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Fix crash in 'remove unreachable code' #72028

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Feb 9, 2024
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Fix crash in 'remove unreachable code' #72028

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232 changes: 118 additions & 114 deletions ...s/CSharp/Analyzers/RemoveUnreachableCode/CSharpRemoveUnreachableCodeDiagnosticAnalyzer.cs
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Expand Up @@ -11,136 +11,140 @@
using Microsoft.CodeAnalysis.Utilities;
using Roslyn.Utilities;

namespace Microsoft.CodeAnalysis.CSharp.RemoveUnreachableCode
namespace Microsoft.CodeAnalysis.CSharp.RemoveUnreachableCode;

[DiagnosticAnalyzer(LanguageNames.CSharp)]
internal class CSharpRemoveUnreachableCodeDiagnosticAnalyzer : AbstractBuiltInUnnecessaryCodeStyleDiagnosticAnalyzer
{
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[DiagnosticAnalyzer(LanguageNames.CSharp)]
internal class CSharpRemoveUnreachableCodeDiagnosticAnalyzer : AbstractBuiltInUnnecessaryCodeStyleDiagnosticAnalyzer
private const string CS0162 = nameof(CS0162); // Unreachable code detected

public const string IsSubsequentSection = nameof(IsSubsequentSection);
private static readonly ImmutableDictionary<string, string?> s_subsequentSectionProperties = ImmutableDictionary<string, string?>.Empty.Add(IsSubsequentSection, "");

public CSharpRemoveUnreachableCodeDiagnosticAnalyzer()
: base(IDEDiagnosticIds.RemoveUnreachableCodeDiagnosticId,
EnforceOnBuildValues.RemoveUnreachableCode,
option: null,
fadingOption: FadingOptions.FadeOutUnreachableCode,
new LocalizableResourceString(nameof(CSharpAnalyzersResources.Unreachable_code_detected), CSharpAnalyzersResources.ResourceManager, typeof(CSharpAnalyzersResources)),
configurable: false)
{
private const string CS0162 = nameof(CS0162); // Unreachable code detected

public const string IsSubsequentSection = nameof(IsSubsequentSection);
private static readonly ImmutableDictionary<string, string?> s_subsequentSectionProperties = ImmutableDictionary<string, string?>.Empty.Add(IsSubsequentSection, "");

public CSharpRemoveUnreachableCodeDiagnosticAnalyzer()
: base(IDEDiagnosticIds.RemoveUnreachableCodeDiagnosticId,
EnforceOnBuildValues.RemoveUnreachableCode,
option: null,
fadingOption: FadingOptions.FadeOutUnreachableCode,
new LocalizableResourceString(nameof(CSharpAnalyzersResources.Unreachable_code_detected), CSharpAnalyzersResources.ResourceManager, typeof(CSharpAnalyzersResources)),
configurable: false)
{
}
}

public override DiagnosticAnalyzerCategory GetAnalyzerCategory()
=> DiagnosticAnalyzerCategory.SemanticSpanAnalysis;
public override DiagnosticAnalyzerCategory GetAnalyzerCategory()
=> DiagnosticAnalyzerCategory.SemanticSpanAnalysis;

protected override void InitializeWorker(AnalysisContext context)
=> context.RegisterSemanticModelAction(AnalyzeSemanticModel);
protected override void InitializeWorker(AnalysisContext context)
=> context.RegisterSemanticModelAction(AnalyzeSemanticModel);

private void AnalyzeSemanticModel(SemanticModelAnalysisContext context)
private void AnalyzeSemanticModel(SemanticModelAnalysisContext context)
{
if (ShouldSkipAnalysis(context, notification: null))
return;

var semanticModel = context.SemanticModel;
var cancellationToken = context.CancellationToken;

// There is no good existing API to check if a statement is unreachable in an efficient
// manner. While there is SemanticModel.AnalyzeControlFlow, it can only operate on a
// statement at a time, and it will reanalyze and allocate on each call.
//
// To avoid this, we simply ask the semantic model for all the diagnostics for this
// block and we look for any reported "unreachable code detected" diagnostics.
//
// This is actually quite fast to do because the compiler does not actually need to
// recompile things to determine the diagnostics. It will have already stashed the
// binding diagnostics directly on the SourceMethodSymbol containing this block, and
// so it can retrieve the diagnostics at practically no cost.
var root = semanticModel.SyntaxTree.GetRoot(cancellationToken);
var diagnostics = semanticModel.GetDiagnostics(context.FilterSpan, cancellationToken);
foreach (var diagnostic in diagnostics)
{
if (ShouldSkipAnalysis(context, notification: null))
return;

var semanticModel = context.SemanticModel;
var cancellationToken = context.CancellationToken;

// There is no good existing API to check if a statement is unreachable in an efficient
// manner. While there is SemanticModel.AnalyzeControlFlow, it can only operate on a
// statement at a time, and it will reanalyze and allocate on each call.
//
// To avoid this, we simply ask the semantic model for all the diagnostics for this
// block and we look for any reported "unreachable code detected" diagnostics.
//
// This is actually quite fast to do because the compiler does not actually need to
// recompile things to determine the diagnostics. It will have already stashed the
// binding diagnostics directly on the SourceMethodSymbol containing this block, and
// so it can retrieve the diagnostics at practically no cost.
var root = semanticModel.SyntaxTree.GetRoot(cancellationToken);
var diagnostics = semanticModel.GetDiagnostics(context.FilterSpan, cancellationToken);
foreach (var diagnostic in diagnostics)
{
cancellationToken.ThrowIfCancellationRequested();
cancellationToken.ThrowIfCancellationRequested();

if (diagnostic.Id == CS0162)
{
ProcessUnreachableDiagnostic(context, root, diagnostic.Location.SourceSpan);
}
if (diagnostic.Id == CS0162)
{
ProcessUnreachableDiagnostic(context, root, diagnostic.Location.SourceSpan);
}
}
}

private void ProcessUnreachableDiagnostic(
SemanticModelAnalysisContext context, SyntaxNode root, TextSpan sourceSpan)
public static StatementSyntax? TryGetContainingStatement(SyntaxNode node)
{
var firstUnreachableStatement = node.FirstAncestorOrSelf<StatementSyntax>();
return firstUnreachableStatement != null && firstUnreachableStatement.SpanStart == node.SpanStart
? firstUnreachableStatement
: null;
}

private void ProcessUnreachableDiagnostic(
SemanticModelAnalysisContext context, SyntaxNode root, TextSpan sourceSpan)
{
var node = root.FindNode(sourceSpan);

// Note: this approach works as the language only supports the concept of
// unreachable statements. If we ever get unreachable subexpressions, then
// we'll need to revise this code accordingly.
var firstUnreachableStatement = TryGetContainingStatement(node);
if (firstUnreachableStatement is null)
return;

// At a high level, we can think about us wanting to fade out a "section" of unreachable
// statements. However, the compiler only reports the first statement in that "section".
// We want to figure out what other statements are in that section and fade them all out
// along with the first statement. This is made somewhat tricky due to the fact that
// subsequent sibling statements possibly being reachable due to explicit gotos+labels.
//
// On top of this, an unreachable section might not be contiguous. This is possible
// when there is unreachable code that contains a local function declaration in-situ.
// This is legal, and the local function declaration may be called from other reachable code.
//
// As such, it's not possible to just get first unreachable statement, and the last, and
// then report that whole region as unreachable. Instead, when we are told about an
// unreachable statement, we simply determine which other statements are also unreachable
// and bucket them into contiguous chunks.
//
// We then fade each of these contiguous chunks, while also having each diagnostic we
// report point back to the first unreachable statement so that we can easily determine
// what to remove if the user fixes the issue. (The fix itself has to go recompute this
// as the total set of statements to remove may be larger than the actual faded code
// that that diagnostic corresponds to).

// Get the location of this first unreachable statement. It will be given to all
// the diagnostics we create off of this single compiler diagnostic so that we always
// know how to find it regardless of which of our diagnostics the user invokes the
// fix off of.
var firstStatementLocation = root.SyntaxTree.GetLocation(firstUnreachableStatement.FullSpan);

// 'additionalLocations' is how we always pass along the locaiton of the first unreachable
// statement in this group.
var additionalLocations = ImmutableArray.Create(firstStatementLocation);

context.ReportDiagnostic(DiagnosticHelper.CreateWithLocationTags(
Descriptor,
firstStatementLocation,
NotificationOption2.ForSeverity(Descriptor.DefaultSeverity),
additionalLocations: [],
additionalUnnecessaryLocations: additionalLocations));

var sections = RemoveUnreachableCodeHelpers.GetSubsequentUnreachableSections(firstUnreachableStatement);
foreach (var section in sections)
{
var node = root.FindNode(sourceSpan);

// Note: this approach works as the language only supports the concept of
// unreachable statements. If we ever get unreachable subexpressions, then
// we'll need to revise this code accordingly.
var firstUnreachableStatement = node.FirstAncestorOrSelf<StatementSyntax>();
if (firstUnreachableStatement == null ||
firstUnreachableStatement.SpanStart != sourceSpan.Start)
{
return;
}
var span = TextSpan.FromBounds(section[0].FullSpan.Start, section.Last().FullSpan.End);
var location = root.SyntaxTree.GetLocation(span);

// At a high level, we can think about us wanting to fade out a "section" of unreachable
// statements. However, the compiler only reports the first statement in that "section".
// We want to figure out what other statements are in that section and fade them all out
// along with the first statement. This is made somewhat tricky due to the fact that
// subsequent sibling statements possibly being reachable due to explicit gotos+labels.
//
// On top of this, an unreachable section might not be contiguous. This is possible
// when there is unreachable code that contains a local function declaration in-situ.
// This is legal, and the local function declaration may be called from other reachable code.
//
// As such, it's not possible to just get first unreachable statement, and the last, and
// then report that whole region as unreachable. Instead, when we are told about an
// unreachable statement, we simply determine which other statements are also unreachable
// and bucket them into contiguous chunks.
//
// We then fade each of these contiguous chunks, while also having each diagnostic we
// report point back to the first unreachable statement so that we can easily determine
// what to remove if the user fixes the issue. (The fix itself has to go recompute this
// as the total set of statements to remove may be larger than the actual faded code
// that that diagnostic corresponds to).

// Get the location of this first unreachable statement. It will be given to all
// the diagnostics we create off of this single compiler diagnostic so that we always
// know how to find it regardless of which of our diagnostics the user invokes the
// fix off of.
var firstStatementLocation = root.SyntaxTree.GetLocation(firstUnreachableStatement.FullSpan);

// 'additionalLocations' is how we always pass along the locaiton of the first unreachable
// statement in this group.
var additionalLocations = ImmutableArray.Create(firstStatementLocation);
// Mark subsequent sections as being 'cascaded'. We don't need to actually process them
// when doing a fix-all as they'll be scooped up when we process the fix for the first
// section.
var additionalUnnecessaryLocations = ImmutableArray.Create(location);

context.ReportDiagnostic(DiagnosticHelper.CreateWithLocationTags(
Descriptor,
firstStatementLocation,
location,
NotificationOption2.ForSeverity(Descriptor.DefaultSeverity),
additionalLocations: [],
additionalUnnecessaryLocations: additionalLocations));

var sections = RemoveUnreachableCodeHelpers.GetSubsequentUnreachableSections(firstUnreachableStatement);
foreach (var section in sections)
{
var span = TextSpan.FromBounds(section[0].FullSpan.Start, section.Last().FullSpan.End);
var location = root.SyntaxTree.GetLocation(span);

// Mark subsequent sections as being 'cascaded'. We don't need to actually process them
// when doing a fix-all as they'll be scooped up when we process the fix for the first
// section.
var additionalUnnecessaryLocations = ImmutableArray.Create(location);

context.ReportDiagnostic(DiagnosticHelper.CreateWithLocationTags(
Descriptor,
location,
NotificationOption2.ForSeverity(Descriptor.DefaultSeverity),
additionalLocations,
additionalUnnecessaryLocations,
s_subsequentSectionProperties));
}
additionalLocations,
additionalUnnecessaryLocations,
s_subsequentSectionProperties));
}
}
}
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