Add expressive diagnostics for Warning::UnusedReturnValue (#7382)

ironcev · web-flow · commit 8fde717752f5 · 2025-09-12T11:19:48.000+10:00
## Description This PR adds expressive diagnostics for the `UnusedReturnValue` warning that explains how to intentionally ignore a returned value by using `let _ = ...`. The reason for adding the diagnostics was an observed confusion among novice Sway developers in the particular case of using `__dbg` intrinsic as a statement: ```sway __dbg("some debug message"); ``` It was unclear to developers why this usage generates the warning, and also how to remove it. Additionally, the PR fixes some `uninlined_format_args` Clippy warnings. ## Checklist - [ ] I have linked to any relevant issues. - [ ] 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. - [ ] 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.
diff --git a/forc-plugins/forc-node/src/util.rs b/forc-plugins/forc-node/src/util.rs
@@ -257,7 +257,7 @@ mod tests {
     fn test_human_readable_config() {
         let config = Config::local_node();
         let human_readable = HumanReadableConfig(&config);
-        let formatted = format!("{}", human_readable);
+        let formatted = format!("{human_readable}");
         let expected = format!(
             r#"Fuel Core Configuration:
   GraphQL Address: {}
diff --git a/sway-core/src/language/ty/declaration/const_generic.rs b/sway-core/src/language/ty/declaration/const_generic.rs
@@ -54,9 +54,7 @@ impl MaterializeConstGenerics for TyConstGenericDecl {
                                 .unwrap()
                                 .cast_value_to_u64()
                                 .unwrap(),
-                        "{:?} {:?}",
-                        v,
-                        value
+                        "{v:?} {value:?}",
                     );
                 }
                 None => {
diff --git a/sway-core/src/semantic_analysis/namespace/trait_map.rs b/sway-core/src/semantic_analysis/namespace/trait_map.rs
@@ -236,24 +236,24 @@ impl DebugWithEngines for TypeRootFilter {
             U64 => write!(f, "u64"),
             U256 => write!(f, "u256"),
             Bool => write!(f, "bool"),
-            Custom(name) => write!(f, "Custom({})", name),
+            Custom(name) => write!(f, "Custom({name})"),
             B256 => write!(f, "b256"),
             Contract => write!(f, "Contract"),
             ErrorRecovery => write!(f, "ErrorRecovery"),
-            Tuple(n) => write!(f, "Tuple(len={})", n),
+            Tuple(n) => write!(f, "Tuple(len={n})"),
             Enum(parsed_id) => {
-                write!(f, "Enum({:?})", parsed_id)
+                write!(f, "Enum({parsed_id:?})")
             }
             Struct(parsed_id) => {
-                write!(f, "Struct({:?})", parsed_id)
+                write!(f, "Struct({parsed_id:?})")
             }
-            ContractCaller(abi_name) => write!(f, "ContractCaller({})", abi_name),
+            ContractCaller(abi_name) => write!(f, "ContractCaller({abi_name})"),
             Array => write!(f, "Array"),
             RawUntypedPtr => write!(f, "RawUntypedPtr"),
             RawUntypedSlice => write!(f, "RawUntypedSlice"),
             Ptr => write!(f, "Ptr"),
             Slice => write!(f, "Slice"),
-            TraitType(name) => write!(f, "TraitType({})", name),
+            TraitType(name) => write!(f, "TraitType({name})"),
         }
     }
 }
@@ -324,14 +324,13 @@ impl DebugWithEngines for TraitMap {
 
                 writeln!(
                     f,
-                    "    impl {} for {} [{}]{} {{",
-                    trait_name_str, ty_str, iface_flag, impl_tparams
+                    "    impl {trait_name_str} for {ty_str} [{iface_flag}]{impl_tparams} {{"
                 )?;
 
                 for (name, item) in &value.trait_items {
                     match item {
                         ResolvedTraitImplItem::Parsed(_p) => {
-                            writeln!(f, "      - {}: <parsed>", name)?;
+                            writeln!(f, "      - {name}: <parsed>")?;
                         }
                         ResolvedTraitImplItem::Typed(ty_item) => {
                             writeln!(f, "      - {}: {:?}", name, engines.help_out(ty_item))?;
diff --git a/sway-error/src/warning.rs b/sway-error/src/warning.rs
@@ -1,7 +1,8 @@
 use crate::{
     diagnostic::{Code, Diagnostic, Hint, Issue, Reason, ToDiagnostic},
     formatting::{
-        did_you_mean_help, num_to_str, sequence_to_list, sequence_to_str, Enclosing, Indent,
+        did_you_mean_help, first_line, num_to_str, sequence_to_list, sequence_to_str, Enclosing,
+        Indent,
     },
 };
 
@@ -245,7 +246,7 @@ impl fmt::Display for Warning {
             },
             UnusedReturnValue { r#type } => write!(
                 f,
-                "This returns a value of type {type}, which is not assigned to anything and is \
+                "This returns a value of type \"{type}\", which is not assigned to anything and is \
                  ignored."
             ),
             SimilarMethodFound { lib, module, name } => write!(
@@ -593,6 +594,25 @@ impl ToDiagnostic for CompileWarning {
                 },
                 help: vec![],
             },
+            UnusedReturnValue { r#type } => Diagnostic {
+                reason: Some(Reason::new(code(1), "Returned value is ignored".to_string())),
+                issue: Issue::warning(
+                    source_engine,
+                    self.span(),
+                    "This returns a value which is not assigned to anything and is ignored.".to_string(),
+                ),
+                hints: vec![
+                    Hint::help(
+                        source_engine,
+                        self.span(),
+                        format!("The returned value has type \"{type}\"."),
+                    )
+                ],
+                help: vec![
+                    "If you want to intentionally ignore the returned value, use `let _ = ...`:".to_string(),
+                    format!("{}let _ = {};", Indent::Single, first_line(self.span.as_str(), true)),
+                ],
+            },
            _ => Diagnostic {
                     // TODO: Temporarily we use self here to achieve backward compatibility.
                     //       In general, self must not be used and will not be used once we
diff --git a/sway-lib-std/src/clone.sw b/sway-lib-std/src/clone.sw
@@ -78,8 +78,7 @@ fn ok_string_array_clone() {
     let a = __to_str_array("abc");
     let b = a.clone();
 
-    let _ = __dbg(a); // TODO __dbg((a, b)) is not working
-    let _ = __dbg(b);
+    let _ = __dbg((a, b));
 
     assert(a == __to_str_array("abc"));
     assert(b == __to_str_array("abc"));
diff --git a/test/src/e2e_vm_tests/test_programs/should_fail/insufficient_type_info/Forc.lock b/test/src/e2e_vm_tests/test_programs/should_fail/insufficient_type_info/Forc.lock
@@ -1,8 +1,3 @@
 [[package]]
 name = "insufficient_type_info"
 source = "member"
-dependencies = ["std"]
-
-[[package]]
-name = "std"
-source = "path+from-root-687D7B6A23A7C9DF"
diff --git a/test/src/e2e_vm_tests/test_programs/should_fail/insufficient_type_info/Forc.toml b/test/src/e2e_vm_tests/test_programs/should_fail/insufficient_type_info/Forc.toml
@@ -3,8 +3,4 @@ authors = ["Fuel Labs <contact@fuel.sh>"]
 entry = "main.sw"
 license = "Apache-2.0"
 name = "insufficient_type_info"
-implicit-std = false
-
-[dependencies]
-std = { path = "../../../reduced_std_libs/sway-lib-std-core" }
-
+implicit-std = false
diff --git a/test/src/e2e_vm_tests/test_programs/should_fail/insufficient_type_info/src/main.sw b/test/src/e2e_vm_tests/test_programs/should_fail/insufficient_type_info/src/main.sw
@@ -1,4 +1,4 @@
-script;
+library;
 
 mod lib;
 
@@ -8,7 +8,7 @@ fn foo<T>() {
     let x = __size_of::<T>();
 }
 
-fn main() {
+pub fn main() {
     foo();
 
     None::<T>;
diff --git a/test/src/e2e_vm_tests/test_programs/should_fail/insufficient_type_info/test.toml b/test/src/e2e_vm_tests/test_programs/should_fail/insufficient_type_info/test.toml
@@ -1,8 +1,11 @@
 category = "fail"
 
-# check: $()warning
+# check: $()Returned value is ignored
 # check: $()None::<T>;
-# nextln: $()This returns a value of type MyOption<T>, which is not assigned to anything and is ignored.
+# nextln: $()This returns a value which is not assigned to anything and is ignored.
+# nextln: $()The returned value has type "MyOption<T>".
+# check: $()If you want to intentionally ignore the returned value, use `let _ = ...`:
+# nextln: $()let _ = None::<T>;
 
 # check: $()error
 # check: $()None::<T>;
diff --git a/test/src/e2e_vm_tests/test_programs/should_pass/language/break_and_continue_block_ret/Forc.lock b/test/src/e2e_vm_tests/test_programs/should_pass/language/break_and_continue_block_ret/Forc.lock
@@ -1,8 +1,3 @@
 [[package]]
 name = "break_and_continue_block_ret"
 source = "member"
-dependencies = ["std"]
-
-[[package]]
-name = "std"
-source = "path+from-root-3052B79D5F74B184"
diff --git a/test/src/e2e_vm_tests/test_programs/should_pass/language/break_and_continue_block_ret/Forc.toml b/test/src/e2e_vm_tests/test_programs/should_pass/language/break_and_continue_block_ret/Forc.toml
@@ -3,7 +3,4 @@ authors = ["Fuel Labs <contact@fuel.sh>"]
 entry = "main.sw"
 license = "Apache-2.0"
 name = "break_and_continue_block_ret"
-implicit-std = false
-
-[dependencies]
-std = { path = "../../../../reduced_std_libs/sway-lib-std-core" }
+implicit-std = false
diff --git a/test/src/e2e_vm_tests/test_programs/should_pass/language/break_and_continue_block_ret/src/main.sw b/test/src/e2e_vm_tests/test_programs/should_pass/language/break_and_continue_block_ret/src/main.sw
@@ -1,6 +1,6 @@
-script;
+library;
 
-fn main() {
+pub fn main() {
     while true {
         if true {
             break;
diff --git a/test/src/e2e_vm_tests/test_programs/should_pass/language/break_and_continue_block_ret/test.toml b/test/src/e2e_vm_tests/test_programs/should_pass/language/break_and_continue_block_ret/test.toml
@@ -1,3 +1,4 @@
 category = "compile"
 
-# not: $()This returns a value of type unknown, which is not assigned to anything and is ignored.
+# not: $()Returned value is ignored
+# not: $()This returns a value which is not assigned to anything and is ignored.
diff --git a/test/src/e2e_vm_tests/test_programs/should_pass/language/const_generics/src/main.sw b/test/src/e2e_vm_tests/test_programs/should_pass/language/const_generics/src/main.sw
@@ -104,7 +104,7 @@ fn return_inner_const<const ZZZ: u64>() -> u64 {
 #[inline(never)]
 fn const_with_const_generics<const B: u64>() {
     const A: u64 = B + 1;
-    __dbg(A);
+    let _ = __dbg(A);
 }
 
 fn main(a: [u64; 2]) {
diff --git a/test/src/e2e_vm_tests/test_programs/should_pass/language/const_generics/stdout.snap b/test/src/e2e_vm_tests/test_programs/should_pass/language/const_generics/stdout.snap
@@ -1,24 +1,13 @@
 ---
 source: test/src/snapshot/mod.rs
+assertion_line: 266
 ---
 > forc build --path test/src/e2e_vm_tests/test_programs/should_pass/language/const_generics --release
 exit status: 1
 output:
     Building test/src/e2e_vm_tests/test_programs/should_pass/language/const_generics
    Compiling library std (sway-lib-std)
    Compiling script const_generics (test/src/e2e_vm_tests/test_programs/should_pass/language/const_generics)
-warning
-   --> test/src/e2e_vm_tests/test_programs/should_pass/language/const_generics/src/main.sw:107:5
-    |
-105 | fn const_with_const_generics<const B: u64>() {
-106 |     const A: u64 = B + 1;
-107 |     __dbg(A);
-    |     -------- This returns a value of type u64, which is not assigned to anything and is ignored.
-108 | }
-109 | 
-    |
-____
-
 warning
   --> test/src/e2e_vm_tests/test_programs/should_pass/language/const_generics/src/main.sw:50:5
    |
@@ -246,7 +235,7 @@ error
 105 | fn const_with_const_generics<const B: u64>() {
     |                                    ^ This needs "const_generics" to be enabled, but it is currently disabled. For more details go to https://github.com/FuelLabs/sway/issues/6860.
 106 |     const A: u64 = B + 1;
-107 |     __dbg(A);
+107 |     let _ = __dbg(A);
     |
 ____
 
@@ -439,18 +428,6 @@ output:
     Building test/src/e2e_vm_tests/test_programs/should_pass/language/const_generics
    Compiling library std (sway-lib-std)
    Compiling script const_generics (test/src/e2e_vm_tests/test_programs/should_pass/language/const_generics)
-warning
-   --> test/src/e2e_vm_tests/test_programs/should_pass/language/const_generics/src/main.sw:107:5
-    |
-105 | fn const_with_const_generics<const B: u64>() {
-106 |     const A: u64 = B + 1;
-107 |     __dbg(A);
-    |     -------- This returns a value of type u64, which is not assigned to anything and is ignored.
-108 | }
-109 | 
-    |
-____
-
 warning
   --> test/src/e2e_vm_tests/test_programs/should_pass/language/const_generics/src/main.sw:87:5
    |
@@ -478,8 +455,8 @@ warning
    |
 ____
 
-  Compiled script "const_generics" with 3 warnings.
-    Finished debug [unoptimized + fuel] target(s) [9.208 KB] in ???
+  Compiled script "const_generics" with 2 warnings.
+    Finished debug [unoptimized + fuel] target(s) [9.216 KB] in ???
      Running 1 test, filtered 0 tests
 
 tested -- const_generics
@@ -500,8 +477,8 @@ tested -- const_generics
 [src/main.sw:167:13] a = "ABCDE"
 [src/main.sw:171:13] a.len() = 70
 [src/main.sw:172:13] a = "1234567890123456789012345678901234567890123456789012345678901234567890"
-[src/main.sw:107:5] A = 2
-[src/main.sw:107:5] A = 6
+[src/main.sw:107:13] A = 2
+[src/main.sw:107:13] A = 6
 
 test result: OK. 1 passed; 0 failed; finished in ???
 
diff --git a/test/src/e2e_vm_tests/test_programs/should_pass/unused_return_value/Forc.lock b/test/src/e2e_vm_tests/test_programs/should_pass/unused_return_value/Forc.lock
@@ -0,0 +1,3 @@
+[[package]]
+name = "unused_return_value"
+source = "member"
diff --git a/test/src/e2e_vm_tests/test_programs/should_pass/unused_return_value/Forc.toml b/test/src/e2e_vm_tests/test_programs/should_pass/unused_return_value/Forc.toml
@@ -0,0 +1,6 @@
+[project]
+authors = ["Fuel Labs <contact@fuel.sh>"]
+entry = "main.sw"
+license = "Apache-2.0"
+name = "unused_return_value"
+implicit-std = false
diff --git a/test/src/e2e_vm_tests/test_programs/should_pass/unused_return_value/snapshot.toml b/test/src/e2e_vm_tests/test_programs/should_pass/unused_return_value/snapshot.toml
diff --git a/test/src/e2e_vm_tests/test_programs/should_pass/unused_return_value/src/main.sw b/test/src/e2e_vm_tests/test_programs/should_pass/unused_return_value/src/main.sw
@@ -0,0 +1,30 @@
+library;
+
+struct A {
+    very_long_field_name: u64,
+}
+
+struct B {
+    very_long_field_name: A,
+}
+
+impl B {
+    fn very_long_method_name(self, x: u64) -> B {
+        B { very_long_field_name: A { very_long_field_name: x } }
+    }
+}
+
+pub fn test() {
+    42;
+    poke(42);
+    B { very_long_field_name: A { very_long_field_name: 0 } }.very_long_method_name(10);
+    B {
+        very_long_field_name: A {
+            very_long_field_name: 0
+        }
+    }.very_long_method_name(10);
+}
+
+fn poke(x: u64) -> u64 {
+    x
+}
diff --git a/test/src/e2e_vm_tests/test_programs/should_pass/unused_return_value/stdout.snap b/test/src/e2e_vm_tests/test_programs/should_pass/unused_return_value/stdout.snap
@@ -0,0 +1,63 @@
+---
+source: test/src/snapshot/mod.rs
+assertion_line: 266
+---
+> forc build --path test/src/e2e_vm_tests/test_programs/should_pass/unused_return_value
+exit status: 0
+output:
+    Building test/src/e2e_vm_tests/test_programs/should_pass/unused_return_value
+   Compiling library unused_return_value (test/src/e2e_vm_tests/test_programs/should_pass/unused_return_value)
+warning: Returned value is ignored
+  --> test/src/e2e_vm_tests/test_programs/should_pass/unused_return_value/src/main.sw:18:5
+   |
+...
+18 |     42;
+   |     -- This returns a value which is not assigned to anything and is ignored.
+   |     -- help: The returned value has type "numeric".
+   |
+   = help: If you want to intentionally ignore the returned value, use `let _ = ...`:
+   = help:   let _ = 42;
+____
+
+warning: Returned value is ignored
+  --> test/src/e2e_vm_tests/test_programs/should_pass/unused_return_value/src/main.sw:19:5
+   |
+...
+19 |     poke(42);
+   |     -------- This returns a value which is not assigned to anything and is ignored.
+   |     -------- help: The returned value has type "u64".
+   |
+   = help: If you want to intentionally ignore the returned value, use `let _ = ...`:
+   = help:   let _ = poke(42);
+____
+
+warning: Returned value is ignored
+  --> test/src/e2e_vm_tests/test_programs/should_pass/unused_return_value/src/main.sw:20:5
+   |
+...
+20 |     B { very_long_field_name: A { very_long_field_name: 0 } }.very_long_method_name(10);
+   |     ----------------------------------------------------------------------------------- This returns a value which is not assigned to anything and is ignored.
+   |     ----------------------------------------------------------------------------------- help: The returned value has type "B".
+   |
+   = help: If you want to intentionally ignore the returned value, use `let _ = ...`:
+   = help:   let _ = B { very_long_field_name: A { very_long_field_name: 0 } }.very_long_method_name(10);
+____
+
+warning: Returned value is ignored
+  --> test/src/e2e_vm_tests/test_programs/should_pass/unused_return_value/src/main.sw:21:5
+   |
+...    
+21 |        B {
+   |   _____-
+   |   _____-
+...  ||
+25 |  |     }.very_long_method_name(10);
+   | ||_______________________________- This returns a value which is not assigned to anything and is ignored.
+   |  |_______________________________- help: The returned value has type "B".
+   |
+   = help: If you want to intentionally ignore the returned value, use `let _ = ...`:
+   = help:   let _ = B {...;
+____
+
+  Compiled library "unused_return_value" with 4 warnings.
+    Finished debug [unoptimized + fuel] target(s) [32 B] in ???
diff --git a/test/src/snapshot/mod.rs b/test/src/snapshot/mod.rs
