Rollup merge of #98784 - compiler-errors:forgot-to-return-binding, r=estebank

Suggest returning local on "expected `ty`, found `()`" due to expr-less block

Putting this up for _initial_ review. Notably, this doesn't consider if the value has possibly been moved, or whether the type is `Copy`. It also provides a structured suggestion if there's one "preferred" binding that matches the type (i.e. one binding in the block or its parent), otherwise it just points them out if there's fewer than 4 of them.

Fixes #98177

r? `@estebank`

5 files changed, 135 insertions, 0 deletions

diff --git a/src/test/ui/suggestions/return-bindings-multi.rs b/src/test/ui/suggestions/return-bindings-multi.rs
new file mode 100644
index 00000000000..8c3bd641e97
--- /dev/null
+++ b/src/test/ui/suggestions/return-bindings-multi.rs
@@ -0,0 +1,9 @@
+fn a(i: i32) -> i32 {
+    //~^ ERROR mismatched types
+    let j = 2i32;
+}
+
+fn b(i: i32, j: i32) -> i32 {}
+//~^ ERROR mismatched types
+
+fn main() {}
diff --git a/src/test/ui/suggestions/return-bindings-multi.stderr b/src/test/ui/suggestions/return-bindings-multi.stderr
new file mode 100644
index 00000000000..738e3f2f4be
--- /dev/null
+++ b/src/test/ui/suggestions/return-bindings-multi.stderr
@@ -0,0 +1,34 @@
+error[E0308]: mismatched types
+  --> $DIR/return-bindings-multi.rs:1:17
+   |
+LL | fn a(i: i32) -> i32 {
+   |    -            ^^^ expected `i32`, found `()`
+   |    |
+   |    implicitly returns `()` as its body has no tail or `return` expression
+   |
+note: consider returning one of these bindings
+  --> $DIR/return-bindings-multi.rs:1:6
+   |
+LL | fn a(i: i32) -> i32 {
+   |      ^
+LL |
+LL |     let j = 2i32;
+   |         ^
+
+error[E0308]: mismatched types
+  --> $DIR/return-bindings-multi.rs:6:25
+   |
+LL | fn b(i: i32, j: i32) -> i32 {}
+   |    -                    ^^^ expected `i32`, found `()`
+   |    |
+   |    implicitly returns `()` as its body has no tail or `return` expression
+   |
+note: consider returning one of these bindings
+  --> $DIR/return-bindings-multi.rs:6:6
+   |
+LL | fn b(i: i32, j: i32) -> i32 {}
+   |      ^       ^
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0308`.
diff --git a/src/test/ui/suggestions/return-bindings.fixed b/src/test/ui/suggestions/return-bindings.fixed
new file mode 100644
index 00000000000..4fabc411abc
--- /dev/null
+++ b/src/test/ui/suggestions/return-bindings.fixed
@@ -0,0 +1,23 @@
+// run-rustfix
+
+#![allow(unused)]
+
+fn a(i: i32) -> i32 { i }
+//~^ ERROR mismatched types
+
+fn b(opt_str: Option<String>) {
+    let s: String = if let Some(s) = opt_str {
+        s
+    //~^ ERROR mismatched types
+    } else {
+        String::new()
+    };
+}
+
+fn c() -> Option<i32> {
+    //~^ ERROR mismatched types
+    let x = Some(1);
+    x
+}
+
+fn main() {}
diff --git a/src/test/ui/suggestions/return-bindings.rs b/src/test/ui/suggestions/return-bindings.rs
new file mode 100644
index 00000000000..d05b4ba27d6
--- /dev/null
+++ b/src/test/ui/suggestions/return-bindings.rs
@@ -0,0 +1,21 @@
+// run-rustfix
+
+#![allow(unused)]
+
+fn a(i: i32) -> i32 {}
+//~^ ERROR mismatched types
+
+fn b(opt_str: Option<String>) {
+    let s: String = if let Some(s) = opt_str {
+        //~^ ERROR mismatched types
+    } else {
+        String::new()
+    };
+}
+
+fn c() -> Option<i32> {
+    //~^ ERROR mismatched types
+    let x = Some(1);
+}
+
+fn main() {}
diff --git a/src/test/ui/suggestions/return-bindings.stderr b/src/test/ui/suggestions/return-bindings.stderr
new file mode 100644
index 00000000000..e5d49255005
--- /dev/null
+++ b/src/test/ui/suggestions/return-bindings.stderr
@@ -0,0 +1,48 @@
+error[E0308]: mismatched types
+  --> $DIR/return-bindings.rs:5:17
+   |
+LL | fn a(i: i32) -> i32 {}
+   |    -            ^^^ expected `i32`, found `()`
+   |    |
+   |    implicitly returns `()` as its body has no tail or `return` expression
+   |
+help: consider returning the local binding `i`
+   |
+LL | fn a(i: i32) -> i32 { i }
+   |                       +
+
+error[E0308]: mismatched types
+  --> $DIR/return-bindings.rs:9:46
+   |
+LL |       let s: String = if let Some(s) = opt_str {
+   |  ______________________________________________^
+LL | |
+LL | |     } else {
+   | |_____^ expected struct `String`, found `()`
+   |
+help: consider returning the local binding `s`
+   |
+LL ~     let s: String = if let Some(s) = opt_str {
+LL +         s
+LL ~
+   |
+
+error[E0308]: mismatched types
+  --> $DIR/return-bindings.rs:16:11
+   |
+LL | fn c() -> Option<i32> {
+   |    -      ^^^^^^^^^^^ expected enum `Option`, found `()`
+   |    |
+   |    implicitly returns `()` as its body has no tail or `return` expression
+   |
+   = note:   expected enum `Option<i32>`
+           found unit type `()`
+help: consider returning the local binding `x`
+   |
+LL ~     let x = Some(1);
+LL +     x
+   |
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0308`.