1 files changed, 777 insertions, 0 deletions

diff --git a/crates/ssr/src/matching.rs b/crates/ssr/src/matching.rs
new file mode 100644
index 000000000..ffc7202ae
--- /dev/null
+++ b/crates/ssr/src/matching.rs
@@ -0,0 +1,777 @@
+//! This module is responsible for matching a search pattern against a node in the AST. In the
+//! process of matching, placeholder values are recorded.
+
+use crate::{
+    parsing::{Constraint, NodeKind, Placeholder},
+    resolving::{ResolvedPattern, ResolvedRule, UfcsCallInfo},
+    SsrMatches,
+};
+use base_db::FileRange;
+use hir::Semantics;
+use rustc_hash::FxHashMap;
+use std::{cell::Cell, iter::Peekable};
+use syntax::ast::{AstNode, AstToken};
+use syntax::{ast, SyntaxElement, SyntaxElementChildren, SyntaxKind, SyntaxNode, SyntaxToken};
+use test_utils::mark;
+
+// Creates a match error. If we're currently attempting to match some code that we thought we were
+// going to match, as indicated by the --debug-snippet flag, then populate the reason field.
+macro_rules! match_error {
+    ($e:expr) => {{
+            MatchFailed {
+                reason: if recording_match_fail_reasons() {
+                    Some(format!("{}", $e))
+                } else {
+                    None
+                }
+            }
+    }};
+    ($fmt:expr, $($arg:tt)+) => {{
+        MatchFailed {
+            reason: if recording_match_fail_reasons() {
+                Some(format!($fmt, $($arg)+))
+            } else {
+                None
+            }
+        }
+    }};
+}
+
+// Fails the current match attempt, recording the supplied reason if we're recording match fail reasons.
+macro_rules! fail_match {
+    ($($args:tt)*) => {return Err(match_error!($($args)*))};
+}
+
+/// Information about a match that was found.
+#[derive(Debug)]
+pub struct Match {
+    pub(crate) range: FileRange,
+    pub(crate) matched_node: SyntaxNode,
+    pub(crate) placeholder_values: FxHashMap<Var, PlaceholderMatch>,
+    pub(crate) ignored_comments: Vec<ast::Comment>,
+    pub(crate) rule_index: usize,
+    /// The depth of matched_node.
+    pub(crate) depth: usize,
+    // Each path in the template rendered for the module in which the match was found.
+    pub(crate) rendered_template_paths: FxHashMap<SyntaxNode, hir::ModPath>,
+}
+
+/// Represents a `$var` in an SSR query.
+#[derive(Debug, Clone, PartialEq, Eq, Hash)]
+pub(crate) struct Var(pub String);
+
+/// Information about a placeholder bound in a match.
+#[derive(Debug)]
+pub(crate) struct PlaceholderMatch {
+    /// The node that the placeholder matched to. If set, then we'll search for further matches
+    /// within this node. It isn't set when we match tokens within a macro call's token tree.
+    pub(crate) node: Option<SyntaxNode>,
+    pub(crate) range: FileRange,
+    /// More matches, found within `node`.
+    pub(crate) inner_matches: SsrMatches,
+}
+
+#[derive(Debug)]
+pub(crate) struct MatchFailureReason {
+    pub(crate) reason: String,
+}
+
+/// An "error" indicating that matching failed. Use the fail_match! macro to create and return this.
+#[derive(Clone)]
+pub(crate) struct MatchFailed {
+    /// The reason why we failed to match. Only present when debug_active true in call to
+    /// `get_match`.
+    pub(crate) reason: Option<String>,
+}
+
+/// Checks if `code` matches the search pattern found in `search_scope`, returning information about
+/// the match, if it does. Since we only do matching in this module and searching is done by the
+/// parent module, we don't populate nested matches.
+pub(crate) fn get_match(
+    debug_active: bool,
+    rule: &ResolvedRule,
+    code: &SyntaxNode,
+    restrict_range: &Option<FileRange>,
+    sema: &Semantics<ide_db::RootDatabase>,
+) -> Result<Match, MatchFailed> {
+    record_match_fails_reasons_scope(debug_active, || {
+        Matcher::try_match(rule, code, restrict_range, sema)
+    })
+}
+
+/// Checks if our search pattern matches a particular node of the AST.
+struct Matcher<'db, 'sema> {
+    sema: &'sema Semantics<'db, ide_db::RootDatabase>,
+    /// If any placeholders come from anywhere outside of this range, then the match will be
+    /// rejected.
+    restrict_range: Option<FileRange>,
+    rule: &'sema ResolvedRule,
+}
+
+/// Which phase of matching we're currently performing. We do two phases because most attempted
+/// matches will fail and it means we can defer more expensive checks to the second phase.
+enum Phase<'a> {
+    /// On the first phase, we perform cheap checks. No state is mutated and nothing is recorded.
+    First,
+    /// On the second phase, we construct the `Match`. Things like what placeholders bind to is
+    /// recorded.
+    Second(&'a mut Match),
+}
+
+impl<'db, 'sema> Matcher<'db, 'sema> {
+    fn try_match(
+        rule: &ResolvedRule,
+        code: &SyntaxNode,
+        restrict_range: &Option<FileRange>,
+        sema: &'sema Semantics<'db, ide_db::RootDatabase>,
+    ) -> Result<Match, MatchFailed> {
+        let match_state = Matcher { sema, restrict_range: restrict_range.clone(), rule };
+        // First pass at matching, where we check that node types and idents match.
+        match_state.attempt_match_node(&mut Phase::First, &rule.pattern.node, code)?;
+        match_state.validate_range(&sema.original_range(code))?;
+        let mut the_match = Match {
+            range: sema.original_range(code),
+            matched_node: code.clone(),
+            placeholder_values: FxHashMap::default(),
+            ignored_comments: Vec::new(),
+            rule_index: rule.index,
+            depth: 0,
+            rendered_template_paths: FxHashMap::default(),
+        };
+        // Second matching pass, where we record placeholder matches, ignored comments and maybe do
+        // any other more expensive checks that we didn't want to do on the first pass.
+        match_state.attempt_match_node(
+            &mut Phase::Second(&mut the_match),
+            &rule.pattern.node,
+            code,
+        )?;
+        the_match.depth = sema.ancestors_with_macros(the_match.matched_node.clone()).count();
+        if let Some(template) = &rule.template {
+            the_match.render_template_paths(template, sema)?;
+        }
+        Ok(the_match)
+    }
+
+    /// Checks that `range` is within the permitted range if any. This is applicable when we're
+    /// processing a macro expansion and we want to fail the match if we're working with a node that
+    /// didn't originate from the token tree of the macro call.
+    fn validate_range(&self, range: &FileRange) -> Result<(), MatchFailed> {
+        if let Some(restrict_range) = &self.restrict_range {
+            if restrict_range.file_id != range.file_id
+                || !restrict_range.range.contains_range(range.range)
+            {
+                fail_match!("Node originated from a macro");
+            }
+        }
+        Ok(())
+    }
+
+    fn attempt_match_node(
+        &self,
+        phase: &mut Phase,
+        pattern: &SyntaxNode,
+        code: &SyntaxNode,
+    ) -> Result<(), MatchFailed> {
+        // Handle placeholders.
+        if let Some(placeholder) = self.get_placeholder(&SyntaxElement::Node(pattern.clone())) {
+            for constraint in &placeholder.constraints {
+                self.check_constraint(constraint, code)?;
+            }
+            if let Phase::Second(matches_out) = phase {
+                let original_range = self.sema.original_range(code);
+                // We validated the range for the node when we started the match, so the placeholder
+                // probably can't fail range validation, but just to be safe...
+                self.validate_range(&original_range)?;
+                matches_out.placeholder_values.insert(
+                    Var(placeholder.ident.to_string()),
+                    PlaceholderMatch::new(code, original_range),
+                );
+            }
+            return Ok(());
+        }
+        // We allow a UFCS call to match a method call, provided they resolve to the same function.
+        if let Some(pattern_ufcs) = self.rule.pattern.ufcs_function_calls.get(pattern) {
+            if let Some(code) = ast::MethodCallExpr::cast(code.clone()) {
+                return self.attempt_match_ufcs_to_method_call(phase, pattern_ufcs, &code);
+            }
+            if let Some(code) = ast::CallExpr::cast(code.clone()) {
+                return self.attempt_match_ufcs_to_ufcs(phase, pattern_ufcs, &code);
+            }
+        }
+        if pattern.kind() != code.kind() {
+            fail_match!(
+                "Pattern had `{}` ({:?}), code had `{}` ({:?})",
+                pattern.text(),
+                pattern.kind(),
+                code.text(),
+                code.kind()
+            );
+        }
+        // Some kinds of nodes have special handling. For everything else, we fall back to default
+        // matching.
+        match code.kind() {
+            SyntaxKind::RECORD_EXPR_FIELD_LIST => {
+                self.attempt_match_record_field_list(phase, pattern, code)
+            }
+            SyntaxKind::TOKEN_TREE => self.attempt_match_token_tree(phase, pattern, code),
+            SyntaxKind::PATH => self.attempt_match_path(phase, pattern, code),
+            _ => self.attempt_match_node_children(phase, pattern, code),
+        }
+    }
+
+    fn attempt_match_node_children(
+        &self,
+        phase: &mut Phase,
+        pattern: &SyntaxNode,
+        code: &SyntaxNode,
+    ) -> Result<(), MatchFailed> {
+        self.attempt_match_sequences(
+            phase,
+            PatternIterator::new(pattern),
+            code.children_with_tokens(),
+        )
+    }
+
+    fn attempt_match_sequences(
+        &self,
+        phase: &mut Phase,
+        pattern_it: PatternIterator,
+        mut code_it: SyntaxElementChildren,
+    ) -> Result<(), MatchFailed> {
+        let mut pattern_it = pattern_it.peekable();
+        loop {
+            match phase.next_non_trivial(&mut code_it) {
+                None => {
+                    if let Some(p) = pattern_it.next() {
+                        fail_match!("Part of the pattern was unmatched: {:?}", p);
+                    }
+                    return Ok(());
+                }
+                Some(SyntaxElement::Token(c)) => {
+                    self.attempt_match_token(phase, &mut pattern_it, &c)?;
+                }
+                Some(SyntaxElement::Node(c)) => match pattern_it.next() {
+                    Some(SyntaxElement::Node(p)) => {
+                        self.attempt_match_node(phase, &p, &c)?;
+                    }
+                    Some(p) => fail_match!("Pattern wanted '{}', code has {}", p, c.text()),
+                    None => fail_match!("Pattern reached end, code has {}", c.text()),
+                },
+            }
+        }
+    }
+
+    fn attempt_match_token(
+        &self,
+        phase: &mut Phase,
+        pattern: &mut Peekable<PatternIterator>,
+        code: &syntax::SyntaxToken,
+    ) -> Result<(), MatchFailed> {
+        phase.record_ignored_comments(code);
+        // Ignore whitespace and comments.
+        if code.kind().is_trivia() {
+            return Ok(());
+        }
+        if let Some(SyntaxElement::Token(p)) = pattern.peek() {
+            // If the code has a comma and the pattern is about to close something, then accept the
+            // comma without advancing the pattern. i.e. ignore trailing commas.
+            if code.kind() == SyntaxKind::COMMA && is_closing_token(p.kind()) {
+                return Ok(());
+            }
+            // Conversely, if the pattern has a comma and the code doesn't, skip that part of the
+            // pattern and continue to match the code.
+            if p.kind() == SyntaxKind::COMMA && is_closing_token(code.kind()) {
+                pattern.next();
+            }
+        }
+        // Consume an element from the pattern and make sure it matches.
+        match pattern.next() {
+            Some(SyntaxElement::Token(p)) => {
+                if p.kind() != code.kind() || p.text() != code.text() {
+                    fail_match!(
+                        "Pattern wanted token '{}' ({:?}), but code had token '{}' ({:?})",
+                        p.text(),
+                        p.kind(),
+                        code.text(),
+                        code.kind()
+                    )
+                }
+            }
+            Some(SyntaxElement::Node(p)) => {
+                // Not sure if this is actually reachable.
+                fail_match!(
+                    "Pattern wanted {:?}, but code had token '{}' ({:?})",
+                    p,
+                    code.text(),
+                    code.kind()
+                );
+            }
+            None => {
+                fail_match!("Pattern exhausted, while code remains: `{}`", code.text());
+            }
+        }
+        Ok(())
+    }
+
+    fn check_constraint(
+        &self,
+        constraint: &Constraint,
+        code: &SyntaxNode,
+    ) -> Result<(), MatchFailed> {
+        match constraint {
+            Constraint::Kind(kind) => {
+                kind.matches(code)?;
+            }
+            Constraint::Not(sub) => {
+                if self.check_constraint(&*sub, code).is_ok() {
+                    fail_match!("Constraint {:?} failed for '{}'", constraint, code.text());
+                }
+            }
+        }
+        Ok(())
+    }
+
+    /// Paths are matched based on whether they refer to the same thing, even if they're written
+    /// differently.
+    fn attempt_match_path(
+        &self,
+        phase: &mut Phase,
+        pattern: &SyntaxNode,
+        code: &SyntaxNode,
+    ) -> Result<(), MatchFailed> {
+        if let Some(pattern_resolved) = self.rule.pattern.resolved_paths.get(pattern) {
+            let pattern_path = ast::Path::cast(pattern.clone()).unwrap();
+            let code_path = ast::Path::cast(code.clone()).unwrap();
+            if let (Some(pattern_segment), Some(code_segment)) =
+                (pattern_path.segment(), code_path.segment())
+            {
+                // Match everything within the segment except for the name-ref, which is handled
+                // separately via comparing what the path resolves to below.
+                self.attempt_match_opt(
+                    phase,
+                    pattern_segment.generic_arg_list(),
+                    code_segment.generic_arg_list(),
+                )?;
+                self.attempt_match_opt(
+                    phase,
+                    pattern_segment.param_list(),
+                    code_segment.param_list(),
+                )?;
+            }
+            if matches!(phase, Phase::Second(_)) {
+                let resolution = self
+                    .sema
+                    .resolve_path(&code_path)
+                    .ok_or_else(|| match_error!("Failed to resolve path `{}`", code.text()))?;
+                if pattern_resolved.resolution != resolution {
+                    fail_match!("Pattern had path `{}` code had `{}`", pattern.text(), code.text());
+                }
+            }
+        } else {
+            return self.attempt_match_node_children(phase, pattern, code);
+        }
+        Ok(())
+    }
+
+    fn attempt_match_opt<T: AstNode>(
+        &self,
+        phase: &mut Phase,
+        pattern: Option<T>,
+        code: Option<T>,
+    ) -> Result<(), MatchFailed> {
+        match (pattern, code) {
+            (Some(p), Some(c)) => self.attempt_match_node(phase, &p.syntax(), &c.syntax()),
+            (None, None) => Ok(()),
+            (Some(p), None) => fail_match!("Pattern `{}` had nothing to match", p.syntax().text()),
+            (None, Some(c)) => {
+                fail_match!("Nothing in pattern to match code `{}`", c.syntax().text())
+            }
+        }
+    }
+
+    /// We want to allow the records to match in any order, so we have special matching logic for
+    /// them.
+    fn attempt_match_record_field_list(
+        &self,
+        phase: &mut Phase,
+        pattern: &SyntaxNode,
+        code: &SyntaxNode,
+    ) -> Result<(), MatchFailed> {
+        // Build a map keyed by field name.
+        let mut fields_by_name = FxHashMap::default();
+        for child in code.children() {
+            if let Some(record) = ast::RecordExprField::cast(child.clone()) {
+                if let Some(name) = record.field_name() {
+                    fields_by_name.insert(name.text().clone(), child.clone());
+                }
+            }
+        }
+        for p in pattern.children_with_tokens() {
+            if let SyntaxElement::Node(p) = p {
+                if let Some(name_element) = p.first_child_or_token() {
+                    if self.get_placeholder(&name_element).is_some() {
+                        // If the pattern is using placeholders for field names then order
+                        // independence doesn't make sense. Fall back to regular ordered
+                        // matching.
+                        return self.attempt_match_node_children(phase, pattern, code);
+                    }
+                    if let Some(ident) = only_ident(name_element) {
+                        let code_record = fields_by_name.remove(ident.text()).ok_or_else(|| {
+                            match_error!(
+                                "Placeholder has record field '{}', but code doesn't",
+                                ident
+                            )
+                        })?;
+                        self.attempt_match_node(phase, &p, &code_record)?;
+                    }
+                }
+            }
+        }
+        if let Some(unmatched_fields) = fields_by_name.keys().next() {
+            fail_match!(
+                "{} field(s) of a record literal failed to match, starting with {}",
+                fields_by_name.len(),
+                unmatched_fields
+            );
+        }
+        Ok(())
+    }
+
+    /// Outside of token trees, a placeholder can only match a single AST node, whereas in a token
+    /// tree it can match a sequence of tokens. Note, that this code will only be used when the
+    /// pattern matches the macro invocation. For matches within the macro call, we'll already have
+    /// expanded the macro.
+    fn attempt_match_token_tree(
+        &self,
+        phase: &mut Phase,
+        pattern: &SyntaxNode,
+        code: &syntax::SyntaxNode,
+    ) -> Result<(), MatchFailed> {
+        let mut pattern = PatternIterator::new(pattern).peekable();
+        let mut children = code.children_with_tokens();
+        while let Some(child) = children.next() {
+            if let Some(placeholder) = pattern.peek().and_then(|p| self.get_placeholder(p)) {
+                pattern.next();
+                let next_pattern_token = pattern
+                    .peek()
+                    .and_then(|p| match p {
+                        SyntaxElement::Token(t) => Some(t.clone()),
+                        SyntaxElement::Node(n) => n.first_token(),
+                    })
+                    .map(|p| p.text().to_string());
+                let first_matched_token = child.clone();
+                let mut last_matched_token = child;
+                // Read code tokens util we reach one equal to the next token from our pattern
+                // or we reach the end of the token tree.
+                while let Some(next) = children.next() {
+                    match &next {
+                        SyntaxElement::Token(t) => {
+                            if Some(t.to_string()) == next_pattern_token {
+                                pattern.next();
+                                break;
+                            }
+                        }
+                        SyntaxElement::Node(n) => {
+                            if let Some(first_token) = n.first_token() {
+                                if Some(first_token.to_string()) == next_pattern_token {
+                                    if let Some(SyntaxElement::Node(p)) = pattern.next() {
+                                        // We have a subtree that starts with the next token in our pattern.
+                                        self.attempt_match_token_tree(phase, &p, &n)?;
+                                        break;
+                                    }
+                                }
+                            }
+                        }
+                    };
+                    last_matched_token = next;
+                }
+                if let Phase::Second(match_out) = phase {
+                    match_out.placeholder_values.insert(
+                        Var(placeholder.ident.to_string()),
+                        PlaceholderMatch::from_range(FileRange {
+                            file_id: self.sema.original_range(code).file_id,
+                            range: first_matched_token
+                                .text_range()
+                                .cover(last_matched_token.text_range()),
+                        }),
+                    );
+                }
+                continue;
+            }
+            // Match literal (non-placeholder) tokens.
+            match child {
+                SyntaxElement::Token(token) => {
+                    self.attempt_match_token(phase, &mut pattern, &token)?;
+                }
+                SyntaxElement::Node(node) => match pattern.next() {
+                    Some(SyntaxElement::Node(p)) => {
+                        self.attempt_match_token_tree(phase, &p, &node)?;
+                    }
+                    Some(SyntaxElement::Token(p)) => fail_match!(
+                        "Pattern has token '{}', code has subtree '{}'",
+                        p.text(),
+                        node.text()
+                    ),
+                    None => fail_match!("Pattern has nothing, code has '{}'", node.text()),
+                },
+            }
+        }
+        if let Some(p) = pattern.next() {
+            fail_match!("Reached end of token tree in code, but pattern still has {:?}", p);
+        }
+        Ok(())
+    }
+
+    fn attempt_match_ufcs_to_method_call(
+        &self,
+        phase: &mut Phase,
+        pattern_ufcs: &UfcsCallInfo,
+        code: &ast::MethodCallExpr,
+    ) -> Result<(), MatchFailed> {
+        use ast::ArgListOwner;
+        let code_resolved_function = self
+            .sema
+            .resolve_method_call(code)
+            .ok_or_else(|| match_error!("Failed to resolve method call"))?;
+        if pattern_ufcs.function != code_resolved_function {
+            fail_match!("Method call resolved to a different function");
+        }
+        if code_resolved_function.has_self_param(self.sema.db) {
+            if let (Some(pattern_type), Some(expr)) = (&pattern_ufcs.qualifier_type, &code.expr()) {
+                self.check_expr_type(pattern_type, expr)?;
+            }
+        }
+        // Check arguments.
+        let mut pattern_args = pattern_ufcs
+            .call_expr
+            .arg_list()
+            .ok_or_else(|| match_error!("Pattern function call has no args"))?
+            .args();
+        self.attempt_match_opt(phase, pattern_args.next(), code.expr())?;
+        let mut code_args =
+            code.arg_list().ok_or_else(|| match_error!("Code method call has no args"))?.args();
+        loop {
+            match (pattern_args.next(), code_args.next()) {
+                (None, None) => return Ok(()),
+                (p, c) => self.attempt_match_opt(phase, p, c)?,
+            }
+        }
+    }
+
+    fn attempt_match_ufcs_to_ufcs(
+        &self,
+        phase: &mut Phase,
+        pattern_ufcs: &UfcsCallInfo,
+        code: &ast::CallExpr,
+    ) -> Result<(), MatchFailed> {
+        use ast::ArgListOwner;
+        // Check that the first argument is the expected type.
+        if let (Some(pattern_type), Some(expr)) = (
+            &pattern_ufcs.qualifier_type,
+            &code.arg_list().and_then(|code_args| code_args.args().next()),
+        ) {
+            self.check_expr_type(pattern_type, expr)?;
+        }
+        self.attempt_match_node_children(phase, pattern_ufcs.call_expr.syntax(), code.syntax())
+    }
+
+    fn check_expr_type(
+        &self,
+        pattern_type: &hir::Type,
+        expr: &ast::Expr,
+    ) -> Result<(), MatchFailed> {
+        use hir::HirDisplay;
+        let code_type = self.sema.type_of_expr(&expr).ok_or_else(|| {
+            match_error!("Failed to get receiver type for `{}`", expr.syntax().text())
+        })?;
+        if !code_type
+            .autoderef(self.sema.db)
+            .any(|deref_code_type| *pattern_type == deref_code_type)
+        {
+            fail_match!(
+                "Pattern type `{}` didn't match code type `{}`",
+                pattern_type.display(self.sema.db),
+                code_type.display(self.sema.db)
+            );
+        }
+        Ok(())
+    }
+
+    fn get_placeholder(&self, element: &SyntaxElement) -> Option<&Placeholder> {
+        only_ident(element.clone()).and_then(|ident| self.rule.get_placeholder(&ident))
+    }
+}
+
+impl Match {
+    fn render_template_paths(
+        &mut self,
+        template: &ResolvedPattern,
+        sema: &Semantics<ide_db::RootDatabase>,
+    ) -> Result<(), MatchFailed> {
+        let module = sema
+            .scope(&self.matched_node)
+            .module()
+            .ok_or_else(|| match_error!("Matched node isn't in a module"))?;
+        for (path, resolved_path) in &template.resolved_paths {
+            if let hir::PathResolution::Def(module_def) = resolved_path.resolution {
+                let mod_path = module.find_use_path(sema.db, module_def).ok_or_else(|| {
+                    match_error!("Failed to render template path `{}` at match location")
+                })?;
+                self.rendered_template_paths.insert(path.clone(), mod_path);
+            }
+        }
+        Ok(())
+    }
+}
+
+impl Phase<'_> {
+    fn next_non_trivial(&mut self, code_it: &mut SyntaxElementChildren) -> Option<SyntaxElement> {
+        loop {
+            let c = code_it.next();
+            if let Some(SyntaxElement::Token(t)) = &c {
+                self.record_ignored_comments(t);
+                if t.kind().is_trivia() {
+                    continue;
+                }
+            }
+            return c;
+        }
+    }
+
+    fn record_ignored_comments(&mut self, token: &SyntaxToken) {
+        if token.kind() == SyntaxKind::COMMENT {
+            if let Phase::Second(match_out) = self {
+                if let Some(comment) = ast::Comment::cast(token.clone()) {
+                    match_out.ignored_comments.push(comment);
+                }
+            }
+        }
+    }
+}
+
+fn is_closing_token(kind: SyntaxKind) -> bool {
+    kind == SyntaxKind::R_PAREN || kind == SyntaxKind::R_CURLY || kind == SyntaxKind::R_BRACK
+}
+
+pub(crate) fn record_match_fails_reasons_scope<F, T>(debug_active: bool, f: F) -> T
+where
+    F: Fn() -> T,
+{
+    RECORDING_MATCH_FAIL_REASONS.with(|c| c.set(debug_active));
+    let res = f();
+    RECORDING_MATCH_FAIL_REASONS.with(|c| c.set(false));
+    res
+}
+
+// For performance reasons, we don't want to record the reason why every match fails, only the bit
+// of code that the user indicated they thought would match. We use a thread local to indicate when
+// we are trying to match that bit of code. This saves us having to pass a boolean into all the bits
+// of code that can make the decision to not match.
+thread_local! {
+    pub static RECORDING_MATCH_FAIL_REASONS: Cell<bool> = Cell::new(false);
+}
+
+fn recording_match_fail_reasons() -> bool {
+    RECORDING_MATCH_FAIL_REASONS.with(|c| c.get())
+}
+
+impl PlaceholderMatch {
+    fn new(node: &SyntaxNode, range: FileRange) -> Self {
+        Self { node: Some(node.clone()), range, inner_matches: SsrMatches::default() }
+    }
+
+    fn from_range(range: FileRange) -> Self {
+        Self { node: None, range, inner_matches: SsrMatches::default() }
+    }
+}
+
+impl NodeKind {
+    fn matches(&self, node: &SyntaxNode) -> Result<(), MatchFailed> {
+        let ok = match self {
+            Self::Literal => {
+                mark::hit!(literal_constraint);
+                ast::Literal::can_cast(node.kind())
+            }
+        };
+        if !ok {
+            fail_match!("Code '{}' isn't of kind {:?}", node.text(), self);
+        }
+        Ok(())
+    }
+}
+
+// If `node` contains nothing but an ident then return it, otherwise return None.
+fn only_ident(element: SyntaxElement) -> Option<SyntaxToken> {
+    match element {
+        SyntaxElement::Token(t) => {
+            if t.kind() == SyntaxKind::IDENT {
+                return Some(t);
+            }
+        }
+        SyntaxElement::Node(n) => {
+            let mut children = n.children_with_tokens();
+            if let (Some(only_child), None) = (children.next(), children.next()) {
+                return only_ident(only_child);
+            }
+        }
+    }
+    None
+}
+
+struct PatternIterator {
+    iter: SyntaxElementChildren,
+}
+
+impl Iterator for PatternIterator {
+    type Item = SyntaxElement;
+
+    fn next(&mut self) -> Option<SyntaxElement> {
+        while let Some(element) = self.iter.next() {
+            if !element.kind().is_trivia() {
+                return Some(element);
+            }
+        }
+        None
+    }
+}
+
+impl PatternIterator {
+    fn new(parent: &SyntaxNode) -> Self {
+        Self { iter: parent.children_with_tokens() }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::{MatchFinder, SsrRule};
+
+    #[test]
+    fn parse_match_replace() {
+        let rule: SsrRule = "foo($x) ==>> bar($x)".parse().unwrap();
+        let input = "fn foo() {} fn bar() {} fn main() { foo(1+2); }";
+
+        let (db, position, selections) = crate::tests::single_file(input);
+        let mut match_finder = MatchFinder::in_context(&db, position, selections);
+        match_finder.add_rule(rule).unwrap();
+        let matches = match_finder.matches();
+        assert_eq!(matches.matches.len(), 1);
+        assert_eq!(matches.matches[0].matched_node.text(), "foo(1+2)");
+        assert_eq!(matches.matches[0].placeholder_values.len(), 1);
+        assert_eq!(
+            matches.matches[0].placeholder_values[&Var("x".to_string())]
+                .node
+                .as_ref()
+                .unwrap()
+                .text(),
+            "1+2"
+        );
+
+        let edits = match_finder.edits();
+        assert_eq!(edits.len(), 1);
+        let edit = &edits[0];
+        let mut after = input.to_string();
+        edit.edit.apply(&mut after);
+        assert_eq!(after, "fn foo() {} fn bar() {} fn main() { bar(1+2); }");
+    }
+}