test: add more tests and const generics documentation

Fill test coverage gaps identified during code review:

- `trybuild`: verify error messages for missing Facet derive,
  non-Debug type params, and const generics (documenting the limitation)
- `insta`: snapshot tests for exact Debug output format verification
  across structs, enums, tuple structs, and nested types
- Bump `facet` dev-dep to 0.44, lower MSRV to 1.87.0

Author: ceejbot

Cargo.toml

@@ -2,7 +2,7 @@
 name = "safe-debug"
 version = "0.1.1"
 edition = "2024"
-rust-version = "1.90.0"
+rust-version = "1.87.0"
 authors = ["C J Silverio <ceejceej@gmail.com>"]
 description = "Derives std::fmt::Debug with automatic redaction for sensitive fields marked with #[facet(sensitive)]"
 license = "MIT OR Apache-2.0"
@@ -27,7 +27,9 @@ quote = "1.0"
 facet-macros-parse = "0.31"
 
 [dev-dependencies]
-facet = { version = "0.43.2", features = ["reflect"] }
+facet = { version = "0.44", features = ["reflect"] }
+insta = "1"
+trybuild = { version = "1", features = ["diff"] }
 
 [[example]]
 name = "enum_example"

tests/compile_fail/const_generic.rs

@@ -0,0 +1,21 @@
+/// Const generics are not supported.
+/// Both `Facet` and `SafeDebug` fail on const generic parameters.
+/// Facet's derive rejects `const N: usize` outright, and SafeDebug's
+/// string-based generics parser produces unparsable tokens.
+use facet::Facet;
+use safe_debug::SafeDebug;
+
+#[derive(Facet, SafeDebug)]
+struct FixedBuffer<const N: usize> {
+    #[facet(sensitive)]
+    data: [u8; N],
+    label: String,
+}
+
+fn main() {
+    let buf: FixedBuffer<4> = FixedBuffer {
+        data: [1, 2, 3, 4],
+        label: "test".to_string(),
+    };
+    println!("{:?}", buf);
+}

tests/compile_fail/const_generic.stderr

@@ -0,0 +1,36 @@
+error: unexpected `const` parameter declaration
+ --> tests/compile_fail/const_generic.rs:9:20
+  |
+9 | struct FixedBuffer<const N: usize> {
+  |                    ^^^^^^^^^^^^^^ expected a `const` expression, not a parameter declaration
+
+error: expected one of `#`, `{`, lifetime, or type, found keyword `const`
+ --> tests/compile_fail/const_generic.rs:8:17
+  |
+8 | #[derive(Facet, SafeDebug)]
+  |                 ^^^^^^^^^ expected one of `#`, `{`, lifetime, or type
+  |
+  = note: this error originates in the derive macro `SafeDebug` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: proc-macro derive produced unparsable tokens
+ --> tests/compile_fail/const_generic.rs:8:17
+  |
+8 | #[derive(Facet, SafeDebug)]
+  |                 ^^^^^^^^^
+
+error[E0277]: `FixedBuffer<4>` doesn't implement `std::fmt::Debug`
+  --> tests/compile_fail/const_generic.rs:20:22
+   |
+20 |     println!("{:?}", buf);
+   |               ----   ^^^ `FixedBuffer<4>` cannot be formatted using `{:?}` because it doesn't implement `std::fmt::Debug`
+   |               |
+   |               required by this formatting parameter
+   |
+   = help: the trait `std::fmt::Debug` is not implemented for `FixedBuffer<4>`
+   = note: add `#[derive(Debug)]` to `FixedBuffer<4>` or manually `impl std::fmt::Debug for FixedBuffer<4>`
+   = note: this error originates in the macro `$crate::format_args_nl` which comes from the expansion of the macro `println` (in Nightly builds, run with -Z macro-backtrace for more info)
+help: consider annotating `FixedBuffer<4>` with `#[derive(Debug)]`
+   |
+ 9 + #[derive(Debug)]
+10 | struct FixedBuffer<const N: usize> {
+   |

tests/compile_fail/missing_facet_derive.rs

@@ -0,0 +1,11 @@
+/// Deriving SafeDebug without Facet should produce a helpful error.
+use safe_debug::SafeDebug;
+
+#[derive(SafeDebug)]
+struct MissingFacet {
+    name: String,
+    #[facet(sensitive)]
+    secret: String,
+}
+
+fn main() {}

tests/compile_fail/missing_facet_derive.stderr

@@ -0,0 +1,20 @@
+error: cannot find attribute `facet` in this scope
+ --> tests/compile_fail/missing_facet_derive.rs:7:7
+  |
+7 |     #[facet(sensitive)]
+  |       ^^^^^
+  |
+  = note: `facet` is in scope, but it is a crate, not an attribute
+
+error[E0599]: no associated item named `SHAPE` found for struct `MissingFacet` in the current scope
+ --> tests/compile_fail/missing_facet_derive.rs:4:10
+  |
+4 | #[derive(SafeDebug)]
+  |          ^^^^^^^^^ associated item not found in `MissingFacet`
+5 | struct MissingFacet {
+  | ------------------- associated item `SHAPE` not found for this struct
+  |
+  = help: items from traits can only be used if the trait is implemented and in scope
+  = note: the following trait defines an item `SHAPE`, perhaps you need to implement it:
+          candidate #1: `Facet`
+  = note: this error originates in the derive macro `SafeDebug` (in Nightly builds, run with -Z macro-backtrace for more info)

tests/compile_fail/non_debug_type_param.rs

@@ -0,0 +1,16 @@
+/// A generic type parameter that doesn't implement Debug should fail to compile.
+use facet::Facet;
+use safe_debug::SafeDebug;
+
+struct NotDebug;
+
+#[derive(Facet, SafeDebug)]
+struct Container<T> {
+    value: T,
+}
+
+fn main() {
+    let c = Container { value: NotDebug };
+    // This should fail because NotDebug doesn't implement Debug
+    println!("{:?}", c);
+}

tests/compile_fail/non_debug_type_param.stderr

@@ -0,0 +1,56 @@
+error[E0277]: `NotDebug` doesn't implement `std::fmt::Debug`
+  --> tests/compile_fail/non_debug_type_param.rs:15:22
+   |
+15 |     println!("{:?}", c);
+   |               ----   ^ `NotDebug` cannot be formatted using `{:?}` because it doesn't implement `std::fmt::Debug`
+   |               |
+   |               required by this formatting parameter
+   |
+   = help: the trait `std::fmt::Debug` is not implemented for `NotDebug`
+   = note: add `#[derive(Debug)]` to `NotDebug` or manually `impl std::fmt::Debug for NotDebug`
+help: the trait `std::fmt::Debug` is implemented for `Container<T>`
+  --> tests/compile_fail/non_debug_type_param.rs:7:17
+   |
+ 7 | #[derive(Facet, SafeDebug)]
+   |                 ^^^^^^^^^
+note: required for `Container<NotDebug>` to implement `std::fmt::Debug`
+  --> tests/compile_fail/non_debug_type_param.rs:7:17
+   |
+ 7 | #[derive(Facet, SafeDebug)]
+   |                 ^^^^^^^^^ unsatisfied trait bound introduced in this `derive` macro
+   = note: this error originates in the macro `$crate::format_args_nl` which comes from the expansion of the derive macro `SafeDebug` (in Nightly builds, run with -Z macro-backtrace for more info)
+help: consider annotating `NotDebug` with `#[derive(Debug)]`
+   |
+ 5 + #[derive(Debug)]
+ 6 | struct NotDebug;
+   |
+
+error[E0277]: the trait bound `for<'__facet> NotDebug: Facet<'__facet>` is not satisfied
+  --> tests/compile_fail/non_debug_type_param.rs:15:22
+   |
+15 |     println!("{:?}", c);
+   |               ----   ^ unsatisfied trait bound
+   |               |
+   |               required by this formatting parameter
+   |
+help: the trait `for<'__facet> Facet<'__facet>` is not implemented for `NotDebug`
+  --> tests/compile_fail/non_debug_type_param.rs:5:1
+   |
+ 5 | struct NotDebug;
+   | ^^^^^^^^^^^^^^^
+   = help: the following other types implement trait `Facet<'facet>`:
+             `&'a T` implements `Facet<'a>`
+             `&'a mut T` implements `Facet<'a>`
+             `()` implements `Facet<'_>`
+             `(T0, T1)` implements `Facet<'a>`
+             `(T0, T1, T2)` implements `Facet<'a>`
+             `(T0, T1, T2, T3)` implements `Facet<'a>`
+             `(T0,)` implements `Facet<'a>`
+             `*const T` implements `Facet<'a>`
+           and $N others
+note: required for `Container<NotDebug>` to implement `std::fmt::Debug`
+  --> tests/compile_fail/non_debug_type_param.rs:7:17
+   |
+ 7 | #[derive(Facet, SafeDebug)]
+   |                 ^^^^^^^^^ unsatisfied trait bound introduced in this `derive` macro
+   = note: this error originates in the macro `$crate::format_args_nl` which comes from the expansion of the derive macro `SafeDebug` (in Nightly builds, run with -Z macro-backtrace for more info)

tests/compile_fail_tests.rs

@@ -0,0 +1,5 @@
+#[test]
+fn compile_fail() {
+    let t = trybuild::TestCases::new();
+    t.compile_fail("tests/compile_fail/*.rs");
+}

tests/snapshot_tests.rs

@@ -0,0 +1,121 @@
+use facet::Facet;
+use safe_debug::SafeDebug;
+
+// --- Type definitions for snapshot tests ---
+
+#[derive(Facet, SafeDebug)]
+struct PatientRecord {
+    id: String,
+    name: String,
+    #[facet(sensitive)]
+    ssn: String,
+    #[facet(sensitive)]
+    medical_history: String,
+}
+
+#[derive(Facet, SafeDebug)]
+struct TupleRecord(String, #[facet(sensitive)] String, i32);
+
+#[derive(Facet, SafeDebug)]
+#[repr(C)]
+enum ApiResponse {
+    Success { code: u16, data: String },
+    Error {
+        code: u16,
+        #[facet(sensitive)]
+        details: String,
+    },
+    Pending,
+    Token(#[facet(sensitive)] String),
+}
+
+#[derive(Facet, SafeDebug)]
+struct Inner {
+    public_data: String,
+    #[facet(sensitive)]
+    secret_data: String,
+}
+
+#[derive(Facet, SafeDebug)]
+struct Outer {
+    id: u32,
+    inner: Inner,
+}
+
+// --- Snapshot tests ---
+
+#[test]
+fn snapshot_named_struct() {
+    let record = PatientRecord {
+        id: "P-12345".to_string(),
+        name: "Jane Doe".to_string(),
+        ssn: "123-45-6789".to_string(),
+        medical_history: "Chronic condition notes".to_string(),
+    };
+    insta::assert_snapshot!(format!("{:?}", record));
+}
+
+#[test]
+fn snapshot_named_struct_pretty() {
+    let record = PatientRecord {
+        id: "P-12345".to_string(),
+        name: "Jane Doe".to_string(),
+        ssn: "123-45-6789".to_string(),
+        medical_history: "Chronic condition notes".to_string(),
+    };
+    insta::assert_snapshot!(format!("{:#?}", record));
+}
+
+#[test]
+fn snapshot_tuple_struct() {
+    let record = TupleRecord("visible".to_string(), "secret-token".to_string(), 42);
+    insta::assert_snapshot!(format!("{:?}", record));
+}
+
+#[test]
+fn snapshot_enum_struct_variant() {
+    let success = ApiResponse::Success {
+        code: 200,
+        data: "OK".to_string(),
+    };
+    insta::assert_snapshot!("enum_success", format!("{:?}", success));
+
+    let error = ApiResponse::Error {
+        code: 500,
+        details: "password leaked in stack trace".to_string(),
+    };
+    insta::assert_snapshot!("enum_error_redacted", format!("{:?}", error));
+}
+
+#[test]
+fn snapshot_enum_unit_and_tuple_variants() {
+    let pending = ApiResponse::Pending;
+    insta::assert_snapshot!("enum_pending", format!("{:?}", pending));
+
+    let token = ApiResponse::Token("secret-bearer-token".to_string());
+    insta::assert_snapshot!("enum_token_redacted", format!("{:?}", token));
+}
+
+#[test]
+fn snapshot_nested_struct() {
+    let data = Outer {
+        id: 999,
+        inner: Inner {
+            public_data: "visible-info".to_string(),
+            secret_data: "classified-payload".to_string(),
+        },
+    };
+    insta::assert_snapshot!(format!("{:?}", data));
+}
+
+#[test]
+fn snapshot_nested_struct_pretty() {
+    let data = Outer {
+        id: 999,
+        inner: Inner {
+            public_data: "visible-info".to_string(),
+            secret_data: "classified-payload".to_string(),
+        },
+    };
+    insta::assert_snapshot!(format!("{:#?}", data));
+}

tests/snapshots/snapshot_tests__enum_error_redacted.snap

@@ -0,0 +1,5 @@
+---
+source: tests/snapshot_tests.rs
+expression: "format!(\"{:?}\", error)"
+---
+ApiResponse::Error { code: 500, details: "[REDACTED]" }