Fixes for specific error message. (#6758)

The added test case was emitting a wrong/confusing error message,
because a for loop only gets type-checked in its desugared form, as a
while loop.

The error message was `A while loop's loop body cannot implicitly return
a value. Try assigning it to a mutable variable declared outside of the
loop instead.`

Corrected to: `A for loop's loop body cannot implicitly return a value.
Try assigning it to a mutable variable declared outside of the loop
instead.`

Fixes #6395

## Description


## Checklist

- [x] I have linked to any relevant issues.
- [x] I have commented my code, particularly in hard-to-understand
areas.
- [ ] I have updated the documentation where relevant (API docs, the
reference, and the Sway book).
- [ ] If my change requires substantial documentation changes, I have
[requested support from the DevRel
team](https://github.com/FuelLabs/devrel-requests/issues/new/choose)
- [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.

Co-authored-by: IGI-111 <igi-111@protonmail.com>

sway-core/src/language/parsed/expression/mod.rs

@@ -370,6 +370,7 @@ impl PartialEqWithEngines for IntrinsicFunctionExpression {
 pub struct WhileLoopExpression {
     pub condition: Box<Expression>,
     pub body: CodeBlock,
+    pub is_desugared_for_loop: bool,
 }
 
 impl EqWithEngines for WhileLoopExpression {}

sway-core/src/semantic_analysis/ast_node/expression/typed_expression.rs

@@ -506,9 +506,18 @@ impl ty::TyExpression {
                 arguments,
                 span,
             ),
-            ExpressionKind::WhileLoop(WhileLoopExpression { condition, body }) => {
-                Self::type_check_while_loop(handler, ctx.by_ref(), condition, body, span)
-            }
+            ExpressionKind::WhileLoop(WhileLoopExpression {
+                condition,
+                body,
+                is_desugared_for_loop,
+            }) => Self::type_check_while_loop(
+                handler,
+                ctx.by_ref(),
+                condition,
+                body,
+                *is_desugared_for_loop,
+                span,
+            ),
             ExpressionKind::ForLoop(ForLoopExpression { desugared }) => {
                 Self::type_check_for_loop(handler, ctx.by_ref(), desugared)
             }
@@ -2131,6 +2140,7 @@ impl ty::TyExpression {
         mut ctx: TypeCheckContext,
         condition: &Expression,
         body: &CodeBlock,
+        is_desugared_for_loop: bool,
         span: Span,
     ) -> Result<Self, ErrorEmitted> {
         let type_engine = ctx.engines.te();
@@ -2144,11 +2154,17 @@ impl ty::TyExpression {
         };
 
         let unit_ty = type_engine.id_of_unit();
-        let mut ctx = ctx.with_type_annotation(unit_ty).with_help_text(
-            "A while loop's loop body cannot implicitly return a value. Try \
+        let mut ctx = ctx
+            .with_type_annotation(unit_ty)
+            .with_help_text(if is_desugared_for_loop {
+                "A for loop's loop body cannot implicitly return a value. Try \
                  assigning it to a mutable variable declared outside of the loop \
-                 instead.",
-        );
+                 instead."
+            } else {
+                "A while loop's loop body cannot implicitly return a value. Try \
+                 assigning it to a mutable variable declared outside of the loop \
+                 instead."
+            });
         let typed_body = ty::TyCodeBlock::type_check(handler, ctx.by_ref(), body, false)?;
 
         let exp = ty::TyExpression {

sway-core/src/transform/to_parsed_lang/convert_parse_tree.rs

@@ -2131,6 +2131,7 @@ fn expr_to_expression(
             kind: ExpressionKind::WhileLoop(WhileLoopExpression {
                 condition: Box::new(expr_to_expression(context, handler, engines, *condition)?),
                 body: braced_code_block_contents_to_code_block(context, handler, engines, block)?,
+                is_desugared_for_loop: false,
             }),
             span,
         },
@@ -3278,6 +3279,7 @@ fn for_expr_to_expression(
                                 span: Span::dummy(),
                             }),
                             body: while_body,
+                            is_desugared_for_loop: true,
                         }),
                         span: Span::dummy(),
                     }),

test/src/e2e_vm_tests/test_programs/should_fail/for_loop_error/Forc.lock

@@ -0,0 +1,13 @@
+[[package]]
+name = "core"
+source = "path+from-root-FF35018BB7DF2595"
+
+[[package]]
+name = "for_loop_error"
+source = "member"
+dependencies = ["std"]
+
+[[package]]
+name = "std"
+source = "path+from-root-FF35018BB7DF2595"
+dependencies = ["core"]

test/src/e2e_vm_tests/test_programs/should_fail/for_loop_error/Forc.toml

@@ -0,0 +1,8 @@
+[project]
+authors = ["Fuel Labs <contact@fuel.sh>"]
+license = "Apache-2.0"
+name = "for_loop_error"
+entry = "main.sw"
+
+[dependencies]
+std = { path = "../../../reduced_std_libs/sway-lib-std-vec" }

test/src/e2e_vm_tests/test_programs/should_fail/for_loop_error/json_abi_oracle.json

@@ -0,0 +1 @@
+[]

test/src/e2e_vm_tests/test_programs/should_fail/for_loop_error/src/main.sw

@@ -0,0 +1,12 @@
+script;
+
+fn main() {
+    let mut v : Vec<u8> = Vec::new();
+    v.push(1);
+    v.push(2);
+    v.push(3);
+    for elem in v.iter() {
+        log(elem);
+        3
+    };
+}

test/src/e2e_vm_tests/test_programs/should_fail/for_loop_error/test.toml

@@ -0,0 +1,13 @@
+category = "fail"
+
+# check: $()3
+# nextln: $()Mismatched types.
+# nextln: $()expected: ()
+# nextln: $()found:    numeric.
+# nextln: $()Implicit return must match up with block's type.
+
+# check: $()3
+# nextln: $()Mismatched types.
+# nextln: $()expected: ()
+# nextln: $()found:    numeric.
+# nextln: $()A for loop's loop body cannot implicitly return a value. Try assigning it to a mutable variable declared outside of the loop instead.