Auto merge of #49371 - scottmcm:catch-wrapping, r=nikomatsakis

Add ok-wrapping to catch blocks, per RFC

Updates the `catch{}` lowering to wrap the result in `Try::from_ok`.

r? @nikomatsakis

Fixes #41414
Fixes #43818

src/test/compile-fail/catch-bad-lifetime.rs

@@ -21,7 +21,6 @@ pub fn main() {
             //~^ ERROR `my_string` does not live long enough
             Err(my_str) ?;
             Err("") ?;
-            Ok(())
         };
     }
 
@@ -32,7 +31,6 @@ pub fn main() {
         let mut j: Result<(), &mut i32> = do catch {
             Err(k) ?;
             i = 10; //~ ERROR cannot assign to `i` because it is borrowed
-            Ok(())
         };
         ::std::mem::drop(k); //~ ERROR use of moved value: `k`
         i = 40; //~ ERROR cannot assign to `i` because it is borrowed

src/test/compile-fail/catch-bad-type.rs

@@ -11,11 +11,18 @@
 #![feature(catch_expr)]
 
 pub fn main() {
-    let res: Result<i32, i32> = do catch {
+    let res: Result<u32, i32> = do catch {
         Err("")?; //~ ERROR the trait bound `i32: std::convert::From<&str>` is not satisfied
-        Ok(5)
+        5
     };
+
     let res: Result<i32, i32> = do catch {
-        Ok("") //~ mismatched types
+        "" //~ ERROR type mismatch
     };
+
+    let res: Result<i32, i32> = do catch { }; //~ ERROR type mismatch
+
+    let res: () = do catch { }; //~ the trait bound `(): std::ops::Try` is not satisfied
+
+    let res: i32 = do catch { 5 }; //~ ERROR the trait bound `i32: std::ops::Try` is not satisfied
 }

src/test/compile-fail/catch-maybe-bad-lifetime.rs

@@ -17,7 +17,7 @@ pub fn main() {
         let mut i = 222;
         let x: Result<&i32, ()> = do catch {
             Err(())?;
-            Ok(&i)
+            &i
         };
         x.ok().cloned();
         i = 0; //~ ERROR cannot assign to `i` because it is borrowed
@@ -29,7 +29,6 @@ pub fn main() {
         let _y: Result<(), ()> = do catch {
             Err(())?;
             ::std::mem::drop(x);
-            Ok(())
         };
         println!("{}", x); //~ ERROR use of moved value: `x`
     }
@@ -42,7 +41,6 @@ pub fn main() {
         let x: Result<(), ()> = do catch {
             Err(())?;
             j = &i;
-            Ok(())
         };
         i = 0; //~ ERROR cannot assign to `i` because it is borrowed
         let _ = i;

src/test/compile-fail/catch-opt-init.rs

@@ -19,7 +19,6 @@ pub fn main() {
         cfg_res = 5;
         Ok::<(), ()>(())?;
         use_val(cfg_res);
-        Ok(())
     };
     assert_eq!(cfg_res, 5); //~ ERROR use of possibly uninitialized variable
 }

src/test/run-pass/catch-expr.rs

@@ -13,11 +13,11 @@
 struct catch {}
 
 pub fn main() {
-    let catch_result = do catch {
+    let catch_result: Option<_> = do catch {
         let x = 5;
         x
     };
-    assert_eq!(catch_result, 5);
+    assert_eq!(catch_result, Some(5));
 
     let mut catch = true;
     while catch { catch = false; }
@@ -30,16 +30,16 @@ pub fn main() {
         _ => {}
     };
 
-    let catch_err = do catch {
+    let catch_err: Result<_, i32> = do catch {
         Err(22)?;
-        Ok(1)
+        1
     };
     assert_eq!(catch_err, Err(22));
 
     let catch_okay: Result<i32, i32> = do catch {
         if false { Err(25)?; }
         Ok::<(), i32>(())?;
-        Ok(28)
+        28
     };
     assert_eq!(catch_okay, Ok(28));
 
@@ -47,14 +47,13 @@ pub fn main() {
         for i in 0..10 {
             if i < 5 { Ok::<i32, i32>(i)?; } else { Err(i)?; }
         }
-        Ok(22)
+        22
     };
     assert_eq!(catch_from_loop, Err(5));
 
     let cfg_init;
     let _res: Result<(), ()> = do catch {
         cfg_init = 5;
-        Ok(())
     };
     assert_eq!(cfg_init, 5);
 
@@ -62,19 +61,19 @@ pub fn main() {
     let _res: Result<(), ()> = do catch {
         cfg_init_2 = 6;
         Err(())?;
-        Ok(())
     };
     assert_eq!(cfg_init_2, 6);
 
     let my_string = "test".to_string();
     let res: Result<&str, ()> = do catch {
-        Ok(&my_string)
+        // Unfortunately, deref doesn't fire here (#49356)
+        &my_string[..]
     };
     assert_eq!(res, Ok("test"));
 
-    do catch {
-        ()
-    }
+    let my_opt: Option<_> = do catch { () };
+    assert_eq!(my_opt, Some(()));
 
-    ();
+    let my_opt: Option<_> = do catch { };
+    assert_eq!(my_opt, Some(()));
 }

src/test/ui/catch-block-type-error.rs

@@ -0,0 +1,26 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![feature(catch_expr)]
+
+fn foo() -> Option<()> { Some(()) }
+
+fn main() {
+    let _: Option<f32> = do catch {
+        foo()?;
+        42
+        //~^ ERROR type mismatch
+    };
+
+    let _: Option<i32> = do catch {
+        foo()?;
+    };
+    //~^ ERROR type mismatch
+}

src/test/ui/catch-block-type-error.stderr

@@ -0,0 +1,21 @@
+error[E0271]: type mismatch resolving `<std::option::Option<f32> as std::ops::Try>::Ok == {integer}`
+  --> $DIR/catch-block-type-error.rs:18:9
+   |
+LL |         42
+   |         ^^ expected f32, found integral variable
+   |
+   = note: expected type `f32`
+              found type `{integer}`
+
+error[E0271]: type mismatch resolving `<std::option::Option<i32> as std::ops::Try>::Ok == ()`
+  --> $DIR/catch-block-type-error.rs:24:5
+   |
+LL |     };
+   |     ^ expected i32, found ()
+   |
+   = note: expected type `i32`
+              found type `()`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0271`.