Add unused-ignore-comment rule

Author: MichaReiser

crates/red_knot_python_semantic/resources/mdtest/suppressions/knot-ignore.md

@@ -14,26 +14,55 @@ a = 4 + test  # knot: ignore
 a = 4 + test  # knot: ignore[unresolved-reference]
 ```
 
-## Useless suppression
-
-TODO: Red Knot should emit an `unused-suppression` diagnostic for the
-`possibly-unresolved-reference` suppression.
+## Unused suppression
 
 ```py
 test = 10
+# error: [unused-ignore-comment] "Unused `knot: ignore` directive: 'possibly-unresolved-reference'"
 a = test + 3  # knot: ignore[possibly-unresolved-reference]
 ```
 
-## Useless suppression if the error codes don't match
-
-TODO: Red Knot should emit a `unused-suppression` diagnostic for the `possibly-unresolved-reference`
-suppression because it doesn't match the actual `unresolved-reference` diagnostic.
+## Unused suppression if the error codes don't match
 
 ```py
 # error: [unresolved-reference]
+# error: [unused-ignore-comment] "Unused `knot: ignore` directive: 'possibly-unresolved-reference'"
 a = test + 3  # knot: ignore[possibly-unresolved-reference]
 ```
 
+## Suppressed unused comment
+
+```py
+# error: [unused-ignore-comment]
+a = 10 / 2  # knot: ignore[division-by-zero]
+a = 10 / 2  # knot: ignore[division-by-zero, unused-ignore-comment]
+a = 10 / 2  # knot: ignore[unused-ignore-comment, division-by-zero]
+a = 10 / 2  # knot: ignore[unused-ignore-comment] # type: ignore
+a = 10 / 2  # type: ignore # knot: ignore[unused-ignore-comment]
+```
+
+## Unused ignore comment
+
+```py
+# error: [unused-ignore-comment] "Unused `knot: ignore` directive: 'unused-ignore-comment'"
+a = 10 / 0 # knot: ignore[division-by-zero, unused-ignore-comment]
+```
+
+## Multiple unused comments
+
+Today, Red Knot emits a diagnostic for every unused code. 
+We might want to group the codes by comment at some point in the future.
+
+```py
+# error: [unused-ignore-comment] "Unused `knot: ignore` directive: 'division-by-zero'"
+# error: [unused-ignore-comment] "Unused `knot: ignore` directive: 'unresolved-reference'"
+a = 10 / 2  # knot: ignore[division-by-zero, unresolved-reference]
+
+# error: [unused-ignore-comment] "Unused `knot: ignore` directive: 'invalid-assignment'"
+# error: [unused-ignore-comment] "Unused `knot: ignore` directive: 'unresolved-reference'"
+a = 10 / 0  # knot: ignore[invalid-assignment, division-by-zero, unresolved-reference]
+```
+
 ## Multiple suppressions
 
 ```py
@@ -47,17 +76,20 @@ def test(a: f"f-string type annotation", b: b"byte-string-type-annotation"): ...
 
 ```py
 # error: [invalid-syntax]
+# error: [unused-ignore-comment]
 def test(  # knot: ignore
 ```
 
 <!-- blacken-docs:on -->
 
 ## Can't suppress `revealed-type` diagnostics
 
+TODO: Emit an error that the rule code is unknown: `unknown-rule`
+
 ```py
 a = 10
 # revealed: Literal[10]
-reveal_type(a)  # knot: ignore
+reveal_type(a)  # knot: ignore[revealed-type]
 ```
 
 ## Extra whitespace in type ignore comments is allowed
@@ -126,6 +158,7 @@ if they should use a different syntax that also supports enabling rules or chang
 severity: `knot: possibly-undefined-reference=error`
 
 ```py
+# error: [unused-ignore-comment]
 # knot: ignore[division-by-zero]
 
 a = 4 / 0  # error: [division-by-zero]

crates/red_knot_python_semantic/resources/mdtest/suppressions/type-ignore.md

@@ -43,6 +43,8 @@ from typing import cast
 
 y = (
     cast(int, "test" +
+            # TODO: Remove the expected error after implementing `invalid-operator` for binary expressions
+            # error: [unused-ignore-comment]
             2 # type: ignore
     )
     + "other"  # TODO: expected-error[invalid-operator]
@@ -118,6 +120,7 @@ in Pyright. Neither Ruff, nor mypy support this and neither does Red Knot.
 
 ```py
 # fmt: off
+# error: [unused-ignore-comment]
 a = (  # type: ignore
     test + 4  # error: [unresolved-reference]
 )
@@ -149,7 +152,7 @@ b = a / 0
 File level suppressions must come before any non-trivia token,
 including module docstrings. 
 """
-
+# error: [unused-ignore-comment] "Unused blanked `type: ignore` directive"
 # type: ignore
 
 a = 10 / 0  # error: [division-by-zero]

crates/red_knot_python_semantic/src/lib.rs

@@ -3,6 +3,7 @@ use std::hash::BuildHasherDefault;
 use rustc_hash::FxHasher;
 
 use crate::lint::{LintRegistry, LintRegistryBuilder};
+use crate::suppression::UNUSED_IGNORE_COMMENT;
 pub use db::Db;
 pub use module_name::ModuleName;
 pub use module_resolver::{resolve_module, system_module_search_paths, KnownModule, Module};
@@ -44,4 +45,5 @@ pub fn default_lint_registry() -> &'static LintRegistry {
 /// Register all known semantic lints.
 pub fn register_lints(registry: &mut LintRegistryBuilder) {
     types::register_lints(registry);
+    registry.register_lint(&UNUSED_IGNORE_COMMENT);
 }