Rollup merge of #121364 - Urgau:unary_precedence, r=compiler-errors

Implement lint against ambiguous negative literals

This PR implements a lint against ambiguous negative literals with a literal and method calls right after it.

## `ambiguous_negative_literals`

(deny-by-default)

The `ambiguous_negative_literals` lint checks for cases that are confusing between a negative literal and a negation that's not part of the literal.

### Example

```rust,compile_fail
-1i32.abs(); // equals -1, while `(-1i32).abs()` equals 1
```

### Explanation

Method calls take precedence over unary precedence. Setting the precedence explicitly makes the code clearer and avoid potential bugs.

<details>
<summary>Old proposed lint</summary>

## `ambiguous_unary_precedence`

(deny-by-default)

The `ambiguous_unary_precedence` lint checks for use the negative unary operator with a literal and method calls.

### Example

```rust
-1i32.abs(); // equals -1, while `(-1i32).abs()` equals 1
```

### Explanation

Unary operations take precedence on binary operations and method calls take precedence over unary precedence. Setting the precedence explicitly makes the code clearer and avoid potential bugs.

</details>

-----

Note: This is a strip down version of https://github.com/rust-lang/rust/pull/117161, without the binary op precedence.

Fixes https://github.com/rust-lang/rust/issues/117155
`@rustbot` labels +I-lang-nominated
cc `@scottmcm`
r? compiler

compiler/rustc_lint/messages.ftl

@@ -5,6 +5,11 @@ lint_ambiguous_glob_reexport = ambiguous glob re-exports
     .label_first_reexport = the name `{$name}` in the {$namespace} namespace is first re-exported here
     .label_duplicate_reexport = but the name `{$name}` in the {$namespace} namespace is also re-exported here
 
+lint_ambiguous_negative_literals = `-` has lower precedence than method calls, which might be unexpected
+    .example = e.g. `-4.abs()` equals `-4`; while `(-4).abs()` equals `4`
+    .negative_literal = add parentheses around the `-` and the literal to call the method on a negative literal
+    .current_behavior = add parentheses around the literal and the method call to keep the current behavior
+
 lint_ambiguous_wide_pointer_comparisons = ambiguous wide pointer comparison, the comparison includes metadata which may not be expected
     .addr_metadata_suggestion = use explicit `std::ptr::eq` method to compare metadata and addresses
     .addr_suggestion = use `std::ptr::addr_eq` or untyped pointers to only compare their addresses

compiler/rustc_lint/src/lib.rs

@@ -73,6 +73,7 @@ mod noop_method_call;
 mod opaque_hidden_inferred_bound;
 mod pass_by_value;
 mod passes;
+mod precedence;
 mod ptr_nulls;
 mod redundant_semicolon;
 mod reference_casting;
@@ -111,6 +112,7 @@ use nonstandard_style::*;
 use noop_method_call::*;
 use opaque_hidden_inferred_bound::*;
 use pass_by_value::*;
+use precedence::*;
 use ptr_nulls::*;
 use redundant_semicolon::*;
 use reference_casting::*;
@@ -174,6 +176,7 @@ early_lint_methods!(
             RedundantSemicolons: RedundantSemicolons,
             UnusedDocComment: UnusedDocComment,
             Expr2024: Expr2024,
+            Precedence: Precedence,
         ]
     ]
 );

compiler/rustc_lint/src/lints.rs

@@ -1499,6 +1499,35 @@ pub struct NonLocalDefinitionsCargoUpdateNote {
     pub crate_name: Symbol,
 }
 
+// precedence.rs
+#[derive(LintDiagnostic)]
+#[diag(lint_ambiguous_negative_literals)]
+#[note(lint_example)]
+pub struct AmbiguousNegativeLiteralsDiag {
+    #[subdiagnostic]
+    pub negative_literal: AmbiguousNegativeLiteralsNegativeLiteralSuggestion,
+    #[subdiagnostic]
+    pub current_behavior: AmbiguousNegativeLiteralsCurrentBehaviorSuggestion,
+}
+
+#[derive(Subdiagnostic)]
+#[multipart_suggestion(lint_negative_literal, applicability = "maybe-incorrect")]
+pub struct AmbiguousNegativeLiteralsNegativeLiteralSuggestion {
+    #[suggestion_part(code = "(")]
+    pub start_span: Span,
+    #[suggestion_part(code = ")")]
+    pub end_span: Span,
+}
+
+#[derive(Subdiagnostic)]
+#[multipart_suggestion(lint_current_behavior, applicability = "maybe-incorrect")]
+pub struct AmbiguousNegativeLiteralsCurrentBehaviorSuggestion {
+    #[suggestion_part(code = "(")]
+    pub start_span: Span,
+    #[suggestion_part(code = ")")]
+    pub end_span: Span,
+}
+
 // pass_by_value.rs
 #[derive(LintDiagnostic)]
 #[diag(lint_pass_by_value)]

compiler/rustc_lint/src/precedence.rs

@@ -0,0 +1,70 @@
+use rustc_ast::token::LitKind;
+use rustc_ast::{Expr, ExprKind, MethodCall, UnOp};
+use rustc_session::{declare_lint, declare_lint_pass};
+
+use crate::lints::{
+    AmbiguousNegativeLiteralsCurrentBehaviorSuggestion, AmbiguousNegativeLiteralsDiag,
+    AmbiguousNegativeLiteralsNegativeLiteralSuggestion,
+};
+use crate::{EarlyContext, EarlyLintPass, LintContext};
+
+declare_lint! {
+    /// The `ambiguous_negative_literals` lint checks for cases that are
+    /// confusing between a negative literal and a negation that's not part
+    /// of the literal.
+    ///
+    /// ### Example
+    ///
+    /// ```rust,compile_fail
+    /// # #![allow(unused)]
+    /// -1i32.abs(); // equals -1, while `(-1i32).abs()` equals 1
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// Method calls take precedence over unary precedence. Setting the
+    /// precedence explicitly makes the code clearer and avoid potential bugs.
+    pub AMBIGUOUS_NEGATIVE_LITERALS,
+    Deny,
+    "ambiguous negative literals operations",
+    report_in_external_macro
+}
+
+declare_lint_pass!(Precedence => [AMBIGUOUS_NEGATIVE_LITERALS]);
+
+impl EarlyLintPass for Precedence {
+    fn check_expr(&mut self, cx: &EarlyContext<'_>, expr: &Expr) {
+        let ExprKind::Unary(UnOp::Neg, operand) = &expr.kind else {
+            return;
+        };
+
+        let mut arg = operand;
+        let mut at_least_one = false;
+        while let ExprKind::MethodCall(box MethodCall { receiver, .. }) = &arg.kind {
+            at_least_one = true;
+            arg = receiver;
+        }
+
+        if at_least_one
+            && let ExprKind::Lit(lit) = &arg.kind
+            && let LitKind::Integer | LitKind::Float = &lit.kind
+        {
+            cx.emit_span_lint(
+                AMBIGUOUS_NEGATIVE_LITERALS,
+                expr.span,
+                AmbiguousNegativeLiteralsDiag {
+                    negative_literal: AmbiguousNegativeLiteralsNegativeLiteralSuggestion {
+                        start_span: expr.span.shrink_to_lo(),
+                        end_span: arg.span.shrink_to_hi(),
+                    },
+                    current_behavior: AmbiguousNegativeLiteralsCurrentBehaviorSuggestion {
+                        start_span: operand.span.shrink_to_lo(),
+                        end_span: operand.span.shrink_to_hi(),
+                    },
+                },
+            );
+        }
+    }
+}