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

## 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.

Author: ironcev

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: {}

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 => {

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))?;

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

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"));

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"

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

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>;

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>;

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"