Improve error reporting: Let as last line in block - fixes dotnet#1162

src/fsharp/FSComp.txt

@@ -411,7 +411,7 @@ parsConsiderUsingSeparateRecordType,"Consider using a separate record type inste
 584,parsSuccessivePatternsShouldBeSpacedOrTupled,"Successive patterns should be separated by spaces or tupled"
 586,parsNoMatchingInForLet,"No matching 'in' found for this 'let'"
 587,parsErrorInReturnForLetIncorrectIndentation,"Error in the return expression for this 'let'. Possible incorrect indentation."
-588,parsExpectedStatementAfterLet,"Block following this '%s' is unfinished. Expect an expression."
+588,parsExpectedExpressionAfterLet,"Every code block is an expression and must have a result. '%s' cannot be the final code element in a block. Consider returning something explicit from this block."
 589,parsIncompleteIf,"Incomplete conditional. Expected 'if <expr> then <expr>' or 'if <expr> then <expr> else <expr>'."
 590,parsAssertIsNotFirstClassValue,"'assert' may not be used as a first class value. Use 'assert <expr>' instead."
 594,parsIdentifierExpected,"Identifier expected"

src/fsharp/pars.fsy

@@ -2957,7 +2957,7 @@ seqExpr:
   | hardwhiteLetBindings %prec prec_args_error
      { let hwlb,m = $1
        let mLetKwd,isUse = match hwlb with (BindingSetPreAttrs(m,_,isUse,_,_))  -> m,isUse
-       reportParseErrorAt mLetKwd (FSComp.SR.parsExpectedStatementAfterLet(if isUse then "use" else "let"))
+       reportParseErrorAt mLetKwd (FSComp.SR.parsExpectedExpressionAfterLet(if isUse then "use" else "let"))
        let fauxRange = m.EndRange // zero width range at end of m
        mkLocalBindings (m,hwlb,arbExpr("seqExpr",fauxRange)) }
 

tests/fsharp/typecheck/sigs/neg75.bsl

@@ -7,4 +7,4 @@ neg75.fsx(154,24,154,27): parse error FS0058: Possible incorrect indentation: th
 
 neg75.fsx(154,24,154,27): parse error FS0058: Possible incorrect indentation: this token is offside of context started at position (153:38). Try indenting this token further or using standard formatting conventions.
 
-neg75.fsx(153,24,153,27): parse error FS0588: Block following this 'let' is unfinished. Expect an expression.
+neg75.fsx(153,24,153,27): parse error FS0588: Every code block is an expression and must have a result. 'let' cannot be the final code element in a block. Consider returning something explicit from this block.

tests/fsharp/typecheck/sigs/neg76.bsl

@@ -7,4 +7,4 @@ neg76.fsx(154,24,154,27): parse error FS0058: Possible incorrect indentation: th
 
 neg76.fsx(154,24,154,27): parse error FS0058: Possible incorrect indentation: this token is offside of context started at position (153:38). Try indenting this token further or using standard formatting conventions.
 
-neg76.fsx(153,24,153,27): parse error FS0588: Block following this 'let' is unfinished. Expect an expression.
+neg76.fsx(153,24,153,27): parse error FS0588: Every code block is an expression and must have a result. 'let' cannot be the final code element in a block. Consider returning something explicit from this block.

tests/fsharp/typecheck/sigs/neg76.vsbsl

@@ -7,7 +7,7 @@ neg76.fsx(154,24,154,27): parse error FS0058: Possible incorrect indentation: th
 
 neg76.fsx(154,24,154,27): parse error FS0058: Possible incorrect indentation: this token is offside of context started at position (153:38). Try indenting this token further or using standard formatting conventions.
 
-neg76.fsx(153,24,153,27): parse error FS0588: Block following this 'let' is unfinished. Expect an expression.
+neg76.fsx(153,24,153,27): parse error FS0588: Every code block is an expression and must have a result. 'let' cannot be the final code element in a block. Consider returning something explicit from this block.
 
 neg76.fsx(154,24,154,27): parse error FS0058: Possible incorrect indentation: this token is offside of context started at position (153:38). Try indenting this token further or using standard formatting conventions.
 
@@ -17,4 +17,4 @@ neg76.fsx(155,24,155,27): parse error FS0058: Possible incorrect indentation: th
 
 neg76.fsx(155,24,155,27): parse error FS0058: Possible incorrect indentation: this token is offside of context started at position (153:38). Try indenting this token further or using standard formatting conventions.
 
-neg76.fsx(153,24,153,27): parse error FS0588: Block following this 'let' is unfinished. Expect an expression.
+neg76.fsx(153,24,153,27): parse error FS0588: Every code block is an expression and must have a result. 'let' cannot be the final code element in a block. Consider returning something explicit from this block.

tests/fsharpqa/Source/Warnings/MissingExpressionAfterLet.fs

@@ -0,0 +1,8 @@
+// #Warnings
+//<Expects status="Error" span="(6,5)" id="FS0588">This value is not mutable. Consider using the mutable keyword, e.g. 'let mutable x = expression'.</Expects>
+
+let sum = 0
+for x in 0 .. 10 do
+    let sum = sum + x
+
+exit 0

tests/fsharpqa/Source/Warnings/env.lst

@@ -1,4 +1,5 @@
 	SOURCE=WrongNumericLiteral.fs # WrongNumericLiteral.fs
 	SOURCE=WarnIfMissingElseBranch.fs # WarnIfMissingElseBranch.fs
 	SOURCE=ElseBranchHasWrongType.fs # ElseBranchHasWrongType.fs
+	SOURCE=MissingExpressionAfterLet.fs # MissingExpressionAfterLet.fs
 	SOURCE=AssignmentOnImmutable.fs # AssignmentOnImmutable.fs

vsintegration/tests/unittests/Tests.LanguageService.ErrorRecovery.fs

@@ -38,7 +38,7 @@ type UsingMSBuild()  =
                 override x.ToString() = ""
             let Main() =
                 let x = MyType()"""
-        let expectedStr = "Block following this 'let' is unfinished"
+        let expectedStr = "Every code block is an expression and must have a result. 'let' cannot be the final code element in a block. Consider returning something explicit from this block."
         this.VerifyErrorListContainedExpectedString(fileContent,expectedStr)
 
     // Not a recovery case, but make sure we get a squiggle at the unfinished Main()
@@ -50,7 +50,7 @@ type UsingMSBuild()  =
                 override x.ToString() = ""
             let Main() =
                 use x = MyType()"""
-        let expectedStr = "Block following this 'use' is unfinished"
+        let expectedStr = "Every code block is an expression and must have a result. 'use' cannot be the final code element in a block. Consider returning something explicit from this block."
         this.VerifyErrorListContainedExpectedString(fileContent,expectedStr)
 
     [<Test>]

vsintegration/tests/unittests/Tests.LanguageService.Squiggles.fs

@@ -544,7 +544,7 @@ type X() =
     [<Test>]
     member public this.``OrphanFs.ParseErrorsStillShow``() =  
         let fileContent = """let foo = let(*Mark*)"""
-        this.VerifySquiggleContainedAtStartOfMarker(fileContent,"(*Mark*)",(Microsoft.VisualStudio.FSharp.LanguageService.Severity.Error, "Block following "))  
+        this.VerifySquiggleContainedAtStartOfMarker(fileContent,"(*Mark*)",(Microsoft.VisualStudio.FSharp.LanguageService.Severity.Error, "Every code block "))
 
     /// FEATURE: If a .fs file has a BuildAction other than "Compile", it behaves like a
     /// single-file-project with regards to intellisense.