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Merge pull request #17525 from geoffw0/unreachable
Rust: Unreachable code query
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‎rust/ql/src/queries/unusedentities/UnreachableCode.qhelp‎

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<!DOCTYPE qhelp PUBLIC
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"-//Semmle//qhelp//EN"
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"qhelp.dtd">
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<qhelp>
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<overview>
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<p>This rule finds code that is never reached. Unused code should be removed to increase readability and avoid confusion.</p>
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</overview>
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<recommendation>
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<p>Remove any unreachable code.</p>
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</recommendation>
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<example>
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<p>In the following example, the final <code>return</code> statement can never be reached:</p>
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<sample src="UnreachableCodeBad.rs" />
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<p>The problem can be fixed simply by removing the unreachable code:</p>
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<sample src="UnreachableCodeGood.rs" />
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</example>
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<references>
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<li>Wikipedia: <a href="https://en.wikipedia.org/wiki/Unreachable_code">Unreachable code</a></li>
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</references>
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</qhelp>

‎rust/ql/src/queries/unusedentities/UnreachableCode.ql‎

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/**
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* @name Unreachable code
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* @description Code that cannot be reached should be deleted.
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* @kind problem
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* @problem.severity recommendation
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* @precision medium
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* @id rust/dead-code
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* @tags maintainability
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*/
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import rust
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import codeql.rust.controlflow.ControlFlowGraph
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import codeql.rust.controlflow.internal.ControlFlowGraphImpl as ControlFlowGraphImpl
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/**
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* Holds if `n` is an AST node that's unreachable.
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*/
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private predicate unreachable(AstNode n) {
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not n = any(CfgNode cfn).getAstNode() and // reachable nodes
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exists(ControlFlowGraphImpl::ControlFlowTree cft |
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// nodes intended to be part of the CFG
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cft.succ(n, _, _)
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or
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cft.succ(_, n, _)
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)
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}
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/**
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* Holds if `n` is an AST node that's unreachable, and is not the successor
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* of an unreachable node (which would be a duplicate result).
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*/
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private predicate firstUnreachable(AstNode n) {
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unreachable(n) and
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(
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// no predecessor -> we are the first unreachable node.
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not ControlFlowGraphImpl::succ(_, n, _)
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or
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// reachable predecessor -> we are the first unreachable node.
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exists(AstNode pred |
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ControlFlowGraphImpl::succ(pred, n, _) and
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not unreachable(pred)
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)
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)
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}
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/**
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* Gets a node we'd prefer not to report as unreachable.
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*/
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predicate skipNode(AstNode n) {
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n instanceof ControlFlowGraphImpl::PostOrderTree or // location is counter-intuitive
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not n instanceof ControlFlowGraphImpl::ControlFlowTree // not expected to be reachable
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}
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/**
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* Gets the `ControlFlowTree` successor of a node we'd prefer not to report.
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*/
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AstNode skipSuccessor(AstNode n) {
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skipNode(n) and
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ControlFlowGraphImpl::succ(n, result, _)
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}
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/**
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* Gets the node `n`, skipping past any nodes we'd prefer not to report.
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*/
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AstNode skipSuccessors(AstNode n) {
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result = skipSuccessor*(n) and
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not skipNode(result)
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}
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from AstNode first, AstNode report
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where
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firstUnreachable(first) and
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report = skipSuccessors(first) and
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exists(report.getFile().getRelativePath()) // in source
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select report, "This code is never reached."

‎rust/ql/src/queries/unusedentities/UnreachableCodeBad.rs‎

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fn fib(input: u32) -> u32 {
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if (input == 0) {
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return 0;
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} else if (input == 1) {
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return 1;
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} else {
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return fib(input - 1) + fib(input - 2);
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}
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return input; // BAD: this code is never reached
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}

‎rust/ql/src/queries/unusedentities/UnreachableCodeGood.rs‎

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fn fib(input: u32) -> u32 {
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if (input == 0) {
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return 0;
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} else if (input == 1) {
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return 1;
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} else {
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return fib(input - 1) + fib(input - 2);
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}
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}

‎rust/ql/test/query-tests/unusedentities/UnreachableCode.expected‎

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| unreachable.rs:12:3:12:17 | ExprStmt | This code is never reached. |
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| unreachable.rs:20:3:20:17 | ExprStmt | This code is never reached. |
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| unreachable.rs:32:3:32:17 | ExprStmt | This code is never reached. |
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| unreachable.rs:39:3:39:17 | ExprStmt | This code is never reached. |
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| unreachable.rs:60:2:60:16 | ExprStmt | This code is never reached. |
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| unreachable.rs:101:3:101:17 | ExprStmt | This code is never reached. |
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| unreachable.rs:109:3:109:17 | ExprStmt | This code is never reached. |
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| unreachable.rs:124:2:124:16 | ExprStmt | This code is never reached. |
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| unreachable.rs:134:2:134:16 | ExprStmt | This code is never reached. |
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| unreachable.rs:141:3:141:17 | ExprStmt | This code is never reached. |
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| unreachable.rs:150:4:150:18 | ExprStmt | This code is never reached. |
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| unreachable.rs:156:3:156:17 | ExprStmt | This code is never reached. |
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| unreachable.rs:162:4:162:18 | ExprStmt | This code is never reached. |
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| unreachable.rs:165:2:165:16 | ExprStmt | This code is never reached. |

‎rust/ql/test/query-tests/unusedentities/UnreachableCode.qlref‎

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queries/unusedentities/UnreachableCode.ql

‎rust/ql/test/query-tests/unusedentities/main.rs‎

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for k // SPURIOUS: unused variable [macros not yet supported]
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in ks
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{
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println!("lets use {}", k);
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println!("lets use {}", k); // [unreachable FALSE POSITIVE]
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}
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}
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}
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}
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// --- main ---
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fn main() {
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locals_1();
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locals_2();
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structs();
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arrays();
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statics();
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println!("lets use result {}", parameters(1, 2, 3));
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loops();
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if_lets_matches();
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shadowing();
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println!("lets use result {}", parameters(1, 2, 3));
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unreachable_if();
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unreachable_panic();
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unreachable_match();
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unreachable_loop();
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unreachable_paren();
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}

‎rust/ql/test/query-tests/unusedentities/unreachable.rs‎

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//fn cond() -> bool;
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//fn get_a_number() -> i32;
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// --- unreachable code --
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fn do_something() {
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}
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fn unreachable_if() {
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if false {
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do_something(); // BAD: unreachable code
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} else {
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do_something();
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}
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if true {
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do_something();
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} else {
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do_something(); // BAD: unreachable code
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}
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let v = get_a_number();
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if v == 1 {
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if v != 1 {
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do_something(); // BAD: unreachable code [NOT DETECTED]
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}
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}
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if cond() {
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return;
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do_something(); // BAD: unreachable code
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}
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if cond() {
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do_something();
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} else {
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return;
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do_something(); // BAD: unreachable code
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}
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do_something();
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if cond() {
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let x = cond();
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if (x) {
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do_something();
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if (!x) {
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do_something(); // BAD: unreachable code [NOT DETECTED]
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}
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do_something();
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}
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}
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if cond() {
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return;
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} else {
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return;
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}
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do_something(); // BAD: unreachable code
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}
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fn unreachable_panic() {
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if cond() {
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do_something();
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panic!("Oh no!!!");
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do_something(); // BAD: unreachable code [NOT DETECTED]
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}
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if cond() {
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do_something();
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unimplemented!();
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do_something(); // BAD: unreachable code [NOT DETECTED]
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}
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if cond() {
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do_something();
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todo!();
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do_something(); // BAD: unreachable code [NOT DETECTED]
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}
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if cond() {
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let mut maybe;
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maybe = Some("Thing");
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_ = maybe.unwrap(); // (safe)
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do_something();
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maybe = if cond() { Some("Other") } else { None };
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_ = maybe.unwrap(); // (might panic)
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do_something();
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maybe = None;
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_ = maybe.unwrap(); // (always panics)
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do_something(); // BAD: unreachable code [NOT DETECTED]
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}
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if cond() {
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do_something();
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_ = false && panic!(); // does not panic due to short-circuiting
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do_something(); // SPURIOUS: unreachable
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_ = false || panic!();
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do_something(); // BAD: unreachable code [NOT DETECTED]
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}
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if cond() {
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do_something();
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_ = true || panic!(); // does not panic due to short-circuiting
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do_something(); // SPURIOUS: unreachable
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_ = true && panic!();
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do_something(); // BAD: unreachable code [NOT DETECTED]
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}
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}
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fn unreachable_match() {
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match get_a_number() {
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1=>{
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return;
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}
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_=>{
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do_something();
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}
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}
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do_something(); // [unreachable FALSE POSITIVE]
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match get_a_number() {
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1=>{
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return;
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}
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_=>{
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return;
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}
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}
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do_something(); // BAD: unreachable code
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}
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fn unreachable_loop() {
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loop {
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do_something();
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break;
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do_something(); // BAD: unreachable code
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}
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if cond() {
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while cond() {
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do_something();
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}
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while false {
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do_something(); // BAD: unreachable code
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}
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while true {
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do_something();
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}
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do_something(); // BAD: unreachable code
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}
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loop {
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if cond() {
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return;
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do_something(); // BAD: unreachable code
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}
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}
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do_something(); // BAD: unreachable code
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do_something();
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do_something();
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}
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fn unreachable_paren() {
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let _ = (((1)));
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}

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