some(_) if let Some(()) = { drop(a); None } => None,
+        x => x, //~ ERROR use of moved value: `a`
+    };
 }

src/test/ui/borrowck/issue-31287-drop-in-guard.stderr

@@ -1,5 +1,5 @@
 error[E0382]: use of moved value: `a`
-  --> $DIR/issue-31287-drop-in-guard.rs:5:9
+  --> $DIR/issue-31287-drop-in-guard.rs:7:9
    |
 LL |     let a = Some("...".to_owned());
    |         - move occurs because `a` has type `Option<String>`, which does not implement the `Copy` trait
@@ -14,6 +14,22 @@ help: consider cloning the value if the performance cost is acceptable
 LL |         Some(_) if { drop(a.clone()); false } => None,
    |                            ++++++++
 
-error: aborting due to previous error
+error[E0382]: use of moved value: `a`
+  --> $DIR/issue-31287-drop-in-guard.rs:13:9
+   |
+LL |     let a = Some("...".to_owned());
+   |         - move occurs because `a` has type `Option<String>`, which does not implement the `Copy` trait
+LL |     let b = match a {
+LL |         Some(_) if let Some(()) = { drop(a); None } => None,
+   |                                          - value moved here
+LL |         x => x,
+   |         ^ value used here after move
+   |
+help: consider cloning the value if the performance cost is acceptable
+   |
+LL |         Some(_) if let Some(()) = { drop(a.clone()); None } => None,
+   |                                           ++++++++
+
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0382`.

src/test/ui/issues/issue-29723.rs

@@ -1,5 +1,7 @@
 // test for https://github.com/rust-lang/rust/issues/29723
 
+#![feature(if_let_guard)]
+
 fn main() {
     let s = String::new();
     let _s = match 0 {
@@ -11,4 +13,10 @@ fn main() {
             //~^ ERROR use of moved value: `s`
         }
     };
+
+    let s = String::new();
+    let _s = match 0 {
+        0 if let Some(()) = { drop(s); None } => String::from("oops"),
+        _ => s //~ ERROR use of moved value: `s`
+    };
 }

src/test/ui/issues/issue-29723.stderr

@@ -1,5 +1,5 @@
 error[E0382]: use of moved value: `s`
-  --> $DIR/issue-29723.rs:10:13
+  --> $DIR/issue-29723.rs:12:13
    |
 LL |     let s = String::new();
    |         - move occurs because `s` has type `String`, which does not implement the `Copy` trait
@@ -15,6 +15,22 @@ help: consider cloning the value if the performance cost is acceptable
 LL |         0 if { drop(s.clone()); false } => String::from("oops"),
    |                      ++++++++
 
-error: aborting due to previous error
+error[E0382]: use of moved value: `s`
+  --> $DIR/issue-29723.rs:20:14
+   |
+LL |     let s = String::new();
+   |         - move occurs because `s` has type `String`, which does not implement the `Copy` trait
+LL |     let _s = match 0 {
+LL |         0 if let Some(()) = { drop(s); None } => String::from("oops"),
+   |                                    - value moved here
+LL |         _ => s
+   |              ^ value used here after move
+   |
+help: consider cloning the value if the performance cost is acceptable
+   |
+LL |         0 if let Some(()) = { drop(s.clone()); None } => String::from("oops"),
+   |                                     ++++++++
+
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0382`.

src/test/ui/nll/issue-24535-allow-mutable-borrow-in-match-guard.rs

@@ -5,6 +5,8 @@
 // See further discussion on rust-lang/rust#24535,
 // rust-lang/rfcs#1006, and rust-lang/rfcs#107
 
+#![feature(if_let_guard)]
+
 fn main() {
     rust_issue_24535();
     rfcs_issue_1006_1();
@@ -23,6 +25,12 @@ fn rust_issue_24535() {
         3 if compare(&a, &mut 3) => (),
         _ => panic!("nope"),
     }
+
+    match a {
+        0 => panic!("nope"),
+        3 if let true = compare(&a, &mut 3) => (),
+        _ => panic!("nope"),
+    }
 }
 
 fn rfcs_issue_1006_1() {

src/test/ui/nll/issue-27282-move-match-input-into-guard.rs

@@ -7,6 +7,8 @@
 // reaches the panic code when executed, despite the compiler warning
 // about that match arm being unreachable.
 
+#![feature(if_let_guard)]
+
 fn main() {
     let b = &mut true;
     match b {
@@ -17,4 +19,16 @@ fn main() {
         &mut true => { println!("You might think we should get here"); },
         _ => panic!("surely we could never get here, since rustc warns it is unreachable."),
     }
+
+    let b = &mut true;
+    match b {
+        //~^ ERROR use of moved value: `b` [E0382]
+        &mut false => {}
+        _ if let Some(()) = {
+            (|| { let bar = b; *bar = false; })();
+            None
+        } => {}
+        &mut true => {}
+        _ => {}
+    }
 }

src/test/ui/nll/issue-27282-move-match-input-into-guard.stderr

@@ -1,5 +1,5 @@
 error[E0382]: use of moved value: `b`
-  --> $DIR/issue-27282-move-match-input-into-guard.rs:12:5
+  --> $DIR/issue-27282-move-match-input-into-guard.rs:14:5
    |
 LL |     let b = &mut true;
    |         - move occurs because `b` has type `&mut bool`, which does not implement the `Copy` trait
@@ -11,6 +11,19 @@ LL |         _ if { (|| { let bar = b; *bar = false; })();
    |                 |
    |                 value moved into closure here
 
-error: aborting due to previous error
+error[E0382]: use of moved value: `b`
+  --> $DIR/issue-27282-move-match-input-into-guard.rs:24:5
+   |
+LL |     let b = &mut true;
+   |         - move occurs because `b` has type `&mut bool`, which does not implement the `Copy` trait
+LL |     match b {
+   |     ^^^^^^^ value used here after move
+...
+LL |             (|| { let bar = b; *bar = false; })();
+   |              --             - variable moved due to use in closure
+   |              |
+   |              value moved into closure here
+
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0382`.

src/test/ui/nll/issue-27282-move-ref-mut-into-guard.rs

@@ -2,6 +2,8 @@
 // mutable borrows in match guards by hiding the mutable borrow in a
 // guard behind a move (of the ref mut pattern id) within a closure.
 
+#![feature(if_let_guard)]
+
 fn main() {
     match Some(&4) {
         None => {},
@@ -10,4 +12,12 @@ fn main() {
         //~^ ERROR cannot move out of `foo` in pattern guard [E0507]
         Some(s) => std::process::exit(*s),
     }
+
+    match Some(&4) {
+        None => {},
+        ref mut foo
+            if let Some(()) = { (|| { let bar = foo; bar.take() })(); None } => {},
+        //~^ ERROR cannot move out of `foo` in pattern guard [E0507]
+        Some(s) => std::process::exit(*s),
+    }
 }

src/test/ui/nll/issue-27282-move-ref-mut-into-guard.stderr

@@ -1,5 +1,5 @@
 error[E0507]: cannot move out of `foo` in pattern guard
-  --> $DIR/issue-27282-move-ref-mut-into-guard.rs:9:19
+  --> $DIR/issue-27282-move-ref-mut-into-guard.rs:11:19
    |
 LL |             if { (|| { let bar = foo; bar.take() })(); false } => {},
    |                   ^^             --- move occurs because `foo` has type `&mut Option<&i32>`, which does not implement the `Copy` trait
@@ -8,6 +8,16 @@ LL |             if { (|| { let bar = foo; bar.take() })(); false } => {},
    |
    = note: variables bound in patterns cannot be moved from until after the end of the pattern guard
 
-error: aborting due to previous error
+error[E0507]: cannot move out of `foo` in pattern guard
+  --> $DIR/issue-27282-move-ref-mut-into-guard.rs:19:34
+   |
+LL |             if let Some(()) = { (|| { let bar = foo; bar.take() })(); None } => {},
+   |                                  ^^             --- move occurs because `foo` has type `&mut Option<&i32>`, which does not implement the `Copy` trait
+   |                                  |
+   |                                  move out of `foo` occurs here
+   |
+   = note: variables bound in patterns cannot be moved from until after the end of the pattern guard
+
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0507`.

src/test/ui/nll/issue-27282-mutation-in-guard.rs

@@ -1,3 +1,5 @@
+#![feature(if_let_guard)]
+
 fn main() {
     match Some(&4) {
         None => {},
@@ -10,4 +12,15 @@ fn main() {
         Some(ref _s) => println!("Note this arm is bogus; the `Some` became `None` in the guard."),
         _ => println!("Here is some supposedly unreachable code."),
     }
+
+    match Some(&4) {
+        None => {},
+        ref mut foo
+            if let Some(()) = {
+                (|| { let bar = foo; bar.take() })();
+                //~^ ERROR cannot move out of `foo` in pattern guard
+                None
+            } => {},
+        Some(_) => {},
+    }
 }

src/test/ui/nll/issue-27282-mutation-in-guard.stderr

@@ -1,5 +1,5 @@
 error[E0507]: cannot move out of `foo` in pattern guard
-  --> $DIR/issue-27282-mutation-in-guard.rs:6:18
+  --> $DIR/issue-27282-mutation-in-guard.rs:8:18
    |
 LL |                 (|| { let bar = foo; bar.take() })();
    |                  ^^             --- move occurs because `foo` has type `&mut Option<&i32>`, which does not implement the `Copy` trait
@@ -8,6 +8,16 @@ LL |                 (|| { let bar = foo; bar.take() })();
    |
    = note: variables bound in patterns cannot be moved from until after the end of the pattern guard
 
-error: aborting due to previous error
+error[E0507]: cannot move out of `foo` in pattern guard
+  --> $DIR/issue-27282-mutation-in-guard.rs:20:18
+   |
+LL |                 (|| { let bar = foo; bar.take() })();
+   |                  ^^             --- move occurs because `foo` has type `&mut Option<&i32>`, which does not implement the `Copy` trait
+   |                  |
+   |                  move out of `foo` occurs here
+   |
+   = note: variables bound in patterns cannot be moved from until after the end of the pattern guard
+
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0507`.