Commit with just personally changed files.

clippy_lints/src/methods/join_absolute_paths.rs

@@ -0,0 +1,30 @@
+use clippy_utils::diagnostics::span_lint_and_then;
+use clippy_utils::ty::is_type_diagnostic_item;
+use rustc_ast::ast::LitKind;
+use rustc_hir::{Expr, ExprKind};
+use rustc_lint::LateContext;
+use rustc_span::symbol::sym::Path;
+
+use super::JOIN_ABSOLUTE_PATHS;
+
+pub(super) fn check<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'tcx>, join_arg: &'tcx Expr<'tcx>) {
+    let ty = cx.typeck_results().expr_ty(expr).peel_refs();
+    if is_type_diagnostic_item(cx, ty, Path)
+        && let ExprKind::Lit(spanned) = &join_arg.kind
+        && let LitKind::Str(symbol, _) = spanned.node
+        && (symbol.as_str().starts_with('/') || symbol.as_str().starts_with('\\'))
+    {
+        span_lint_and_then(
+            cx,
+            JOIN_ABSOLUTE_PATHS,
+            join_arg.span,
+            "argument to `Path::join` starts with a path separator",
+            |diag| {
+                diag
+                      .note("joining a path starting with separator will replace the path instead")
+                      .help(r#"if this is unintentional, try removing the starting separator"#)
+                      .help(r#"if this is intentional, try creating a new Path instead"#);
+            },
+        );
+    }
+}