[ty] disable division-by-zero by default (#18220)

## Summary

I think `division-by-zero` is a low-value diagnostic in general; most
real division-by-zero errors (especially those that are less obvious to
the human eye) will occur on values typed as `int`, in which case we
don't issue the diagnostic anyway. Mypy and pyright do not emit this
diagnostic.

Currently the diagnostic is prone to false positives because a) we do
not silence it in unreachable code, and b) we do not implement narrowing
of literals from inequality checks. We will probably fix (a) regardless,
but (b) is low priority apart from division-by-zero.

I think we have many more important things to do and should not allow
false positives on a low-value diagnostic to be a distraction. Not
opposed to re-enabling this diagnostic in future when we can prioritize
reducing its false positives.

References https://github.com/astral-sh/ty/issues/443

## Test Plan

Existing tests.

.github/mypy-primer-ty.toml

@@ -5,3 +5,4 @@
 [rules]
 possibly-unresolved-reference = "warn"
 unused-ignore-comment = "warn"
+division-by-zero = "warn"

crates/ty/docs/rules.md

@@ -176,29 +176,6 @@ class B(A): ...
 * [View source](https://github.com/astral-sh/ruff/blob/main/crates%2Fty_python_semantic%2Fsrc%2Ftypes%2Fdiagnostic.rs#L195)
 </details>
 
-## `division-by-zero`
-
-**Default level**: error
-
-<details>
-<summary>detects division by zero</summary>
-
-### What it does
-It detects division by zero.
-
-### Why is this bad?
-Dividing by zero raises a `ZeroDivisionError` at runtime.
-
-### Examples
-```python
-5 / 0
-```
-
-### Links
-* [Related issues](https://github.com/astral-sh/ty/issues?q=sort%3Aupdated-desc%20is%3Aissue%20is%3Aopen%20division-by-zero)
-* [View source](https://github.com/astral-sh/ruff/blob/main/crates%2Fty_python_semantic%2Fsrc%2Ftypes%2Fdiagnostic.rs#L221)
-</details>
-
 ## `duplicate-base`
 
 **Default level**: error
@@ -1743,6 +1720,29 @@ a = 20 / 0  # ty: ignore[division-by-zero]
 * [View source](https://github.com/astral-sh/ruff/blob/main/crates%2Fty_python_semantic%2Fsrc%2Fsuppression.rs#L40)
 </details>
 
+## `division-by-zero`
+
+**Default level**: ignore
+
+<details>
+<summary>detects division by zero</summary>
+
+### What it does
+It detects division by zero.
+
+### Why is this bad?
+Dividing by zero raises a `ZeroDivisionError` at runtime.
+
+### Examples
+```python
+5 / 0
+```
+
+### Links
+* [Related issues](https://github.com/astral-sh/ty/issues?q=sort%3Aupdated-desc%20is%3Aissue%20is%3Aopen%20division-by-zero)
+* [View source](https://github.com/astral-sh/ruff/blob/main/crates%2Fty_python_semantic%2Fsrc%2Ftypes%2Fdiagnostic.rs#L221)
+</details>
+
 ## `possibly-unresolved-reference`
 
 **Default level**: ignore

crates/ty/tests/cli.rs

@@ -387,22 +387,11 @@ fn configuration_rule_severity() -> anyhow::Result<()> {
             "#,
     )?;
 
-    // Assert that there's an `unresolved-reference` diagnostic (error)
-    // and a `division-by-zero` diagnostic (error).
-    assert_cmd_snapshot!(case.command(), @r"
+    // Assert that there's an `unresolved-reference` diagnostic (error).
+    assert_cmd_snapshot!(case.command(), @r###"
     success: false
     exit_code: 1
     ----- stdout -----
-    error[division-by-zero]: Cannot divide object of type `Literal[4]` by zero
-     --> test.py:2:5
-      |
-    2 | y = 4 / 0
-      |     ^^^^^
-    3 |
-    4 | for a in range(0, int(y)):
-      |
-    info: rule `division-by-zero` is enabled by default
-
     error[unresolved-reference]: Name `prin` used when not defined
      --> test.py:7:1
       |
@@ -413,17 +402,17 @@ fn configuration_rule_severity() -> anyhow::Result<()> {
       |
     info: rule `unresolved-reference` is enabled by default
 
-    Found 2 diagnostics
+    Found 1 diagnostic
 
     ----- stderr -----
     WARN ty is pre-release software and not ready for production use. Expect to encounter bugs, missing features, and fatal errors.
-    ");
+    "###);
 
     case.write_file(
         "pyproject.toml",
         r#"
         [tool.ty.rules]
-        division-by-zero = "warn" # demote to warn
+        division-by-zero = "warn" # promote to warn
         unresolved-reference = "ignore"
     "#,
     )?;
@@ -468,9 +457,9 @@ fn cli_rule_severity() -> anyhow::Result<()> {
         "#,
     )?;
 
-    // Assert that there's an `unresolved-reference` diagnostic (error),
-    // a `division-by-zero` (error) and a unresolved-import (error) diagnostic by default.
-    assert_cmd_snapshot!(case.command(), @r"
+    // Assert that there's an `unresolved-reference` diagnostic (error)
+    // and an unresolved-import (error) diagnostic by default.
+    assert_cmd_snapshot!(case.command(), @r###"
     success: false
     exit_code: 1
     ----- stdout -----
@@ -485,18 +474,6 @@ fn cli_rule_severity() -> anyhow::Result<()> {
     info: make sure your Python environment is properly configured: https://github.com/astral-sh/ty/blob/main/docs/README.md#python-environment
     info: rule `unresolved-import` is enabled by default
 
-    error[division-by-zero]: Cannot divide object of type `Literal[4]` by zero
-     --> test.py:4:5
-      |
-    2 | import does_not_exit
-    3 |
-    4 | y = 4 / 0
-      |     ^^^^^
-    5 |
-    6 | for a in range(0, int(y)):
-      |
-    info: rule `division-by-zero` is enabled by default
-
     error[unresolved-reference]: Name `prin` used when not defined
      --> test.py:9:1
       |
@@ -507,11 +484,11 @@ fn cli_rule_severity() -> anyhow::Result<()> {
       |
     info: rule `unresolved-reference` is enabled by default
 
-    Found 3 diagnostics
+    Found 2 diagnostics
 
     ----- stderr -----
     WARN ty is pre-release software and not ready for production use. Expect to encounter bugs, missing features, and fatal errors.
-    ");
+    "###);
 
     assert_cmd_snapshot!(
         case
@@ -575,22 +552,11 @@ fn cli_rule_severity_precedence() -> anyhow::Result<()> {
         "#,
     )?;
 
-    // Assert that there's a `unresolved-reference` diagnostic (error)
-    // and a `division-by-zero` (error) by default.
-    assert_cmd_snapshot!(case.command(), @r"
+    // Assert that there's a `unresolved-reference` diagnostic (error) by default.
+    assert_cmd_snapshot!(case.command(), @r###"
     success: false
     exit_code: 1
     ----- stdout -----
-    error[division-by-zero]: Cannot divide object of type `Literal[4]` by zero
-     --> test.py:2:5
-      |
-    2 | y = 4 / 0
-      |     ^^^^^
-    3 |
-    4 | for a in range(0, int(y)):
-      |
-    info: rule `division-by-zero` is enabled by default
-
     error[unresolved-reference]: Name `prin` used when not defined
      --> test.py:7:1
       |
@@ -601,11 +567,11 @@ fn cli_rule_severity_precedence() -> anyhow::Result<()> {
       |
     info: rule `unresolved-reference` is enabled by default
 
-    Found 2 diagnostics
+    Found 1 diagnostic
 
     ----- stderr -----
     WARN ty is pre-release software and not ready for production use. Expect to encounter bugs, missing features, and fatal errors.
-    ");
+    "###);
 
     assert_cmd_snapshot!(
         case
@@ -614,7 +580,6 @@ fn cli_rule_severity_precedence() -> anyhow::Result<()> {
             .arg("unresolved-reference")
             .arg("--warn")
             .arg("division-by-zero")
-            // Override the error severity with warning
             .arg("--ignore")
             .arg("unresolved-reference"),
         @r"
@@ -1103,18 +1068,10 @@ fn check_specific_paths() -> anyhow::Result<()> {
 
     assert_cmd_snapshot!(
         case.command(),
-        @r"
+        @r###"
     success: false
     exit_code: 1
     ----- stdout -----
-    error[division-by-zero]: Cannot divide object of type `Literal[4]` by zero
-     --> project/main.py:2:5
-      |
-    2 | y = 4 / 0  # error: division-by-zero
-      |     ^^^^^
-      |
-    info: rule `division-by-zero` is enabled by default
-
     error[unresolved-import]: Cannot resolve imported module `main2`
      --> project/other.py:2:6
       |
@@ -1135,11 +1092,11 @@ fn check_specific_paths() -> anyhow::Result<()> {
     info: make sure your Python environment is properly configured: https://github.com/astral-sh/ty/blob/main/docs/README.md#python-environment
     info: rule `unresolved-import` is enabled by default
 
-    Found 3 diagnostics
+    Found 2 diagnostics
 
     ----- stderr -----
     WARN ty is pre-release software and not ready for production use. Expect to encounter bugs, missing features, and fatal errors.
-    "
+    "###
     );
 
     // Now check only the `tests` and `other.py` files.

crates/ty_python_semantic/src/types/diagnostic.rs

@@ -232,7 +232,7 @@ declare_lint! {
     pub(crate) static DIVISION_BY_ZERO = {
         summary: "detects division by zero",
         status: LintStatus::preview("1.0.0"),
-        default_level: Level::Error,
+        default_level: Level::Ignore,
     }
 }
 

ty.schema.json

@@ -293,7 +293,7 @@
         "division-by-zero": {
           "title": "detects division by zero",
           "description": "## What it does\nIt detects division by zero.\n\n## Why is this bad?\nDividing by zero raises a `ZeroDivisionError` at runtime.\n\n## Examples\n```python\n5 / 0\n```",
-          "default": "error",
+          "default": "ignore",
           "oneOf": [
             {
               "$ref": "#/definitions/Level"