Rollup merge of #120479 - estebank:issue-61788, r=wesleywiser

Suggest turning `if let` into irrefutable `let` if appropriate

When encountering an `if let` tail expression without an `else` arm for an enum with a single variant, suggest writing an irrefutable `let` binding instead.

```
error[E0317]: `if` may be missing an `else` clause
  --> $DIR/irrefutable-if-let-without-else.rs:8:5
   |
LL |   fn foo(x: Enum) -> i32 {
   |                      --- expected `i32` because of this return type
LL | /     if let Enum::Variant(value) = x {
LL | |         value
LL | |     }
   | |_____^ expected `i32`, found `()`
   |
   = note: `if` expressions without `else` evaluate to `()`
   = help: consider adding an `else` block that evaluates to the expected type
help: consider using an irrefutable `let` binding instead
   |
LL ~     let Enum::Variant(value) = x;
LL ~         value
   |
```

Fix #61788.

3 files changed, 114 insertions, 0 deletions

diff --git a/tests/ui/binding/irrefutable-if-let-without-else.fixed b/tests/ui/binding/irrefutable-if-let-without-else.fixed
new file mode 100644
index 00000000000..3d7f4695ca8
--- /dev/null
+++ b/tests/ui/binding/irrefutable-if-let-without-else.fixed
@@ -0,0 +1,25 @@
+// run-rustfix
+enum Enum {
+    Variant(i32),
+}
+struct Struct(i32);
+
+fn foo(x: Enum) -> i32 {
+    let Enum::Variant(value) = x;
+        value
+}
+fn bar(x: Enum) -> i32 {
+    let Enum::Variant(value) = x;
+        let x = value + 1;
+        x
+}
+fn baz(x: Struct) -> i32 {
+    let Struct(value) = x;
+        let x = value + 1;
+        x
+}
+fn main() {
+    let _ = foo(Enum::Variant(42));
+    let _ = bar(Enum::Variant(42));
+    let _ = baz(Struct(42));
+}
diff --git a/tests/ui/binding/irrefutable-if-let-without-else.rs b/tests/ui/binding/irrefutable-if-let-without-else.rs
new file mode 100644
index 00000000000..5aaf4ace3f8
--- /dev/null
+++ b/tests/ui/binding/irrefutable-if-let-without-else.rs
@@ -0,0 +1,28 @@
+// run-rustfix
+enum Enum {
+    Variant(i32),
+}
+struct Struct(i32);
+
+fn foo(x: Enum) -> i32 {
+    if let Enum::Variant(value) = x { //~ ERROR `if` may be missing an `else` clause
+        value
+    }
+}
+fn bar(x: Enum) -> i32 {
+    if let Enum::Variant(value) = x { //~ ERROR `if` may be missing an `else` clause
+        let x = value + 1;
+        x
+    }
+}
+fn baz(x: Struct) -> i32 {
+    if let Struct(value) = x { //~ ERROR `if` may be missing an `else` clause
+        let x = value + 1;
+        x
+    }
+}
+fn main() {
+    let _ = foo(Enum::Variant(42));
+    let _ = bar(Enum::Variant(42));
+    let _ = baz(Struct(42));
+}
diff --git a/tests/ui/binding/irrefutable-if-let-without-else.stderr b/tests/ui/binding/irrefutable-if-let-without-else.stderr
new file mode 100644
index 00000000000..e234cfdd945
--- /dev/null
+++ b/tests/ui/binding/irrefutable-if-let-without-else.stderr
@@ -0,0 +1,61 @@
+error[E0317]: `if` may be missing an `else` clause
+  --> $DIR/irrefutable-if-let-without-else.rs:8:5
+   |
+LL |   fn foo(x: Enum) -> i32 {
+   |                      --- expected `i32` because of this return type
+LL | /     if let Enum::Variant(value) = x {
+LL | |         value
+LL | |     }
+   | |_____^ expected `i32`, found `()`
+   |
+   = note: `if` expressions without `else` evaluate to `()`
+   = help: consider adding an `else` block that evaluates to the expected type
+help: consider using an irrefutable `let` binding instead
+   |
+LL ~     let Enum::Variant(value) = x;
+LL ~         value
+   |
+
+error[E0317]: `if` may be missing an `else` clause
+  --> $DIR/irrefutable-if-let-without-else.rs:13:5
+   |
+LL |   fn bar(x: Enum) -> i32 {
+   |                      --- expected `i32` because of this return type
+LL | /     if let Enum::Variant(value) = x {
+LL | |         let x = value + 1;
+LL | |         x
+LL | |     }
+   | |_____^ expected `i32`, found `()`
+   |
+   = note: `if` expressions without `else` evaluate to `()`
+   = help: consider adding an `else` block that evaluates to the expected type
+help: consider using an irrefutable `let` binding instead
+   |
+LL ~     let Enum::Variant(value) = x;
+LL ~         let x = value + 1;
+LL ~         x
+   |
+
+error[E0317]: `if` may be missing an `else` clause
+  --> $DIR/irrefutable-if-let-without-else.rs:19:5
+   |
+LL |   fn baz(x: Struct) -> i32 {
+   |                        --- expected `i32` because of this return type
+LL | /     if let Struct(value) = x {
+LL | |         let x = value + 1;
+LL | |         x
+LL | |     }
+   | |_____^ expected `i32`, found `()`
+   |
+   = note: `if` expressions without `else` evaluate to `()`
+   = help: consider adding an `else` block that evaluates to the expected type
+help: consider using an irrefutable `let` binding instead
+   |
+LL ~     let Struct(value) = x;
+LL ~         let x = value + 1;
+LL ~         x
+   |
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0317`.