From 5a440f1bc29c06fdaa93ce8e05da70fc081f74ba Mon Sep 17 00:00:00 2001
From: varkor <github@varkor.com>
Date: Tue, 29 Sep 2020 01:03:02 +0100
Subject: [PATCH 1/3] Fix control flow check for breaking with diverging values

---
 compiler/rustc_typeck/src/check/expr.rs  |  5 ++++-
 src/test/ui/break-diverging-value.rs     | 13 +++++++++++++
 src/test/ui/break-diverging-value.stderr | 11 +++++++++++
 3 files changed, 28 insertions(+), 1 deletion(-)
 create mode 100644 src/test/ui/break-diverging-value.rs
 create mode 100644 src/test/ui/break-diverging-value.stderr

diff --git a/compiler/rustc_typeck/src/check/expr.rs b/compiler/rustc_typeck/src/check/expr.rs
index 9990f86a36b13..15820d367c0a6 100644
--- a/compiler/rustc_typeck/src/check/expr.rs
+++ b/compiler/rustc_typeck/src/check/expr.rs
@@ -627,7 +627,10 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 assert!(expr_opt.is_none() || self.tcx.sess.has_errors());
             }
 
-            ctxt.may_break = true;
+            // If we encountered a `break`, then (no surprise) it may be possible to break from the
+            // loop... unless the value being returned from the loop diverges itself, e.g.
+            // `break return 5` or `break loop {}`.
+            ctxt.may_break |= !e_ty.is_never();
 
             // the type of a `break` is always `!`, since it diverges
             tcx.types.never
diff --git a/src/test/ui/break-diverging-value.rs b/src/test/ui/break-diverging-value.rs
new file mode 100644
index 0000000000000..0498580c7bd8a
--- /dev/null
+++ b/src/test/ui/break-diverging-value.rs
@@ -0,0 +1,13 @@
+fn loop_break_return() -> i32 {
+    let loop_value = loop { break return 0 }; // ok
+}
+
+fn loop_break_loop() -> i32 {
+    let loop_value = loop { break loop {} }; // ok
+}
+
+fn loop_break_break() -> i32 { //~ ERROR mismatched types
+    let loop_value = loop { break break };
+}
+
+fn main() {}
diff --git a/src/test/ui/break-diverging-value.stderr b/src/test/ui/break-diverging-value.stderr
new file mode 100644
index 0000000000000..78a8d78ed3400
--- /dev/null
+++ b/src/test/ui/break-diverging-value.stderr
@@ -0,0 +1,11 @@
+error[E0308]: mismatched types
+  --> $DIR/break-diverging-value.rs:9:26
+   |
+LL | fn loop_break_break() -> i32 {
+   |    ----------------      ^^^ expected `i32`, found `()`
+   |    |
+   |    implicitly returns `()` as its body has no tail or `return` expression
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.

From d415fae8b76cddbcd7825cb6642186a833453b3c Mon Sep 17 00:00:00 2001
From: varkor <github@varkor.com>
Date: Wed, 21 Oct 2020 22:52:41 +0100
Subject: [PATCH 2/3] Add tests for uninhabited types

---
 src/test/ui/break-diverging-value.rs     | 25 ++++++++++++++++++++
 src/test/ui/break-diverging-value.stderr | 29 +++++++++++++++++++++---
 2 files changed, 51 insertions(+), 3 deletions(-)

diff --git a/src/test/ui/break-diverging-value.rs b/src/test/ui/break-diverging-value.rs
index 0498580c7bd8a..0117abeb105cf 100644
--- a/src/test/ui/break-diverging-value.rs
+++ b/src/test/ui/break-diverging-value.rs
@@ -1,3 +1,5 @@
+#![feature(never_type)]
+
 fn loop_break_return() -> i32 {
     let loop_value = loop { break return 0 }; // ok
 }
@@ -10,4 +12,27 @@ fn loop_break_break() -> i32 { //~ ERROR mismatched types
     let loop_value = loop { break break };
 }
 
+fn loop_break_return_2() -> i32 { //~ ERROR mismatched types
+    let loop_value = loop { break { return; () } };
+    //~^ ERROR `return;` in a function whose return type is not `()`
+}
+
+enum Void {}
+
+fn get_void() -> Void {
+    panic!()
+}
+
+fn loop_break_void() -> i32 { //~ ERROR mismatched types
+    let loop_value = loop { break get_void() };
+}
+
+fn get_never() -> ! {
+    panic!()
+}
+
+fn loop_break_never() -> i32 {
+    let loop_value = loop { break get_never() }; // ok
+}
+
 fn main() {}
diff --git a/src/test/ui/break-diverging-value.stderr b/src/test/ui/break-diverging-value.stderr
index 78a8d78ed3400..de3f9bd778a9d 100644
--- a/src/test/ui/break-diverging-value.stderr
+++ b/src/test/ui/break-diverging-value.stderr
@@ -1,11 +1,34 @@
 error[E0308]: mismatched types
-  --> $DIR/break-diverging-value.rs:9:26
+  --> $DIR/break-diverging-value.rs:11:26
    |
 LL | fn loop_break_break() -> i32 {
    |    ----------------      ^^^ expected `i32`, found `()`
    |    |
    |    implicitly returns `()` as its body has no tail or `return` expression
 
-error: aborting due to previous error
+error[E0069]: `return;` in a function whose return type is not `()`
+  --> $DIR/break-diverging-value.rs:16:37
+   |
+LL |     let loop_value = loop { break { return; () } };
+   |                                     ^^^^^^ return type is not `()`
+
+error[E0308]: mismatched types
+  --> $DIR/break-diverging-value.rs:15:29
+   |
+LL | fn loop_break_return_2() -> i32 {
+   |    -------------------      ^^^ expected `i32`, found `()`
+   |    |
+   |    implicitly returns `()` as its body has no tail or `return` expression
+
+error[E0308]: mismatched types
+  --> $DIR/break-diverging-value.rs:26:25
+   |
+LL | fn loop_break_void() -> i32 {
+   |    ---------------      ^^^ expected `i32`, found `()`
+   |    |
+   |    implicitly returns `()` as its body has no tail or `return` expression
+
+error: aborting due to 4 previous errors
 
-For more information about this error, try `rustc --explain E0308`.
+Some errors have detailed explanations: E0069, E0308.
+For more information about an error, try `rustc --explain E0069`.

From d1c2815d6aa131182aa93603de215d63c52d0c53 Mon Sep 17 00:00:00 2001
From: varkor <github@varkor.com>
Date: Thu, 22 Oct 2020 23:12:46 +0100
Subject: [PATCH 3/3] Use `diverges` instead of `!`-type

---
 compiler/rustc_typeck/src/check/expr.rs  |  2 +-
 src/test/ui/break-diverging-value.rs     |  5 ++---
 src/test/ui/break-diverging-value.stderr | 21 +++------------------
 3 files changed, 6 insertions(+), 22 deletions(-)

diff --git a/compiler/rustc_typeck/src/check/expr.rs b/compiler/rustc_typeck/src/check/expr.rs
index 15820d367c0a6..dce3be9399dd6 100644
--- a/compiler/rustc_typeck/src/check/expr.rs
+++ b/compiler/rustc_typeck/src/check/expr.rs
@@ -630,7 +630,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             // If we encountered a `break`, then (no surprise) it may be possible to break from the
             // loop... unless the value being returned from the loop diverges itself, e.g.
             // `break return 5` or `break loop {}`.
-            ctxt.may_break |= !e_ty.is_never();
+            ctxt.may_break |= !self.diverges.get().is_always();
 
             // the type of a `break` is always `!`, since it diverges
             tcx.types.never
diff --git a/src/test/ui/break-diverging-value.rs b/src/test/ui/break-diverging-value.rs
index 0117abeb105cf..d070fddaffc19 100644
--- a/src/test/ui/break-diverging-value.rs
+++ b/src/test/ui/break-diverging-value.rs
@@ -12,9 +12,8 @@ fn loop_break_break() -> i32 { //~ ERROR mismatched types
     let loop_value = loop { break break };
 }
 
-fn loop_break_return_2() -> i32 { //~ ERROR mismatched types
-    let loop_value = loop { break { return; () } };
-    //~^ ERROR `return;` in a function whose return type is not `()`
+fn loop_break_return_2() -> i32 {
+    let loop_value = loop { break { return 0; () } }; // ok
 }
 
 enum Void {}
diff --git a/src/test/ui/break-diverging-value.stderr b/src/test/ui/break-diverging-value.stderr
index de3f9bd778a9d..69edcd2408002 100644
--- a/src/test/ui/break-diverging-value.stderr
+++ b/src/test/ui/break-diverging-value.stderr
@@ -6,29 +6,14 @@ LL | fn loop_break_break() -> i32 {
    |    |
    |    implicitly returns `()` as its body has no tail or `return` expression
 
-error[E0069]: `return;` in a function whose return type is not `()`
-  --> $DIR/break-diverging-value.rs:16:37
-   |
-LL |     let loop_value = loop { break { return; () } };
-   |                                     ^^^^^^ return type is not `()`
-
-error[E0308]: mismatched types
-  --> $DIR/break-diverging-value.rs:15:29
-   |
-LL | fn loop_break_return_2() -> i32 {
-   |    -------------------      ^^^ expected `i32`, found `()`
-   |    |
-   |    implicitly returns `()` as its body has no tail or `return` expression
-
 error[E0308]: mismatched types
-  --> $DIR/break-diverging-value.rs:26:25
+  --> $DIR/break-diverging-value.rs:25:25
    |
 LL | fn loop_break_void() -> i32 {
    |    ---------------      ^^^ expected `i32`, found `()`
    |    |
    |    implicitly returns `()` as its body has no tail or `return` expression
 
-error: aborting due to 4 previous errors
+error: aborting due to 2 previous errors
 
-Some errors have detailed explanations: E0069, E0308.
-For more information about an error, try `rustc --explain E0069`.
+For more information about this error, try `rustc --explain E0308`.