Add match_str_case_mismatch lint

8 files changed, 323 insertions, 0 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 13067e4e92a..1f79d9e73db 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2837,6 +2837,7 @@ Released 2018-09-13
 [`match_result_ok`]: https://rust-lang.github.io/rust-clippy/master/index.html#match_result_ok
 [`match_same_arms`]: https://rust-lang.github.io/rust-clippy/master/index.html#match_same_arms
 [`match_single_binding`]: https://rust-lang.github.io/rust-clippy/master/index.html#match_single_binding
+[`match_str_case_mismatch`]: https://rust-lang.github.io/rust-clippy/master/index.html#match_str_case_mismatch
 [`match_wild_err_arm`]: https://rust-lang.github.io/rust-clippy/master/index.html#match_wild_err_arm
 [`match_wildcard_for_single_variants`]: https://rust-lang.github.io/rust-clippy/master/index.html#match_wildcard_for_single_variants
 [`maybe_infinite_iter`]: https://rust-lang.github.io/rust-clippy/master/index.html#maybe_infinite_iter
diff --git a/clippy_lints/src/lib.register_all.rs b/clippy_lints/src/lib.register_all.rs
index 3e6e0244754..e544c6607e8 100644
--- a/clippy_lints/src/lib.register_all.rs
+++ b/clippy_lints/src/lib.register_all.rs
@@ -118,6 +118,7 @@ store.register_group(true, "clippy::all", Some("clippy_all"), vec![
     LintId::of(map_unit_fn::OPTION_MAP_UNIT_FN),
     LintId::of(map_unit_fn::RESULT_MAP_UNIT_FN),
     LintId::of(match_result_ok::MATCH_RESULT_OK),
+    LintId::of(match_str_case_mismatch::MATCH_STR_CASE_MISMATCH),
     LintId::of(matches::INFALLIBLE_DESTRUCTURING_MATCH),
     LintId::of(matches::MATCH_AS_REF),
     LintId::of(matches::MATCH_LIKE_MATCHES_MACRO),
diff --git a/clippy_lints/src/lib.register_correctness.rs b/clippy_lints/src/lib.register_correctness.rs
index e0ef7b3b8af..1c2441d1af2 100644
--- a/clippy_lints/src/lib.register_correctness.rs
+++ b/clippy_lints/src/lib.register_correctness.rs
@@ -36,6 +36,7 @@ store.register_group(true, "clippy::correctness", Some("clippy_correctness"), ve
     LintId::of(loops::ITER_NEXT_LOOP),
     LintId::of(loops::NEVER_LOOP),
     LintId::of(loops::WHILE_IMMUTABLE_CONDITION),
+    LintId::of(match_str_case_mismatch::MATCH_STR_CASE_MISMATCH),
     LintId::of(mem_discriminant::MEM_DISCRIMINANT_NON_ENUM),
     LintId::of(mem_replace::MEM_REPLACE_WITH_UNINIT),
     LintId::of(methods::CLONE_DOUBLE_REF),
diff --git a/clippy_lints/src/lib.register_lints.rs b/clippy_lints/src/lib.register_lints.rs
index 3c4b720671a..d5673ad2c7c 100644
--- a/clippy_lints/src/lib.register_lints.rs
+++ b/clippy_lints/src/lib.register_lints.rs
@@ -226,6 +226,7 @@ store.register_lints(&[
     map_unit_fn::RESULT_MAP_UNIT_FN,
     match_on_vec_items::MATCH_ON_VEC_ITEMS,
     match_result_ok::MATCH_RESULT_OK,
+    match_str_case_mismatch::MATCH_STR_CASE_MISMATCH,
     matches::INFALLIBLE_DESTRUCTURING_MATCH,
     matches::MATCH_AS_REF,
     matches::MATCH_BOOL,
diff --git a/clippy_lints/src/lib.rs b/clippy_lints/src/lib.rs
index 7e1bcbbd0ed..73d21eb5b52 100644
--- a/clippy_lints/src/lib.rs
+++ b/clippy_lints/src/lib.rs
@@ -265,6 +265,7 @@ mod map_err_ignore;
 mod map_unit_fn;
 mod match_on_vec_items;
 mod match_result_ok;
+mod match_str_case_mismatch;
 mod matches;
 mod mem_discriminant;
 mod mem_forget;
@@ -771,6 +772,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
     let enable_raw_pointer_heuristic_for_send = conf.enable_raw_pointer_heuristic_for_send;
     store.register_late_pass(move || Box::new(non_send_fields_in_send_ty::NonSendFieldInSendTy::new(enable_raw_pointer_heuristic_for_send)));
     store.register_late_pass(move || Box::new(undocumented_unsafe_blocks::UndocumentedUnsafeBlocks::default()));
+    store.register_late_pass(|| Box::new(match_str_case_mismatch::MatchStrCaseMismatch));
 }
 
 #[rustfmt::skip]
diff --git a/clippy_lints/src/match_str_case_mismatch.rs b/clippy_lints/src/match_str_case_mismatch.rs
new file mode 100644
index 00000000000..dd8b5cb4a21
--- /dev/null
+++ b/clippy_lints/src/match_str_case_mismatch.rs
@@ -0,0 +1,166 @@
+use clippy_utils::diagnostics::span_lint_and_help;
+use clippy_utils::ty::is_type_diagnostic_item;
+use rustc_ast::ast::LitKind;
+use rustc_hir::intravisit::{walk_expr, NestedVisitorMap, Visitor};
+use rustc_hir::{Arm, Expr, ExprKind, MatchSource, PatKind};
+use rustc_lint::{LateContext, LateLintPass};
+use rustc_middle::hir::map::Map;
+use rustc_middle::lint::in_external_macro;
+use rustc_middle::ty;
+use rustc_session::{declare_lint_pass, declare_tool_lint};
+use rustc_span::{sym, Span};
+
+declare_clippy_lint! {
+    /// ### What it does
+    /// Checks for `match` expressions modifying the case of a string with non-compliant arms
+    ///
+    /// ### Why is this bad?
+    /// The arm is unreachable, which is likely a mistake
+    ///
+    /// ### Example
+    /// ```rust
+    /// match &*text.to_ascii_lowercase() {
+    ///     "foo" => {},
+    ///     "Bar" => {},
+    ///     _ => {},
+    /// }
+    /// ```
+    /// Use instead:
+    /// ```rust
+    /// match &*text.to_ascii_lowercase() {
+    ///     "foo" => {},
+    ///     "bar" => {},
+    ///     _ => {},
+    /// }
+    /// ```
+    pub MATCH_STR_CASE_MISMATCH,
+    correctness,
+    "creation of a case altering match expression with non-compliant arms"
+}
+
+declare_lint_pass!(MatchStrCaseMismatch => [MATCH_STR_CASE_MISMATCH]);
+
+#[derive(Debug)]
+enum CaseMethod {
+    LowerCase,
+    AsciiLowerCase,
+    UpperCase,
+    AsciiUppercase,
+}
+
+impl LateLintPass<'_> for MatchStrCaseMismatch {
+    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
+        if_chain! {
+            if !in_external_macro(cx.tcx.sess, expr.span);
+            if let ExprKind::Match(match_expr, arms, MatchSource::Normal) = expr.kind;
+            if let ty::Ref(_, ty, _) = cx.typeck_results().expr_ty(match_expr).kind();
+            if let ty::Str = ty.kind();
+            then {
+                let mut visitor = MatchExprVisitor {
+                    cx,
+                    case_method: None,
+                };
+
+                visitor.visit_expr(match_expr);
+
+                if let Some(case_method) = visitor.case_method {
+                    if let Some(bad_case) = verify_case(&case_method, arms) {
+                        lint(cx, expr.span, &case_method, bad_case);
+                    }
+                }
+            }
+        }
+    }
+}
+
+struct MatchExprVisitor<'a, 'tcx> {
+    cx: &'a LateContext<'tcx>,
+    case_method: Option<CaseMethod>,
+}
+
+impl<'a, 'tcx> Visitor<'tcx> for MatchExprVisitor<'a, 'tcx> {
+    type Map = Map<'tcx>;
+
+    fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
+        NestedVisitorMap::None
+    }
+
+    fn visit_expr(&mut self, ex: &'tcx Expr<'_>) {
+        match ex.kind {
+            ExprKind::MethodCall(segment, _, [receiver], _)
+                if self.case_altered(&*segment.ident.as_str(), receiver) => {},
+            _ => walk_expr(self, ex),
+        }
+    }
+}
+
+impl<'a, 'tcx> MatchExprVisitor<'a, 'tcx> {
+    fn case_altered(&mut self, segment_ident: &str, receiver: &Expr<'_>) -> bool {
+        if let Some(case_method) = get_case_method(segment_ident) {
+            let ty = self.cx.typeck_results().expr_ty(receiver).peel_refs();
+
+            if is_type_diagnostic_item(self.cx, ty, sym::String) || ty.kind() == &ty::Str {
+                self.case_method = Some(case_method);
+                return true;
+            }
+        }
+
+        false
+    }
+}
+
+fn get_case_method(segment_ident_str: &str) -> Option<CaseMethod> {
+    match segment_ident_str {
+        "to_lowercase" => Some(CaseMethod::LowerCase),
+        "to_ascii_lowercase" => Some(CaseMethod::AsciiLowerCase),
+        "to_uppercase" => Some(CaseMethod::UpperCase),
+        "to_ascii_uppercase" => Some(CaseMethod::AsciiUppercase),
+        _ => None,
+    }
+}
+
+fn verify_case(case_method: &CaseMethod, arms: &'_ [Arm<'_>]) -> Option<Span> {
+    let mut bad_case = None;
+
+    let case_check = match case_method {
+        CaseMethod::LowerCase => |input: &str| -> bool { input.chars().all(char::is_lowercase) },
+        CaseMethod::AsciiLowerCase => |input: &str| -> bool { input.chars().all(|c| matches!(c, 'a'..='z')) },
+        CaseMethod::UpperCase => |input: &str| -> bool { input.chars().all(char::is_uppercase) },
+        CaseMethod::AsciiUppercase => |input: &str| -> bool { input.chars().all(|c| matches!(c, 'A'..='Z')) },
+    };
+
+    for arm in arms {
+        if_chain! {
+            if let PatKind::Lit(Expr {
+                                kind: ExprKind::Lit(lit),
+                                ..
+                            }) = arm.pat.kind;
+            if let LitKind::Str(symbol, _) = lit.node;
+            if !case_check(&symbol.as_str());
+            then {
+                bad_case = Some(lit.span);
+                break;
+            }
+        }
+    }
+
+    bad_case
+}
+
+fn lint(cx: &LateContext<'_>, expr_span: Span, case_method: &CaseMethod, bad_case_span: Span) {
+    let method_str = match case_method {
+        CaseMethod::LowerCase => "to_lower_case",
+        CaseMethod::AsciiLowerCase => "to_ascii_lowercase",
+        CaseMethod::UpperCase => "to_uppercase",
+        CaseMethod::AsciiUppercase => "to_ascii_uppercase",
+    };
+
+    span_lint_and_help(
+        cx,
+        MATCH_STR_CASE_MISMATCH,
+        expr_span,
+        "this `match` expression alters case, but has non-compliant arms",
+        Some(bad_case_span),
+        &*format!("consider changing the case of this arm to respect `{}`", method_str),
+    );
+}
diff --git a/tests/ui/match_str_case_mismatch.rs b/tests/ui/match_str_case_mismatch.rs
new file mode 100644
index 00000000000..208a4bba3d2
--- /dev/null
+++ b/tests/ui/match_str_case_mismatch.rs
@@ -0,0 +1,98 @@
+#![warn(clippy::match_str_case_mismatch)]
+
+// Valid
+
+fn as_str_match() {
+    let var = "BAR";
+
+    match var.to_ascii_lowercase().as_str() {
+        "foo" => {},
+        "bar" => {},
+        _ => {},
+    }
+}
+
+fn addrof_unary_match() {
+    let var = "BAR";
+
+    match &*var.to_ascii_lowercase() {
+        "foo" => {},
+        "bar" => {},
+        _ => {},
+    }
+}
+
+fn alternating_chain() {
+    let var = "BAR";
+
+    match &*var
+        .to_ascii_lowercase()
+        .to_uppercase()
+        .to_lowercase()
+        .to_ascii_uppercase()
+    {
+        "FOO" => {},
+        "BAR" => {},
+        _ => {},
+    }
+}
+
+fn unrelated_method() {
+    struct Item {
+        a: String,
+    }
+
+    impl Item {
+        #[allow(clippy::wrong_self_convention)]
+        fn to_lowercase(self) -> String {
+            self.a
+        }
+    }
+
+    let item = Item { a: String::from("BAR") };
+
+    match &*item.to_lowercase() {
+        "FOO" => {},
+        "BAR" => {},
+        _ => {},
+    }
+}
+
+// Invalid
+
+fn as_str_match_mismatch() {
+    let var = "BAR";
+
+    match var.to_ascii_lowercase().as_str() {
+        "foo" => {},
+        "Bar" => {},
+        _ => {},
+    }
+}
+
+fn addrof_unary_match_mismatch() {
+    let var = "BAR";
+
+    match &*var.to_ascii_lowercase() {
+        "foo" => {},
+        "Bar" => {},
+        _ => {},
+    }
+}
+
+fn alternating_chain_mismatch() {
+    let var = "BAR";
+
+    match &*var
+        .to_ascii_lowercase()
+        .to_uppercase()
+        .to_lowercase()
+        .to_ascii_uppercase()
+    {
+        "FOO" => {},
+        "bAR" => {},
+        _ => {},
+    }
+}
+
+fn main() {}
diff --git a/tests/ui/match_str_case_mismatch.stderr b/tests/ui/match_str_case_mismatch.stderr
new file mode 100644
index 00000000000..a5eab1f72f3
--- /dev/null
+++ b/tests/ui/match_str_case_mismatch.stderr
@@ -0,0 +1,53 @@
+error: this `match` expression alters case, but has non-compliant arms
+  --> $DIR/match_str_case_mismatch.rs:66:5
+   |
+LL | /     match var.to_ascii_lowercase().as_str() {
+LL | |         "foo" => {},
+LL | |         "Bar" => {},
+LL | |         _ => {},
+LL | |     }
+   | |_____^
+   |
+   = note: `-D clippy::match-str-case-mismatch` implied by `-D warnings`
+help: consider changing the case of this arm to respect `to_ascii_lowercase`
+  --> $DIR/match_str_case_mismatch.rs:68:9
+   |
+LL |         "Bar" => {},
+   |         ^^^^^
+
+error: this `match` expression alters case, but has non-compliant arms
+  --> $DIR/match_str_case_mismatch.rs:76:5
+   |
+LL | /     match &*var.to_ascii_lowercase() {
+LL | |         "foo" => {},
+LL | |         "Bar" => {},
+LL | |         _ => {},
+LL | |     }
+   | |_____^
+   |
+help: consider changing the case of this arm to respect `to_ascii_lowercase`
+  --> $DIR/match_str_case_mismatch.rs:78:9
+   |
+LL |         "Bar" => {},
+   |         ^^^^^
+
+error: this `match` expression alters case, but has non-compliant arms
+  --> $DIR/match_str_case_mismatch.rs:86:5
+   |
+LL | /     match &*var
+LL | |         .to_ascii_lowercase()
+LL | |         .to_uppercase()
+LL | |         .to_lowercase()
+...  |
+LL | |         _ => {},
+LL | |     }
+   | |_____^
+   |
+help: consider changing the case of this arm to respect `to_ascii_uppercase`
+  --> $DIR/match_str_case_mismatch.rs:93:9
+   |
+LL |         "bAR" => {},
+   |         ^^^^^
+
+error: aborting due to 3 previous errors
+