[clang-tools-extra] [clang-tidy] Add readability-use-span-first-last check (PR #118074)

[Julian Schmidt via cfe-commits]

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@@ -0,0 +1,104 @@
+//===--- UseSpanFirstLastCheck.cpp - clang-tidy -----------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "UseSpanFirstLastCheck.h"
+#include "../utils/ASTUtils.h"
+#include "../utils/Matchers.h"
+#include "clang/AST/ASTContext.h"
+#include "clang/AST/RecursiveASTVisitor.h"
+#include "clang/ASTMatchers/ASTMatchFinder.h"
+#include "clang/ASTMatchers/ASTMatchers.h"
+#include "clang/Lex/Lexer.h"
+
+using namespace clang::ast_matchers;
+using namespace clang::tidy::matchers;
+
+namespace clang::tidy::readability {
+
+void UseSpanFirstLastCheck::registerMatchers(MatchFinder *Finder) {
+  const auto HasSpanType =
+      hasType(hasUnqualifiedDesugaredType(recordType(hasDeclaration(
+          classTemplateSpecializationDecl(hasName("::std::span"))))));
+
+  // Match span.subspan(0, n) -> first(n)
+  Finder->addMatcher(
+      cxxMemberCallExpr(
+          callee(memberExpr(hasDeclaration(cxxMethodDecl(hasName("subspan"))))),
+          on(expr(HasSpanType).bind("span_object")),
+          hasArgument(0, integerLiteral(equals(0))),
+          hasArgument(1, expr().bind("count")), argumentCountIs(2))
+          .bind("first_subspan"),
+      this);
+
+  // Match span.subspan(span.size() - n) or span.subspan(std::ranges::size(span)
+  // - n)
+  // -> last(n)
+  const auto SizeCall = anyOf(
+      cxxMemberCallExpr(
+          callee(memberExpr(hasDeclaration(cxxMethodDecl(hasName("size"))))),
+          on(expr(isStatementIdenticalToBoundNode("span_object")))),
+      callExpr(callee(functionDecl(
+                   hasAnyName("::std::size", "::std::ranges::size"))),
+               hasArgument(
+                   0, expr(isStatementIdenticalToBoundNode("span_object")))));
+
+  Finder->addMatcher(
+      cxxMemberCallExpr(
+          callee(memberExpr(hasDeclaration(cxxMethodDecl(hasName("subspan"))))),
+          on(expr(HasSpanType).bind("span_object")),
+          hasArgument(0, binaryOperator(hasOperatorName("-"), hasLHS(SizeCall),
+                                        hasRHS(expr().bind("count")))),
+          argumentCountIs(1))
+          .bind("last_subspan"),
+      this);
+}
+
+void UseSpanFirstLastCheck::check(const MatchFinder::MatchResult &Result) {
+  const auto *SpanObj = Result.Nodes.getNodeAs<Expr>("span_object");
+  if (!SpanObj)
+    return;
+
+  StringRef SpanText = Lexer::getSourceText(
+      CharSourceRange::getTokenRange(SpanObj->getSourceRange()),
+      *Result.SourceManager, Result.Context->getLangOpts());
+
+  if (const auto *FirstCall =
----------------
5chmidti wrote:

Except for which of the two functions should replace the matched code, these two `if` statements are identical. You could bind both cases to the same name, retrieve the `CXXMemberCallExpr`, and compare the number of arguments to determine which case it is:

```c++
    const auto IsFirst = SubSpan->getNumArgs() == 2;
    const StringRef FirstOrLast = IsFirst ? "first" : "last";

    std::string Replacement =
        (SpanText + "." + FirstOrLast + "(" + CountText + ")").str();

    diag(SubSpan->getBeginLoc(), "prefer 'span::%0()' over 'subspan()'")
        << FirstOrLast
        << FixItHint::CreateReplacement(SubSpan->getSourceRange(), Replacement);
```

https://github.com/llvm/llvm-project/pull/118074