Improve overloading diagnostics

.github/workflows/check.yml

@@ -27,7 +27,7 @@ jobs:
           # Use the same components in every step for caching
           components: cargo,clippy,rustfmt
       - name: Build each Rust crate
-        run: cargo build --workspace --all-targets --all-features
+        run: cargo build --all-targets --all-features
 
   test-rust:
     name: 2. Test Rust crates
@@ -39,7 +39,9 @@ jobs:
           toolchain: nightly
           components: cargo,clippy,rustfmt
       - name: Test each Rust crate
-        run: cargo test --workspace --all-targets --all-features
+        run: |
+            cargo test --all-targets --all-features
+            cargo test --doc --all-features
 
   run:
     name: 3. Run Rust example binaries
@@ -69,7 +71,7 @@ jobs:
           toolchain: nightly
           components: cargo,clippy,rustfmt
       - name: Run clippy on Rust crates
-        run: cargo clippy --workspace --all-targets --all-features -- --deny warnings
+        run: cargo clippy --all-targets --all-features -- --deny warnings
 
   doc-rust:
     name: 5. Doc lints on Rust crates
@@ -81,7 +83,7 @@ jobs:
           toolchain: nightly
           components: cargo,clippy,rustfmt
       - name: Run doc checks on Rust crates
-        run: cargo doc --workspace --all-features
+        run: cargo doc --all-features
 
   fmt-rust:
     name: 6. Code format on Rust crates
@@ -107,6 +109,44 @@ jobs:
       - name: Dry run publishing
         run: cargo publish --dry-run
 
+  diagnostics-rust:
+    name: 8. Diagnostics tests on Rust crates
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+      - uses: actions-rust-lang/setup-rust-toolchain@166cdcfd11aee3cb47222f9ddb555ce30ddb9659 # v1.17.0
+        with:
+          toolchain: nightly
+          # Use the same components in every step for caching
+          components: cargo,clippy,rustfmt
+      - name: Build each Rust crate
+        # To bless (update the expected diagnostics), change '/tmp' to 'splat-overload-diagnostics/src/bin',
+        # and disable color output.
+        # FIXME: turn this into a script so we can run it locally.
+        run: |
+            export CARGO_TERM_COLOR=never
+            set -o pipefail
+            TEST_NAMES=$(find splat-overload-diagnostics/src/bin -name "*.rs" | \
+                sed 's|.*/\([^/]*\).rs|\1|g' )
+            echo "testing: $TEST_NAMES"
+            for test_name in $TEST_NAMES; do
+                echo "testing: $test_name"
+                # We expect cargo to fail, so don't exit the shell when it does
+                set +e
+                # Ignore build progress, if it's cached then the built crates change
+                cargo build --all-features --package splat-overload-diagnostics --bin "$test_name" 2>&1 | \
+                    grep --invert-match -e 'Updating .* index' -e 'Compiling [^ ]* .*' \
+                    > "/tmp/$test_name.stderr"
+                CARGO_EXIT_CODE=$?
+                set -e
+                if [[ $CARGO_EXIT_CODE -ne 101 ]]; then
+                    echo "error: $test_name compilation unexpectedly returned exit code $CARGO_EXIT_CODE rather than 101"
+                    exit 1
+                fi
+                echo "comparing diagnostics for: $test_name"
+                diff --unified=5 "splat-overload-diagnostics/src/bin/$test_name.stderr" /tmp/"$test_name.stderr"
+            done
+
   all:
     name: All checks
     # Always run this job, even if earlier steps were skipped (or failed):
@@ -121,6 +161,7 @@ jobs:
       - doc-rust
       - fmt-rust
       - publish-dry-run
+      - diagnostics-rust
     steps:
       - name: Fail if any other job failed
         # Every job status needs to be checked here, because `always()` stops failures from propagating automatically
@@ -133,3 +174,4 @@ jobs:
           [[ "${{ needs.doc-rust.result }}" == "success" ]] || exit 1
           [[ "${{ needs.fmt-rust.result }}" == "success" ]] || exit 1
           [[ "${{ needs.publish-dry-run.result }}" == "success" ]] || exit 1
+          [[ "${{ needs.diagnostics-rust.result }}" == "success" ]] || exit 1

Cargo.toml

@@ -2,11 +2,14 @@
 resolver = "3"
 members = [
     "splat-overload",
-    "splat-overload-test"
+    "splat-overload-test",
+    "splat-overload-diagnostics",
 ]
 default-members = [
     "splat-overload",
-    "splat-overload-test"
+    "splat-overload-test",
+    # Excluded from default members because it deliberately fails compilation
+    #"splat-overload-diagnostics",
 ]
 
 [workspace.package]
@@ -23,11 +26,11 @@
     "interop",
 ]
 license = "MIT OR APACHE-2.0"
 readme = "README.md"
 repository = "https://github.com/rustfoundation/overloading-macros"
-# Requires #[rustc_splat] which was renamed in nightly-2026-07-31 (rust-lang/rust#159817)
-# FIXME: we can't specify nightly dates or the latest nightly here, update to 1.100 once 1.99 beta branches (20 August 2026)
-rust-version = "1.99"
+# Requires nightly `#[rustc_splat]` which was renamed in nightly-2026-07-31 (rust-lang/rust#159817)
+# We can't specify nightly here, because version suffixes aren't supported.
+rust-version = "1.100"
 
 [workspace.dependencies]
 syn = { version = "2.0.119", features = ["full"] }

splat-overload-diagnostics/Cargo.toml

@@ -0,0 +1,16 @@
+[package]
+name = "splat-overload-diagnostics"
+description = "Diagnostics test crate for the experimental splat-overload proc-macro"
+publish = false
+version.workspace = true
+authors.workspace = true
+edition.workspace = true
+homepage.workspace = true
+keywords.workspace = true
+license.workspace = true
+readme.workspace = true
+repository.workspace = true
+rust-version.workspace = true
+
+[dependencies]
+splat-overload = { path = "../splat-overload" }

splat-overload-diagnostics/rust-toolchain.toml

@@ -0,0 +1,2 @@
+[toolchain]
+channel = "nightly"

splat-overload-diagnostics/src/README.md

@@ -0,0 +1,2 @@
+These example binaries are automatically run in CI, they are expected to fail, with output in the
+corresponding `.stderr` file.

splat-overload-diagnostics/src/bin/diag-function-return-values.rs

@@ -0,0 +1,21 @@
+#![feature(splat)]
+#![feature(tuple_trait)]
+#![allow(incomplete_features)]
+#![allow(unused_braces)]
+
+use splat_overload::overload;
+
+overload! {
+    fn foo(x: i32) -> i32 { x }
+    fn foo(x: f64) -> f64 { x }
+}
+
+fn main() {
+    // Empty argument list
+    foo();
+    foo("wrong type");
+    // Wrong number of arguments
+    foo(1_i32, 2, 3);
+    // Wrong return type
+    let a: i32 = foo(1.0_f64);
+}

splat-overload-diagnostics/src/bin/diag-function-return-values.stderr

@@ -0,0 +1,171 @@
+error[E0277]: missing overload for arguments `()`
+  --> splat-overload-diagnostics/src/bin/diag-function-return-values.rs:15:5
+   |
+15 |     foo();
+   |     ^^^ the argument types `()` do not match any overload
+   |
+   = help: the trait `FooArgs` is not implemented for `()`
+   = note: check for missing or extra arguments, and check argument types
+   = note: consider adding a new overload in the overload! { ... } block
+help: the following other types implement trait `FooArgs`
+  --> splat-overload-diagnostics/src/bin/diag-function-return-values.rs:8:1
+   |
+ 8 | / overload! {
+ 9 | |     fn foo(x: i32) -> i32 { x }
+10 | |     fn foo(x: f64) -> f64 { x }
+11 | | }
+   | | ^
+   | | |
+   | |_`(f64,)`
+   |   `(i32,)`
+note: required by a bound in `foo`
+  --> splat-overload-diagnostics/src/bin/diag-function-return-values.rs:8:1
+   |
+ 8 | / overload! {
+ 9 | |     fn foo(x: i32) -> i32 { x }
+10 | |     fn foo(x: f64) -> f64 { x }
+11 | | }
+   | |_^ required by this bound in `foo`
+   = note: this error originates in the macro `overload` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error[E0277]: missing overload for arguments `(&str,)`
+  --> splat-overload-diagnostics/src/bin/diag-function-return-values.rs:16:9
+   |
+16 |     foo("wrong type");
+   |     --- ^^^^^^^^^^^^ the argument types `(&str,)` do not match any overload
+   |     |
+   |     required by a bound introduced by this call
+   |
+   = help: the trait `FooArgs` is not implemented for `(&str,)`
+   = note: check for missing or extra arguments, and check argument types
+   = note: consider adding a new overload in the overload! { ... } block
+help: the following other types implement trait `FooArgs`
+  --> splat-overload-diagnostics/src/bin/diag-function-return-values.rs:8:1
+   |
+ 8 | / overload! {
+ 9 | |     fn foo(x: i32) -> i32 { x }
+10 | |     fn foo(x: f64) -> f64 { x }
+11 | | }
+   | | ^
+   | | |
+   | |_`(f64,)`
+   |   `(i32,)`
+note: required by a bound in `foo`
+  --> splat-overload-diagnostics/src/bin/diag-function-return-values.rs:8:1
+   |
+ 8 | / overload! {
+ 9 | |     fn foo(x: i32) -> i32 { x }
+10 | |     fn foo(x: f64) -> f64 { x }
+11 | | }
+   | |_^ required by this bound in `foo`
+   = note: this error originates in the macro `overload` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error[E0277]: missing overload for arguments `(_, _, _)`
+  --> splat-overload-diagnostics/src/bin/diag-function-return-values.rs:18:9
+   |
+18 |     foo(1_i32, 2, 3);
+   |     --- ^^^^^ the argument types `(_, _, _)` do not match any overload
+   |     |
+   |     required by a bound introduced by this call
+   |
+   = help: the trait `FooArgs` is not implemented for `(_, _, _)`
+   = note: check for missing or extra arguments, and check argument types
+   = note: consider adding a new overload in the overload! { ... } block
+help: the following other types implement trait `FooArgs`
+  --> splat-overload-diagnostics/src/bin/diag-function-return-values.rs:8:1
+   |
+ 8 | / overload! {
+ 9 | |     fn foo(x: i32) -> i32 { x }
+10 | |     fn foo(x: f64) -> f64 { x }
+11 | | }
+   | | ^
+   | | |
+   | |_`(f64,)`
+   |   `(i32,)`
+note: required by a bound in `foo`
+  --> splat-overload-diagnostics/src/bin/diag-function-return-values.rs:8:1
+   |
+ 8 | / overload! {
+ 9 | |     fn foo(x: i32) -> i32 { x }
+10 | |     fn foo(x: f64) -> f64 { x }
+11 | | }
+   | |_^ required by this bound in `foo`
+   = note: this error originates in the macro `overload` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error[E0308]: mismatched types
+  --> splat-overload-diagnostics/src/bin/diag-function-return-values.rs:20:18
+   |
+20 |     let a: i32 = foo(1.0_f64);
+   |            ---   ^^^^^^^^^^^^ expected `i32`, found `f64`
+   |            |
+   |            expected due to this
+
+error[E0277]: missing overload for arguments `()`
+  --> splat-overload-diagnostics/src/bin/diag-function-return-values.rs:15:5
+   |
+15 |     foo();
+   |     ^^^^^ the argument types `()` do not match any overload
+   |
+   = help: the trait `FooArgs` is not implemented for `()`
+   = note: check for missing or extra arguments, and check argument types
+   = note: consider adding a new overload in the overload! { ... } block
+help: the following other types implement trait `FooArgs`
+  --> splat-overload-diagnostics/src/bin/diag-function-return-values.rs:8:1
+   |
+ 8 | / overload! {
+ 9 | |     fn foo(x: i32) -> i32 { x }
+10 | |     fn foo(x: f64) -> f64 { x }
+11 | | }
+   | | ^
+   | | |
+   | |_`(f64,)`
+   |   `(i32,)`
+   = note: this error originates in the macro `overload` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error[E0277]: missing overload for arguments `(&str,)`
+  --> splat-overload-diagnostics/src/bin/diag-function-return-values.rs:16:5
+   |
+16 |     foo("wrong type");
+   |     ^^^^^^^^^^^^^^^^^ the argument types `(&str,)` do not match any overload
+   |
+   = help: the trait `FooArgs` is not implemented for `(&str,)`
+   = note: check for missing or extra arguments, and check argument types
+   = note: consider adding a new overload in the overload! { ... } block
+help: the following other types implement trait `FooArgs`
+  --> splat-overload-diagnostics/src/bin/diag-function-return-values.rs:8:1
+   |
+ 8 | / overload! {
+ 9 | |     fn foo(x: i32) -> i32 { x }
+10 | |     fn foo(x: f64) -> f64 { x }
+11 | | }
+   | | ^
+   | | |
+   | |_`(f64,)`
+   |   `(i32,)`
+   = note: this error originates in the macro `overload` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error[E0277]: missing overload for arguments `(i32, {integer}, {integer})`
+  --> splat-overload-diagnostics/src/bin/diag-function-return-values.rs:18:5
+   |
+18 |     foo(1_i32, 2, 3);
+   |     ^^^^^^^^^^^^^^^^ the argument types `(i32, {integer}, {integer})` do not match any overload
+   |
+   = help: the trait `FooArgs` is not implemented for `(i32, {integer}, {integer})`
+   = note: check for missing or extra arguments, and check argument types
+   = note: consider adding a new overload in the overload! { ... } block
+help: the following other types implement trait `FooArgs`
+  --> splat-overload-diagnostics/src/bin/diag-function-return-values.rs:8:1
+   |
+ 8 | / overload! {
+ 9 | |     fn foo(x: i32) -> i32 { x }
+10 | |     fn foo(x: f64) -> f64 { x }
+11 | | }
+   | | ^
+   | | |
+   | |_`(f64,)`
+   |   `(i32,)`
+   = note: this error originates in the macro `overload` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+Some errors have detailed explanations: E0277, E0308.
+For more information about an error, try `rustc --explain E0277`.
+error: could not compile `splat-overload-diagnostics` (bin "diag-function-return-values") due to 7 previous errors

splat-overload-diagnostics/src/bin/diag-functions.rs

@@ -0,0 +1,19 @@
+#![feature(splat)]
+#![feature(tuple_trait)]
+#![allow(incomplete_features, clippy::approx_constant)]
+
+use splat_overload::overload;
+
+overload! {
+    fn foo(_x: i32, _y: f64) {}
+    fn foo(_x: bool, _y: i32, _z: f64) {}
+    fn foo(_a: i32, _b: f64, _c: bool, _d: u8) {}
+}
+
+fn main() {
+    // Empty argument list
+    foo();
+    foo("one wrong type", 2.0_f64);
+    // Wrong number of arguments
+    foo(1_i32, 2_f64, true, 4_u8, 5);
+}