Start uplifting clippy::for_loops_over_fallibles

I refactored the code:
- Removed handling of methods, as it felt entirely unnecessary
- Removed clippy utils (obviously...)
- Used some shiny compiler features
  (let-else is very handy for lints 👀)
- I also renamed the lint to `for_loop_over_fallibles` (note: no `s`).
  I'm not sure what's the naming convention here, so maybe I'm wrong.

compiler/rustc_lint/src/for_loop_over_fallibles.rs

@@ -0,0 +1,99 @@
+use crate::{LateContext, LateLintPass, LintContext};
+
+use hir::{Expr, Pat};
+use rustc_hir as hir;
+use rustc_middle::ty;
+use rustc_span::sym;
+
+declare_lint! {
+    /// ### What it does
+    ///
+    /// Checks for `for` loops over `Option` or `Result` values.
+    ///
+    /// ### Why is this bad?
+    /// Readability. This is more clearly expressed as an `if
+    /// let`.
+    ///
+    /// ### Example
+    ///
+    /// ```rust
+    /// # let opt = Some(1);
+    /// # let res: Result<i32, std::io::Error> = Ok(1);
+    /// for x in opt {
+    ///     // ..
+    /// }
+    ///
+    /// for x in &res {
+    ///     // ..
+    /// }
+    ///
+    /// for x in res.iter() {
+    ///     // ..
+    /// }
+    /// ```
+    ///
+    /// Use instead:
+    /// ```rust
+    /// # let opt = Some(1);
+    /// # let res: Result<i32, std::io::Error> = Ok(1);
+    /// if let Some(x) = opt {
+    ///     // ..
+    /// }
+    ///
+    /// if let Ok(x) = res {
+    ///     // ..
+    /// }
+    /// ```
+    pub FOR_LOOP_OVER_FALLIBLES,
+    Warn,
+    "for-looping over an `Option` or a `Result`, which is more clearly expressed as an `if let`"
+}
+
+declare_lint_pass!(ForLoopOverFallibles => [FOR_LOOP_OVER_FALLIBLES]);
+
+impl<'tcx> LateLintPass<'tcx> for ForLoopOverFallibles {
+    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
+        let Some((pat, arg)) = extract_for_loop(expr) else { return };
+
+        let ty = cx.typeck_results().expr_ty(arg);
+
+        let ty::Adt(adt, _) = ty.kind() else { return };
+
+        let (article, ty, var) = match adt.did() {
+            did if cx.tcx.is_diagnostic_item(sym::Option, did) => ("an", "Option", "Some"),
+            did if cx.tcx.is_diagnostic_item(sym::Result, did) => ("a", "Result", "Ok"),
+            _ => return,
+        };
+
+        let Ok(pat_snip) = cx.sess().source_map().span_to_snippet(pat.span) else { return };
+        let Ok(arg_snip) = cx.sess().source_map().span_to_snippet(arg.span) else { return };
+
+        let help_string = format!(
+            "consider replacing `for {pat_snip} in {arg_snip}` with `if let {var}({pat_snip}) = {arg_snip}`"
+        );
+        let msg = format!(
+            "for loop over `{arg_snip}`, which is {article} `{ty}`. This is more readably written as an `if let` statement",
+        );
+
+        cx.struct_span_lint(FOR_LOOP_OVER_FALLIBLES, arg.span, |diag| {
+            diag.build(msg).help(help_string).emit()
+        })
+    }
+}
+
+fn extract_for_loop<'tcx>(expr: &Expr<'tcx>) -> Option<(&'tcx Pat<'tcx>, &'tcx Expr<'tcx>)> {
+    if let hir::ExprKind::DropTemps(e) = expr.kind
+    && let hir::ExprKind::Match(iterexpr, [arm], hir::MatchSource::ForLoopDesugar) = e.kind
+    && let hir::ExprKind::Call(_, [arg]) = iterexpr.kind
+    && let hir::ExprKind::Loop(block, ..) = arm.body.kind
+    && let [stmt] = block.stmts
+    && let hir::StmtKind::Expr(e) = stmt.kind
+    && let hir::ExprKind::Match(_, [_, some_arm], _) = e.kind
+    && let hir::PatKind::Struct(_, [field], _) = some_arm.pat.kind 
+    {
+        Some((field.pat, arg))
+    } else {
+        None
+    }
+
+}

compiler/rustc_lint/src/lib.rs

@@ -52,6 +52,7 @@ mod early;
 mod enum_intrinsics_non_enums;
 mod errors;
 mod expect;
+mod for_loop_over_fallibles;
 pub mod hidden_unicode_codepoints;
 mod internal;
 mod late;
@@ -86,6 +87,7 @@ use rustc_span::Span;
 use array_into_iter::ArrayIntoIter;
 use builtin::*;
 use enum_intrinsics_non_enums::EnumIntrinsicsNonEnums;
+use for_loop_over_fallibles::*;
 use hidden_unicode_codepoints::*;
 use internal::*;
 use let_underscore::*;
@@ -188,6 +190,7 @@ macro_rules! late_lint_mod_passes {
         $macro!(
             $args,
             [
+                ForLoopOverFallibles: ForLoopOverFallibles,
                 HardwiredLints: HardwiredLints,
                 ImproperCTypesDeclarations: ImproperCTypesDeclarations,
                 ImproperCTypesDefinitions: ImproperCTypesDefinitions,