Sema: remove unresolved inferred allocs

Closes #2557

src/Module.zig

@@ -5612,6 +5612,18 @@ pub fn analyzeFnBody(mod: *Module, func: *Fn, arena: Allocator) SemaError!Air {
         else => |e| return e,
     };
 
+    {
+        var it = sema.unresolved_inferred_allocs.keyIterator();
+        while (it.next()) |ptr_inst| {
+            // The lack of a resolve_inferred_alloc means that this instruction
+            // is unused so it just has to be a no-op.
+            sema.air_instructions.set(ptr_inst.*, .{
+                .tag = .alloc,
+                .data = .{ .ty = Type.initTag(.single_const_pointer_to_comptime_int) },
+            });
+        }
+    }
+
     // If we don't get an error return trace from a caller, create our own.
     if (func.calls_or_awaits_errorable_fn and
         mod.comp.bin_file.options.error_return_tracing and

src/Sema.zig

@@ -82,6 +82,8 @@ is_generic_instantiation: bool = false,
 /// function types will emit generic poison instead of a partial type.
 no_partial_func_ty: bool = false,
 
+unresolved_inferred_allocs: std.AutoHashMapUnmanaged(Air.Inst.Index, void) = .{},
+
 const std = @import("std");
 const math = std.math;
 const mem = std.mem;
@@ -579,6 +581,7 @@ pub fn deinit(sema: *Sema) void {
         }
         sema.post_hoc_blocks.deinit(gpa);
     }
+    sema.unresolved_inferred_allocs.deinit(gpa);
     sema.* = undefined;
 }
 
@@ -3238,6 +3241,7 @@ fn zirAllocExtended(
     );
     try sema.requireFunctionBlock(block, src);
     try block.instructions.append(sema.gpa, Air.refToIndex(result).?);
+    try sema.unresolved_inferred_allocs.putNoClobber(sema.gpa, Air.refToIndex(result).?, {});
     return result;
 }
 
@@ -3414,6 +3418,7 @@ fn zirAllocInferred(
     );
     try sema.requireFunctionBlock(block, src);
     try block.instructions.append(sema.gpa, Air.refToIndex(result).?);
+    try sema.unresolved_inferred_allocs.putNoClobber(sema.gpa, Air.refToIndex(result).?, {});
     return result;
 }
 
@@ -3463,6 +3468,7 @@ fn zirResolveInferredAlloc(sema: *Sema, block: *Block, inst: Zir.Inst.Index) Com
             }
         },
         .inferred_alloc => {
+            assert(sema.unresolved_inferred_allocs.remove(ptr_inst));
             const inferred_alloc = ptr_val.castTag(.inferred_alloc).?;
             const peer_inst_list = inferred_alloc.data.prongs.items(.stored_inst);
             const final_elem_ty = try sema.resolvePeerTypes(block, ty_src, peer_inst_list, .none);

test/behavior.zig

@@ -41,6 +41,7 @@ test {
     _ = @import("behavior/bugs/2006.zig");
     _ = @import("behavior/bugs/2114.zig");
     _ = @import("behavior/bugs/2346.zig");
+    _ = @import("behavior/bugs/2557.zig");
     _ = @import("behavior/bugs/2578.zig");
     _ = @import("behavior/bugs/2692.zig");
     _ = @import("behavior/bugs/2889.zig");

test/behavior/bugs/2557.zig

@@ -0,0 +1,6 @@
+test {
+    var a = if (true) {
+        return;
+    } else true;
+    _ = a;
+}