Auto merge of #6843 - Jarcho:match_on_same_arms_macro, r=Manishearth

Compare empty blocks for equality based on tokens

fixes: #1390

This only considers empty blocks for now, though we should also catch something like this:

```rust
match 0 {
    0 => {
        do_something();
        trace!(0);
        0
    }
    1 => {
        do_something();
        trace!(1);
        1
    }
    x => x,
}
```

As far as I can tell there aren't any negative effects on other lints. These blocks only happen to be the same for a given compilation, not all compilations.

changelog: Fix `match_on_same_arms` and others. Only consider empty blocks equal if the tokens contained are the same.

4 files changed, 130 insertions, 8 deletions

diff --git a/clippy_utils/src/hir_utils.rs b/clippy_utils/src/hir_utils.rs
index 81be9254cbe..e28ad27d9a6 100644
--- a/clippy_utils/src/hir_utils.rs
+++ b/clippy_utils/src/hir_utils.rs
@@ -1,5 +1,5 @@
 use crate::consts::{constant_context, constant_simple};
-use crate::differing_macro_contexts;
+use crate::{differing_macro_contexts, snippet_opt};
 use rustc_ast::ast::InlineAsmTemplatePiece;
 use rustc_data_structures::fx::FxHashMap;
 use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
@@ -9,6 +9,7 @@ use rustc_hir::{
     GenericArg, GenericArgs, Guard, HirId, InlineAsmOperand, Lifetime, LifetimeName, ParamName, Pat, PatKind, Path,
     PathSegment, QPath, Stmt, StmtKind, Ty, TyKind, TypeBinding,
 };
+use rustc_lexer::{tokenize, TokenKind};
 use rustc_lint::LateContext;
 use rustc_middle::ich::StableHashingContextProvider;
 use rustc_middle::ty::TypeckResults;
@@ -110,8 +111,54 @@ impl HirEqInterExpr<'_, '_, '_> {
 
     /// Checks whether two blocks are the same.
     fn eq_block(&mut self, left: &Block<'_>, right: &Block<'_>) -> bool {
-        over(&left.stmts, &right.stmts, |l, r| self.eq_stmt(l, r))
-            && both(&left.expr, &right.expr, |l, r| self.eq_expr(l, r))
+        match (left.stmts, left.expr, right.stmts, right.expr) {
+            ([], None, [], None) => {
+                // For empty blocks, check to see if the tokens are equal. This will catch the case where a macro
+                // expanded to nothing, or the cfg attribute was used.
+                let (left, right) = match (
+                    snippet_opt(self.inner.cx, left.span),
+                    snippet_opt(self.inner.cx, right.span),
+                ) {
+                    (Some(left), Some(right)) => (left, right),
+                    _ => return true,
+                };
+                let mut left_pos = 0;
+                let left = tokenize(&left)
+                    .map(|t| {
+                        let end = left_pos + t.len;
+                        let s = &left[left_pos..end];
+                        left_pos = end;
+                        (t, s)
+                    })
+                    .filter(|(t, _)| {
+                        !matches!(
+                            t.kind,
+                            TokenKind::LineComment { .. } | TokenKind::BlockComment { .. } | TokenKind::Whitespace
+                        )
+                    })
+                    .map(|(_, s)| s);
+                let mut right_pos = 0;
+                let right = tokenize(&right)
+                    .map(|t| {
+                        let end = right_pos + t.len;
+                        let s = &right[right_pos..end];
+                        right_pos = end;
+                        (t, s)
+                    })
+                    .filter(|(t, _)| {
+                        !matches!(
+                            t.kind,
+                            TokenKind::LineComment { .. } | TokenKind::BlockComment { .. } | TokenKind::Whitespace
+                        )
+                    })
+                    .map(|(_, s)| s);
+                left.eq(right)
+            },
+            _ => {
+                over(&left.stmts, &right.stmts, |l, r| self.eq_stmt(l, r))
+                    && both(&left.expr, &right.expr, |l, r| self.eq_expr(l, r))
+            },
+        }
     }
 
     #[allow(clippy::similar_names)]
@@ -131,7 +178,10 @@ impl HirEqInterExpr<'_, '_, '_> {
             }
         }
 
-        let is_eq = match (&reduce_exprkind(&left.kind), &reduce_exprkind(&right.kind)) {
+        let is_eq = match (
+            &reduce_exprkind(self.inner.cx, &left.kind),
+            &reduce_exprkind(self.inner.cx, &right.kind),
+        ) {
             (&ExprKind::AddrOf(lb, l_mut, ref le), &ExprKind::AddrOf(rb, r_mut, ref re)) => {
                 lb == rb && l_mut == r_mut && self.eq_expr(le, re)
             },
@@ -360,11 +410,30 @@ impl HirEqInterExpr<'_, '_, '_> {
 }
 
 /// Some simple reductions like `{ return }` => `return`
-fn reduce_exprkind<'hir>(kind: &'hir ExprKind<'hir>) -> &ExprKind<'hir> {
+fn reduce_exprkind<'hir>(cx: &LateContext<'_>, kind: &'hir ExprKind<'hir>) -> &'hir ExprKind<'hir> {
     if let ExprKind::Block(block, _) = kind {
         match (block.stmts, block.expr) {
+            // From an `if let` expression without an `else` block. The arm for the implicit wild pattern is an empty
+            // block with an empty span.
+            ([], None) if block.span.is_empty() => &ExprKind::Tup(&[]),
             // `{}` => `()`
-            ([], None) => &ExprKind::Tup(&[]),
+            ([], None) => match snippet_opt(cx, block.span) {
+                // Don't reduce if there are any tokens contained in the braces
+                Some(snip)
+                    if tokenize(&snip)
+                        .map(|t| t.kind)
+                        .filter(|t| {
+                            !matches!(
+                                t,
+                                TokenKind::LineComment { .. } | TokenKind::BlockComment { .. } | TokenKind::Whitespace
+                            )
+                        })
+                        .ne([TokenKind::OpenBrace, TokenKind::CloseBrace].iter().cloned()) =>
+                {
+                    kind
+                },
+                _ => &ExprKind::Tup(&[]),
+            },
             ([], Some(expr)) => match expr.kind {
                 // `{ return .. }` => `return ..`
                 ExprKind::Ret(..) => &expr.kind,
diff --git a/clippy_utils/src/lib.rs b/clippy_utils/src/lib.rs
index 00455d4102f..0395079901c 100644
--- a/clippy_utils/src/lib.rs
+++ b/clippy_utils/src/lib.rs
@@ -14,6 +14,7 @@ extern crate rustc_errors;
 extern crate rustc_hir;
 extern crate rustc_hir_pretty;
 extern crate rustc_infer;
+extern crate rustc_lexer;
 extern crate rustc_lint;
 extern crate rustc_middle;
 extern crate rustc_mir;
diff --git a/tests/ui/match_same_arms2.rs b/tests/ui/match_same_arms2.rs
index 06d91497242..da4e3020d5b 100644
--- a/tests/ui/match_same_arms2.rs
+++ b/tests/ui/match_same_arms2.rs
@@ -120,6 +120,35 @@ fn match_same_arms() {
         },
     }
 
+    // False positive #1390
+    macro_rules! empty {
+        ($e:expr) => {};
+    }
+    match 0 {
+        0 => {
+            empty!(0);
+        },
+        1 => {
+            empty!(1);
+        },
+        x => {
+            empty!(x);
+        },
+    };
+
+    // still lint if the tokens are the same
+    match 0 {
+        0 => {
+            empty!(0);
+        },
+        1 => {
+            empty!(0);
+        },
+        x => {
+            empty!(x);
+        },
+    }
+
     match_expr_like_matches_macro_priority();
 }
 
diff --git a/tests/ui/match_same_arms2.stderr b/tests/ui/match_same_arms2.stderr
index fccaf805616..95f9494cdc9 100644
--- a/tests/ui/match_same_arms2.stderr
+++ b/tests/ui/match_same_arms2.stderr
@@ -141,8 +141,31 @@ LL |         Ok(3) => println!("ok"),
    |         ^^^^^
    = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
 
+error: this `match` has identical arm bodies
+  --> $DIR/match_same_arms2.rs:144:14
+   |
+LL |           1 => {
+   |  ______________^
+LL | |             empty!(0);
+LL | |         },
+   | |_________^
+   |
+note: same as this
+  --> $DIR/match_same_arms2.rs:141:14
+   |
+LL |           0 => {
+   |  ______________^
+LL | |             empty!(0);
+LL | |         },
+   | |_________^
+help: consider refactoring into `0 | 1`
+  --> $DIR/match_same_arms2.rs:141:9
+   |
+LL |         0 => {
+   |         ^
+
 error: match expression looks like `matches!` macro
-  --> $DIR/match_same_arms2.rs:133:16
+  --> $DIR/match_same_arms2.rs:162:16
    |
 LL |       let _ans = match x {
    |  ________________^
@@ -154,5 +177,5 @@ LL | |     };
    |
    = note: `-D clippy::match-like-matches-macro` implied by `-D warnings`
 
-error: aborting due to 8 previous errors
+error: aborting due to 9 previous errors