Rollup merge of #100094 - lyming2007:issue-98982, r=estebank

Detect type mismatch due to loop that might never iterate

When loop as tail expression causes a miss match type E0308 error, recursively get the return statement and add diagnostic information on it.

3 files changed, 41 insertions, 0 deletions

diff --git a/src/test/ui/for-loop-while/break-while-condition.stderr b/src/test/ui/for-loop-while/break-while-condition.stderr
index 6960c4fd867..e79f6a75fde 100644
--- a/src/test/ui/for-loop-while/break-while-condition.stderr
+++ b/src/test/ui/for-loop-while/break-while-condition.stderr
@@ -31,6 +31,14 @@ LL | |             }
    |
    = note:   expected type `!`
            found unit type `()`
+note: the function expects a value to always be returned, but loops might run zero times
+  --> $DIR/break-while-condition.rs:24:13
+   |
+LL |             while false {
+   |             ^^^^^^^^^^^ this might have zero elements to iterate on
+LL |                 return
+   |                 ------ if the loop doesn't execute, this value would never get returned
+   = help: return a value for the case when the loop has zero elements to iterate on, or consider changing the return type to account for that possibility
 
 error: aborting due to 3 previous errors
 
diff --git a/src/test/ui/typeck/issue-98982.rs b/src/test/ui/typeck/issue-98982.rs
new file mode 100644
index 00000000000..2553824bbfe
--- /dev/null
+++ b/src/test/ui/typeck/issue-98982.rs
@@ -0,0 +1,9 @@
+fn foo() -> i32 {
+    for i in 0..0 {
+    //~^ ERROR: mismatched types [E0308]
+        return i;
+    }
+    //~| help: return a value for the case when the loop has zero elements to iterate on, or consider changing the return type to account for that possibility
+}
+
+fn main() {}
diff --git a/src/test/ui/typeck/issue-98982.stderr b/src/test/ui/typeck/issue-98982.stderr
new file mode 100644
index 00000000000..3c9806ac965
--- /dev/null
+++ b/src/test/ui/typeck/issue-98982.stderr
@@ -0,0 +1,24 @@
+error[E0308]: mismatched types
+  --> $DIR/issue-98982.rs:2:5
+   |
+LL |   fn foo() -> i32 {
+   |               --- expected `i32` because of return type
+LL | /     for i in 0..0 {
+LL | |
+LL | |         return i;
+LL | |     }
+   | |_____^ expected `i32`, found `()`
+   |
+note: the function expects a value to always be returned, but loops might run zero times
+  --> $DIR/issue-98982.rs:2:5
+   |
+LL |     for i in 0..0 {
+   |     ^^^^^^^^^^^^^ this might have zero elements to iterate on
+LL |
+LL |         return i;
+   |         -------- if the loop doesn't execute, this value would never get returned
+   = help: return a value for the case when the loop has zero elements to iterate on, or consider changing the return type to account for that possibility
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.