83 lines
3.8 KiB
Rust
83 lines
3.8 KiB
Rust
use rustc::lint::*;
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use rustc_front::hir::*;
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use utils::{snippet, span_lint, span_help_and_lint, in_external_macro, expr_block};
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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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declare_lint!(pub MATCH_REF_PATS, Warn,
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"a match has all arms prefixed with `&`; the match expression can be \
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dereferenced 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, MATCH_REF_PATS)
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}
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}
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impl LateLintPass for MatchPass {
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fn check_expr(&mut self, cx: &LateContext, expr: &Expr) {
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if let ExprMatch(ref ex, ref arms, MatchSource::Normal) = expr.node {
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// check preconditions for SINGLE_MATCH
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// 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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if in_external_macro(cx, expr.span) {return;}
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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 destructuring a \
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single pattern. Consider using `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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expr_block(cx, &arms[0].body, None, "..")));
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}
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// check preconditions for MATCH_REF_PATS
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if has_only_ref_pats(arms) {
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if in_external_macro(cx, expr.span) { return; }
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if let ExprAddrOf(Mutability::MutImmutable, ref inner) = ex.node {
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span_lint(cx, MATCH_REF_PATS, expr.span, &format!(
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"you don't need to add `&` to both the expression to match \
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and the patterns: use `match {} {{ ...`", snippet(cx, inner.span, "..")));
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} else {
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span_lint(cx, MATCH_REF_PATS, expr.span, &format!(
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"instead of prefixing all patterns with `&`, you can dereference the \
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expression to match: `match *{} {{ ...`", snippet(cx, ex.span, "..")));
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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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fn has_only_ref_pats(arms: &[Arm]) -> bool {
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let mapped = arms.iter().flat_map(|a| &a.pats).map(|p| match p.node {
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PatRegion(..) => Some(true), // &-patterns
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PatWild(..) => Some(false), // an "anything" wildcard is also fine
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_ => None, // any other pattern is not fine
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}).collect::<Option<Vec<bool>>>();
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// look for Some(v) where there's at least one true element
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mapped.map_or(false, |v| v.iter().any(|el| *el))
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}
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