Inlining heuristic: return value demotion adds arg (FuelLabs#4518)

## Description

We currently don't support compiling functions with more than 6 args.
Such functions are inlined. However, the inliner heuristic that checks
this fails to say "inline" if there are 6 args, but the return type will
be demoted later on, adding an additional arg. Fix that.

Fixes FuelLabs#4477.

## Checklist

- [x] I have linked to any relevant issues.
- [x] I have commented my code, particularly in hard-to-understand
areas.
- [x] I have updated the documentation where relevant (API docs, the
reference, and the Sway book).
- [x] I have added tests that prove my fix is effective or that my
feature works.
- [x] I have added (or requested a maintainer to add) the necessary
`Breaking*` or `New Feature` labels where relevant.
- [x] I have done my best to ensure that my PR adheres to [the Fuel Labs
Code Review
Standards](https://github.com/FuelLabs/rfcs/blob/master/text/code-standards/external-contributors.md).
- [x] I have requested a review from the relevant team or maintainers.

sway-ir/src/optimize/inline.rs

@@ -157,9 +157,21 @@ pub fn inline_in_non_predicate_module(
             None => {}
         }
 
+        let ret_type = func.get_return_type(ctx);
+        let num_args = {
+            func.args_iter(ctx).count()
+                + if super::target_fuel::is_demotable_type(ctx, &ret_type) {
+                    // The return type will be demoted to memory,
+                    // which means that there'll be an additional return arg.
+                    1
+                } else {
+                    0
+                }
+        };
+
         // For now, pending improvements to ASMgen for calls, we must inline any function which has
         // too many args.
-        if func.args_iter(ctx).count() as u8 > NUM_ARG_REGISTERS {
+        if num_args as u8 > NUM_ARG_REGISTERS {
             return true;
         }
 

test/src/e2e_vm_tests/test_programs/should_pass/language/arg_demotion_inline/Forc.lock

@@ -0,0 +1,16 @@
+[[package]]
+name = 'arg_demotion_inline'
+source = 'member'
+dependencies = [
+    'core',
+    'std',
+]
+
+[[package]]
+name = 'core'
+source = 'path+from-root-603115901A793008'
+
+[[package]]
+name = 'std'
+source = 'path+from-root-603115901A793008'
+dependencies = ['core']

test/src/e2e_vm_tests/test_programs/should_pass/language/arg_demotion_inline/Forc.toml

@@ -0,0 +1,9 @@
+[project]
+authors = ["Fuel Labs <[email protected]>"]
+entry = "main.sw"
+license = "Apache-2.0"
+name = "arg_demotion_inline"
+
+[dependencies]
+core = { path = "../../../../../../../sway-lib-core" }
+std = { path = "../../../../../../../sway-lib-std" }

test/src/e2e_vm_tests/test_programs/should_pass/language/arg_demotion_inline/src/main.sw

@@ -0,0 +1,42 @@
+contract;
+
+abi MyContract {
+    fn test_function() -> bool;
+}
+
+struct Foo {
+    a: u64,
+    b: u64,
+    c: u64,
+    d: u64,
+    e: u64,
+    f: u64,
+}
+
+impl Foo {
+    fn new(
+        a: u64,
+        b: u64,
+        c: u64,
+        d: u64,
+        e: u64,
+        f: u64,
+    ) -> Self {
+        Self {
+            a,
+            b,
+            c,
+            d,
+            e,
+            f,
+        }
+    }
+}
+
+impl MyContract for Contract {
+    fn test_function() -> bool {
+        let bar1 = Foo::new(0,0,0,0,0,0);
+        let bar2 = Foo::new(0,0,0,0,0,0);
+        true
+    }
+}

test/src/e2e_vm_tests/test_programs/should_pass/language/arg_demotion_inline/test.toml

@@ -0,0 +1,2 @@
+category = "compile"
+expected_warnings = 8

test/src/e2e_vm_tests/test_programs/should_pass/unit_tests/stack_indexing_overflow/Forc.lock

@@ -1,13 +1,13 @@
 [[package]]
 name = 'core'
-source = 'path+from-root-6BF9CEE717879469'
+source = 'path+from-root-E733504800A298D4'
 
 [[package]]
-name = 'script_multi_test'
+name = 'stack_indexing_overflow'
 source = 'member'
 dependencies = ['std']
 
 [[package]]
 name = 'std'
-source = 'path+from-root-6BF9CEE717879469'
+source = 'path+from-root-E733504800A298D4'
 dependencies = ['core']

test/src/e2e_vm_tests/test_programs/should_pass/unit_tests/stack_indexing_overflow/Forc.toml

@@ -2,7 +2,7 @@
 authors = ["Fuel Labs <[email protected]>"]
 entry = "main.sw"
 license = "Apache-2.0"
-name = "script_multi_test"
+name = "stack_indexing_overflow"
 
 [dependencies]
 std = { path = "../../../../../../../sway-lib-std" }