Auto merge of #121421 - saethlin:smarter-mono, r=oli-obk

Avoid lowering code under dead SwitchInt targets

The objective of this PR is to detect and eliminate code which is guarded by an `if false`, even if that `false` is a constant which is not known until monomorphization, or is `intrinsics::debug_assertions()`.

The effect of this is that we generate no LLVM IR the standard library's unsafe preconditions, when they are compiled in a build where they should be immediately optimized out. This mono-time optimization ensures that builds which disable debug assertions do not grow a linkage requirement against `core`, which compiler-builtins currently needs: https://github.com/rust-lang/rust/issues/121552

This revives the codegen side of https://github.com/rust-lang/rust/pull/91222 as planned in https://github.com/rust-lang/rust/issues/120848.

2 files changed, 20 insertions, 1 deletions

diff --git a/compiler/rustc_codegen_ssa/src/mir/block.rs b/compiler/rustc_codegen_ssa/src/mir/block.rs
index d3f5de25d9a..9bb2a528265 100644
--- a/compiler/rustc_codegen_ssa/src/mir/block.rs
+++ b/compiler/rustc_codegen_ssa/src/mir/block.rs
@@ -1237,6 +1237,16 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
         }
     }
 
+    pub fn codegen_block_as_unreachable(&mut self, bb: mir::BasicBlock) {
+        let llbb = match self.try_llbb(bb) {
+            Some(llbb) => llbb,
+            None => return,
+        };
+        let bx = &mut Bx::build(self.cx, llbb);
+        debug!("codegen_block_as_unreachable({:?})", bb);
+        bx.unreachable();
+    }
+
     fn codegen_terminator(
         &mut self,
         bx: &mut Bx,
diff --git a/compiler/rustc_codegen_ssa/src/mir/mod.rs b/compiler/rustc_codegen_ssa/src/mir/mod.rs
index a6fcf1fd38c..bac10f31336 100644
--- a/compiler/rustc_codegen_ssa/src/mir/mod.rs
+++ b/compiler/rustc_codegen_ssa/src/mir/mod.rs
@@ -256,13 +256,22 @@ pub fn codegen_mir<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
     // Apply debuginfo to the newly allocated locals.
     fx.debug_introduce_locals(&mut start_bx);
 
+    let reachable_blocks = mir.reachable_blocks_in_mono(cx.tcx(), instance);
+
     // The builders will be created separately for each basic block at `codegen_block`.
     // So drop the builder of `start_llbb` to avoid having two at the same time.
     drop(start_bx);
 
     // Codegen the body of each block using reverse postorder
     for (bb, _) in traversal::reverse_postorder(mir) {
-        fx.codegen_block(bb);
+        if reachable_blocks.contains(bb) {
+            fx.codegen_block(bb);
+        } else {
+            // This may have references to things we didn't monomorphize, so we
+            // don't actually codegen the body. We still create the block so
+            // terminators in other blocks can reference it without worry.
+            fx.codegen_block_as_unreachable(bb);
+        }
     }
 }