feat(lint): add multi-contract-file rule

crates/config/src/lint.rs

@@ -26,11 +26,8 @@ pub struct LinterConfig {
     /// Defaults to true. Set to false to disable automatic linting during builds.
     pub lint_on_build: bool,
 
-    /// Configurable patterns that should be excluded when performing `mixedCase` lint checks.
-    ///
-    /// Default's to ["ERC", "URI"] to allow common names like `rescueERC20`, `ERC721TokenReceiver`
-    /// or `tokenURI`.
-    pub mixed_case_exceptions: Vec<String>,
+    /// Configuration specific to individual lints.
+    pub lint_specific: LintSpecificConfig,
 }
 
 impl Default for LinterConfig {
@@ -40,7 +37,44 @@ impl Default for LinterConfig {
             severity: Vec::new(),
             exclude_lints: Vec::new(),
             ignore: Vec::new(),
+            lint_specific: LintSpecificConfig::default(),
+        }
+    }
+}
+
+/// Contract types that can be exempted from the multi-contract-file lint.
+#[derive(Clone, Debug, PartialEq, Eq, Serialize, Deserialize)]
+#[serde(rename_all = "snake_case")]
+pub enum ContractException {
+    Interface,
+    Library,
+    AbstractContract,
+}
+
+/// Configuration specific to individual lints.
+#[derive(Clone, Debug, PartialEq, Eq, Serialize, Deserialize)]
+#[serde(default)]
+pub struct LintSpecificConfig {
+    /// Configurable patterns that should be excluded when performing `mixedCase` lint checks.
+    ///
+    /// Defaults to ["ERC", "URI"] to allow common names like `rescueERC20`, `ERC721TokenReceiver`
+    /// or `tokenURI`.
+    pub mixed_case_exceptions: Vec<String>,
+
+    /// Contract types that are allowed to appear multiple times in the same file.
+    ///
+    /// Valid values: "interface", "library", "abstract_contract"
+    ///
+    /// Defaults to an empty array (all contract types are flagged when multiple exist).
+    /// Note: Regular contracts cannot be exempted and will always be flagged when multiple exist.
+    pub multi_contract_file_exceptions: Vec<ContractException>,
+}
+
+impl Default for LintSpecificConfig {
+    fn default() -> Self {
+        Self {
             mixed_case_exceptions: vec!["ERC".to_string(), "URI".to_string()],
+            multi_contract_file_exceptions: Vec::new(),
         }
     }
 }

crates/forge/src/cmd/build.rs

@@ -151,7 +151,7 @@ impl BuildArgs {
                             .collect(),
                     )
                 })
-                .with_mixed_case_exceptions(&config.lint.mixed_case_exceptions);
+                .with_lint_specific(&config.lint.lint_specific);
 
             // Expand ignore globs and canonicalize from the get go
             let ignored = expand_globs(&config.root, config.lint.ignore.iter())?

crates/forge/src/cmd/lint.rs

@@ -104,7 +104,7 @@ impl LintArgs {
             .with_lints(include)
             .without_lints(exclude)
             .with_severity(if severity.is_empty() { None } else { Some(severity) })
-            .with_mixed_case_exceptions(&config.lint.mixed_case_exceptions);
+            .with_lint_specific(&config.lint.lint_specific);
 
         let output = ProjectCompiler::new().files(input.iter().cloned()).compile(&project)?;
         let solar_sources = get_solar_sources_from_compile_output(&config, &output, Some(&input))?;

crates/forge/tests/cli/config.rs

@@ -146,10 +146,13 @@ severity = []
 exclude_lints = []
 ignore = []
 lint_on_build = true
+
+[lint.lint_specific]
 mixed_case_exceptions = [
     "ERC",
     "URI",
 ]
+multi_contract_file_exceptions = []
 
 [doc]
 out = "docs"
@@ -1339,10 +1342,13 @@ forgetest_init!(test_default_config, |prj, cmd| {
     "exclude_lints": [],
     "ignore": [],
     "lint_on_build": true,
-    "mixed_case_exceptions": [
-      "ERC",
-      "URI"
-    ]
+    "lint_specific": {
+      "mixed_case_exceptions": [
+        "ERC",
+        "URI"
+      ],
+      "multi_contract_file_exceptions": []
+    }
   },
   "doc": {
     "out": "docs",