Edition 2024: don't special case diverging blocks

Author: WaffleLapkin

compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs

@@ -46,19 +46,18 @@ use rustc_trait_selection::traits::{self, ObligationCauseCode, SelectionContext}
 use std::iter;
 use std::mem;
 
-#[derive(Clone, Copy, Default)]
+#[derive(Clone, Copy)]
 pub enum DivergingBlockBehavior {
-    /// This is the current stable behavior:
+    /// This is edition <= 2021 behavior:
     ///
     /// ```rust
     /// {
     ///     return;
     /// } // block has type = !, even though we are supposedly dropping it with `;`
     /// ```
-    #[default]
     Never,
 
-    /// Alternative behavior:
+    /// This is edition >= 2024 behavior:
     ///
     /// ```ignore (very-unstable-new-attribute)
     /// #![rustc_never_type_options(diverging_block_default = "unit")]

compiler/rustc_hir_typeck/src/fn_ctxt/mod.rs

@@ -441,7 +441,16 @@ fn parse_never_type_options_attr(
         if tcx.features().never_type_fallback { FallbackToNiko } else { FallbackToUnit }
     });
 
-    let block = block.unwrap_or_default();
+    let block = block.unwrap_or_else(|| default_diverging_block_behavior(tcx));
 
     (fallback, block)
 }
+
+/// Returns the edition-based default diverging block behavior
+fn default_diverging_block_behavior(tcx: TyCtxt<'_>) -> DivergingBlockBehavior {
+    if tcx.sess.edition().at_least_rust_2024() {
+        return DivergingBlockBehavior::Unit;
+    }
+
+    DivergingBlockBehavior::Never
+}

tests/ui/editions/diverging-block.e2024.stderr

@@ -0,0 +1,24 @@
+error[E0308]: mismatched types
+  --> $DIR/diverging-block.rs:15:18
+   |
+LL |       let _: u32 = {
+   |  __________________^
+LL | |         return;
+LL | |     };
+   | |_____^ expected `u32`, found `()`
+
+error[E0308]: `if` and `else` have incompatible types
+  --> $DIR/diverging-block.rs:25:9
+   |
+LL | /     if true {
+LL | |         return;
+   | |         ------- expected because of this
+LL | |     } else {
+LL | |         0_u32
+   | |         ^^^^^ expected `()`, found `u32`
+LL | |     };
+   | |_____- `if` and `else` have incompatible types
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0308`.

tests/ui/editions/diverging-block.rs

@@ -0,0 +1,27 @@
+//@ revisions: e2021 e2024
+//
+//@[e2021] edition: 2021
+//@[e2024] edition: 2024
+//@[e2024] compile-flags: -Zunstable-options
+//
+//@[e2021] check-pass
+//@[e2024] check-fail
+
+fn main() {
+    // a diverging block, with no tail expression.
+    //
+    // edition <= 2021: the block has type `!`, which then can be coerced.
+    // edition >= 2024: the block has type `()`, as with any block with no tail.
+    let _: u32 = { //[e2024]~ error: mismatched types
+        return;
+    };
+}
+
+fn _f() {
+    // Same as the above, but with an if
+    if true {
+        return;
+    } else {
+        0_u32 //[e2024]~ error: `if` and `else` have incompatible types
+    };
+}