62 lines
2.5 KiB
Rust
62 lines
2.5 KiB
Rust
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use rustc::lint::*;
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use syntax::ast;
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use syntax::ast::*;
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use std::borrow::Cow;
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use utils::{snippet, snippet_block, span_help_and_lint};
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declare_lint!(pub SINGLE_MATCH, Warn,
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"a match statement with a single nontrivial arm (i.e, where the other arm \
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is `_ => {}`) is used; recommends `if let` instead");
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#[allow(missing_copy_implementations)]
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pub struct MatchPass;
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impl LintPass for MatchPass {
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fn get_lints(&self) -> LintArray {
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lint_array!(SINGLE_MATCH)
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}
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fn check_expr(&mut self, cx: &Context, expr: &Expr) {
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if let ExprMatch(ref ex, ref arms, ast::MatchSource::Normal) = expr.node {
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// check preconditions: only two arms
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if arms.len() == 2 &&
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// both of the arms have a single pattern and no guard
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arms[0].pats.len() == 1 && arms[0].guard.is_none() &&
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arms[1].pats.len() == 1 && arms[1].guard.is_none() &&
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// and the second pattern is a `_` wildcard: this is not strictly necessary,
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// since the exhaustiveness check will ensure the last one is a catch-all,
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// but in some cases, an explicit match is preferred to catch situations
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// when an enum is extended, so we don't consider these cases
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arms[1].pats[0].node == PatWild(PatWildSingle) &&
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// finally, we don't want any content in the second arm (unit or empty block)
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is_unit_expr(&*arms[1].body)
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{
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let body_code = snippet_block(cx, arms[0].body.span, "..");
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let body_code = if let ExprBlock(_) = arms[0].body.node {
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body_code
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} else {
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Cow::Owned(format!("{{ {} }}", body_code))
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};
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span_help_and_lint(cx, SINGLE_MATCH, expr.span,
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"you seem to be trying to use match for \
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destructuring a single pattern. Did you mean to \
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use `if let`?",
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&*format!("try\nif let {} = {} {}",
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snippet(cx, arms[0].pats[0].span, ".."),
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snippet(cx, ex.span, ".."),
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body_code)
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);
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}
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}
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}
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}
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fn is_unit_expr(expr: &Expr) -> bool {
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match expr.node {
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ExprTup(ref v) if v.is_empty() => true,
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ExprBlock(ref b) if b.stmts.is_empty() && b.expr.is_none() => true,
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_ => false,
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}
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}
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