Rollup merge of #122843 - WaffleLapkin:semicolon-vs-the-never, r=compiler-errors

Add a never type option to make diverging blocks `()`

More experiments for ~~the blood god~~ T-lang!

Usage example:
```rust
 #![allow(internal_features)]
 #![feature(never_type, rustc_attrs)]
 #![rustc_never_type_options(diverging_block_default = "unit")]

fn main() {
    let _: u8 = { //~ error: expected `u8`, found `()`
        return;
    };
}
```

r? compiler-errors

I'm not sure how I feel about parsing the attribute every time we create `FnCtxt`. There must be a better way to do this, right?

compiler/rustc_feature/src/builtin_attrs.rs

@@ -597,9 +597,12 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
     ),
 
     rustc_attr!(
-        rustc_never_type_mode, Normal, template!(NameValueStr: "fallback_to_unit|fallback_to_niko|fallback_to_never|no_fallback"), ErrorFollowing,
+        rustc_never_type_options,
+        Normal,
+        template!(List: r#"/*opt*/ fallback = "unit|niko|never|no""#),
+        ErrorFollowing,
         EncodeCrossCrate::No,
-        "`rustc_never_type_fallback` is used to experiment with never type fallback and work on \
+        "`rustc_never_type_options` is used to experiment with never type fallback and work on \
          never type stabilization, and will never be stable"
     ),
 

compiler/rustc_hir_typeck/src/fallback.rs

@@ -4,12 +4,11 @@ use rustc_data_structures::{
     graph::{iterate::DepthFirstSearch, vec_graph::VecGraph},
     unord::{UnordBag, UnordMap, UnordSet},
 };
-use rustc_hir::def_id::CRATE_DEF_ID;
 use rustc_infer::infer::{DefineOpaqueTypes, InferOk};
 use rustc_middle::ty::{self, Ty};
-use rustc_span::sym;
 
-enum DivergingFallbackBehavior {
+#[derive(Copy, Clone)]
+pub enum DivergingFallbackBehavior {
     /// Always fallback to `()` (aka "always spontaneous decay")
     FallbackToUnit,
     /// Sometimes fallback to `!`, but mainly fallback to `()` so that most of the crates are not broken.
@@ -78,9 +77,8 @@ impl<'tcx> FnCtxt<'_, 'tcx> {
             return false;
         }
 
-        let diverging_behavior = self.diverging_fallback_behavior();
+        let diverging_fallback = self
-        let diverging_fallback =
+            .calculate_diverging_fallback(&unresolved_variables, self.diverging_fallback_behavior);
-            self.calculate_diverging_fallback(&unresolved_variables, diverging_behavior);
 
         // We do fallback in two passes, to try to generate
         // better error messages.
@@ -94,32 +92,6 @@ impl<'tcx> FnCtxt<'_, 'tcx> {
         fallback_occurred
     }
 
-    fn diverging_fallback_behavior(&self) -> DivergingFallbackBehavior {
-        let Some((mode, span)) = self
-            .tcx
-            .get_attr(CRATE_DEF_ID, sym::rustc_never_type_mode)
-            .map(|attr| (attr.value_str().unwrap(), attr.span))
-        else {
-            if self.tcx.features().never_type_fallback {
-                return DivergingFallbackBehavior::FallbackToNiko;
-            }
-
-            return DivergingFallbackBehavior::FallbackToUnit;
-        };
-
-        match mode {
-            sym::fallback_to_unit => DivergingFallbackBehavior::FallbackToUnit,
-            sym::fallback_to_niko => DivergingFallbackBehavior::FallbackToNiko,
-            sym::fallback_to_never => DivergingFallbackBehavior::FallbackToNever,
-            sym::no_fallback => DivergingFallbackBehavior::NoFallback,
-            _ => {
-                self.tcx.dcx().span_err(span, format!("unknown never type mode: `{mode}` (supported: `fallback_to_unit`, `fallback_to_niko`, `fallback_to_never` and `no_fallback`)"));
-
-                DivergingFallbackBehavior::FallbackToUnit
-            }
-        }
-    }
-
     fn fallback_effects(&self) -> bool {
         let unsolved_effects = self.unsolved_effects();
 

compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs

@@ -45,6 +45,29 @@ use rustc_trait_selection::traits::{self, ObligationCauseCode, SelectionContext}
 use std::iter;
 use std::mem;
 
+#[derive(Clone, Copy, Default)]
+pub enum DivergingBlockBehavior {
+    /// This is the current stable behavior:
+    ///
+    /// ```rust
+    /// {
+    ///     return;
+    /// } // block has type = !, even though we are supposedly dropping it with `;`
+    /// ```
+    #[default]
+    Never,
+
+    /// Alternative behavior:
+    ///
+    /// ```ignore (very-unstable-new-attribute)
+    /// #![rustc_never_type_options(diverging_block_default = "unit")]
+    /// {
+    ///     return;
+    /// } // block has type = (), since we are dropping `!` from `return` with `;`
+    /// ```
+    Unit,
+}
+
 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     pub(in super::super) fn check_casts(&mut self) {
         // don't hold the borrow to deferred_cast_checks while checking to avoid borrow checker errors
@@ -1710,7 +1733,9 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 //
                 // #41425 -- label the implicit `()` as being the
                 // "found type" here, rather than the "expected type".
-                if !self.diverges.get().is_always() {
+                if !self.diverges.get().is_always()
+                    || matches!(self.diverging_block_behavior, DivergingBlockBehavior::Unit)
+                {
                     // #50009 -- Do not point at the entire fn block span, point at the return type
                     // span, as it is the cause of the requirement, and
                     // `consider_hint_about_removing_semicolon` will point at the last expression

compiler/rustc_hir_typeck/src/fn_ctxt/mod.rs

@@ -5,7 +5,10 @@ mod checks;
 mod suggestions;
 
 use crate::coercion::DynamicCoerceMany;
+use crate::fallback::DivergingFallbackBehavior;
+use crate::fn_ctxt::checks::DivergingBlockBehavior;
 use crate::{CoroutineTypes, Diverges, EnclosingBreakables, Inherited};
+use hir::def_id::CRATE_DEF_ID;
 use rustc_errors::{DiagCtxt, ErrorGuaranteed};
 use rustc_hir as hir;
 use rustc_hir::def_id::{DefId, LocalDefId};
@@ -18,7 +21,7 @@ use rustc_middle::infer::unify_key::{ConstVariableOrigin, ConstVariableOriginKin
 use rustc_middle::ty::{self, Const, Ty, TyCtxt, TypeVisitableExt};
 use rustc_session::Session;
 use rustc_span::symbol::Ident;
-use rustc_span::{self, Span, DUMMY_SP};
+use rustc_span::{self, sym, Span, DUMMY_SP};
 use rustc_trait_selection::traits::{ObligationCause, ObligationCauseCode, ObligationCtxt};
 
 use std::cell::{Cell, RefCell};
@@ -108,6 +111,9 @@ pub struct FnCtxt<'a, 'tcx> {
     pub(super) inh: &'a Inherited<'tcx>,
 
     pub(super) fallback_has_occurred: Cell<bool>,
+
+    pub(super) diverging_fallback_behavior: DivergingFallbackBehavior,
+    pub(super) diverging_block_behavior: DivergingBlockBehavior,
 }
 
 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
@@ -116,6 +122,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         param_env: ty::ParamEnv<'tcx>,
         body_id: LocalDefId,
     ) -> FnCtxt<'a, 'tcx> {
+        let (diverging_fallback_behavior, diverging_block_behavior) =
+            parse_never_type_options_attr(inh.tcx);
         FnCtxt {
             body_id,
             param_env,
@@ -131,6 +139,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             }),
             inh,
             fallback_has_occurred: Cell::new(false),
+            diverging_fallback_behavior,
+            diverging_block_behavior,
         }
     }
 
@@ -374,3 +384,64 @@ impl<'tcx> LoweredTy<'tcx> {
         LoweredTy { raw, normalized }
     }
 }
+
+fn parse_never_type_options_attr(
+    tcx: TyCtxt<'_>,
+) -> (DivergingFallbackBehavior, DivergingBlockBehavior) {
+    use DivergingFallbackBehavior::*;
+
+    // Error handling is dubious here (unwraps), but that's probably fine for an internal attribute.
+    // Just don't write incorrect attributes <3
+
+    let mut fallback = None;
+    let mut block = None;
+
+    let items = tcx
+        .get_attr(CRATE_DEF_ID, sym::rustc_never_type_options)
+        .map(|attr| attr.meta_item_list().unwrap())
+        .unwrap_or_default();
+
+    for item in items {
+        if item.has_name(sym::fallback) && fallback.is_none() {
+            let mode = item.value_str().unwrap();
+            match mode {
+                sym::unit => fallback = Some(FallbackToUnit),
+                sym::niko => fallback = Some(FallbackToNiko),
+                sym::never => fallback = Some(FallbackToNever),
+                sym::no => fallback = Some(NoFallback),
+                _ => {
+                    tcx.dcx().span_err(item.span(), format!("unknown never type fallback mode: `{mode}` (supported: `unit`, `niko`, `never` and `no`)"));
+                }
+            };
+            continue;
+        }
+
+        if item.has_name(sym::diverging_block_default) && fallback.is_none() {
+            let mode = item.value_str().unwrap();
+            match mode {
+                sym::unit => block = Some(DivergingBlockBehavior::Unit),
+                sym::never => block = Some(DivergingBlockBehavior::Never),
+                _ => {
+                    tcx.dcx().span_err(item.span(), format!("unknown diverging block default: `{mode}` (supported: `unit` and `never`)"));
+                }
+            };
+            continue;
+        }
+
+        tcx.dcx().span_err(
+            item.span(),
+            format!(
+                "unknown never type option: `{}` (supported: `fallback`)",
+                item.name_or_empty()
+            ),
+        );
+    }
+
+    let fallback = fallback.unwrap_or_else(|| {
+        if tcx.features().never_type_fallback { FallbackToNiko } else { FallbackToUnit }
+    });
+
+    let block = block.unwrap_or_default();
+
+    (fallback, block)
+}

compiler/rustc_span/src/symbol.rs

@@ -691,6 +691,7 @@ symbols! {
         dispatch_from_dyn,
         div,
         div_assign,
+        diverging_block_default,
         do_not_recommend,
         doc,
         doc_alias,
@@ -816,9 +817,7 @@ symbols! {
         fadd_algebraic,
         fadd_fast,
         fake_variadic,
-        fallback_to_never,
+        fallback,
-        fallback_to_niko,
-        fallback_to_unit,
         fdiv_algebraic,
         fdiv_fast,
         feature,
@@ -1230,6 +1229,7 @@ symbols! {
         new_v1,
         new_v1_formatted,
         next,
+        niko,
         nll,
         no,
         no_builtins,
@@ -1238,7 +1238,6 @@ symbols! {
         no_crate_inject,
         no_debug,
         no_default_passes,
-        no_fallback,
         no_implicit_prelude,
         no_inline,
         no_link,
@@ -1556,7 +1555,7 @@ symbols! {
         rustc_mir,
         rustc_must_implement_one_of,
         rustc_never_returns_null_ptr,
-        rustc_never_type_mode,
+        rustc_never_type_options,
         rustc_no_mir_inline,
         rustc_nonnull_optimization_guaranteed,
         rustc_nounwind,